C++ 与C的区别

[TOC]

C++ 与C的区别

C++中函数的形参的列表为void表面函数不接受任何参数，这与括号里面空着等效；
C中函数的形参的列表为void表面函数不接受任何参数，让括号里面空着意外着对是否接受参数保存沉默；

need to know

'\n'的性能高于std::cout
计算机内存的基本单位是位（bit），可以看作电子开关，可以开也可以关；
字节（bytes）通常指的是8位的内存单元;
sizeof(short):2 (bytes ) sizeof(int):4 sizeof(long):4 sizeof(long):8

计算机在存储数据时候必须跟踪的三种基本属性：

信息存储在何处
存储的值为多少
存储的信息是什么类型

Fundamental Data Types

*整型常量*	*所需类型*
10	代表int类型
10l, 10L	代表long类型
10ll, 10LL	代表long long类型
10u, 10U	代表unsigned int类型
10ul, 10UL	代表unsigned long类型
10ull, 10ULL	代表unsigned long long类型

char

'9' - '0'的类型为int

Now that fixed-width integers have been added to C++, the best practice for integers in C++ is as follows:

int should be preferred when the size of the integer doesn’t matter (e.g. the number will always fit within the range of a 2 byte signed integer). For example, if you're asking the user to enter their age, or counting from 1 to 10, it doesn’t matter whether int is 16 or 32 bits (the numbers will fit either way). This will cover the vast majority of the cases you’re likely to run across.
If you need a variable guaranteed to be a particular size and want to favor performance, use std::int_fast#_t.
If you need a variable guaranteed to be a particular size and want to favor memory conservation over performance, use std::int_least#_t. This is used most often when allocating lots of variables.

Floating point number

Favor double over float unless space is at a premium, as the lack of precision in a float will often lead to inaccuracies.

const

Any variable that should not be modifiable after initialization and whose initializer is known at compile-time should be declared as constexpr.
Any variable that should not be modifiable after initialization and whose initializer is not known at compile-time should be declared as const.

Debug

//Using g++, you can use the -I option to specify an alternate include directory.
//g++ -o main -I/source/includes main.cpp

When using print statements, use std::cerr instead of std::cout.
C++ cout利用控制符dec、hex和oct，分别输出十进制、十六进制和八进制显示整数

literals

Data Type	Suffix	Meaning
int	u or U	unsigned int
int	l or L	long
int	ul, uL, Ul, UL, lu, lU, Lu, or LU	unsigned long
int	ll or LL	long long
int	ull, uLL, Ull, ULL, llu, llU, LLu, or LLU	unsigned long long
double	f or F	float
double	l or L	long double

Operators

Table of operators
scope-duration-and-linkage-summary
The modulus operator can also work with negative operands. x % y always returns results with the sign of x.
Avoid the postfix versions of these operators whenever possible.
double x{ 0.1 + 0.1 + 0.1 }; return (x == 0.3); return false;
De Morgan’s law says that if we distribute a NOT, we need to flip ORs and ANDs to the other. So ~(option4 | option5) becomes ~option4 & ~option5.
In C++, the term inline has evolved to mean “multiple definitions are allowed”.
When applied to a global variable, the static keyword defines the global variable as having internal linkage, meaning the variable cannot be exported to other files.
When applied to a local variable, the static keyword defines the local variable as having static duration, meaning the variable will only be created once, and will not be destroyed until the end of the program.
int8_t is actually a type alias for signed char
Don’t assign the same value to two enumerators in the same enumeration unless there’s a very good reason.

static

for local variables , keep their values and are not destroyed even after they go out of scope
for global variables,it gives them internal linkage (which restricts them from being seen/used outside of the file they are defined in).

static member variable

static member variables are shared by all objects of the class;static members exist even if no objects of the class have been instantiated!
Much like global variables, they are created when the program starts, and destroyed when the program ends.
static member definition is not subject to access controls: you can define and initialize the value even if it’s declared as private (or protected) in the class.

static member functions

static member functions are not attached to any particular object
Static member functions have no *this pointer
tic member functions can directly access other static members (variables or functions), but not non-static members. This is because non-static members must belong to a class object, and static member functions have no class object to work with!

ObjectScopeandConversions

std::string fullName{};
    //std::cin>>fullName;
    std::getline(std::cin,fullName);

std::cin.ignore(32767, '\n'); // ignore up to 32767 characters until a \n is removed
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');


int getInput()
{
    int inputNumber{};
    while (true)
    {
		std::cout << "Enter a integer value: ";
        std::cin >> inputNumber;
        if (std::cin.fail())
        {
            std::cin.clear();
            std::cin.ignore(32767, '\n');
            std::cout << "error,input again:";
        }
        else
        {
            std::cin.ignore(32767, '\n');
            return inputNumber;
        }
    }
}
#include <iostream>
#include <cstdlib> // for std::rand() and std::srand()
#include <ctime>   // for std::time()
#include <random>  // for std::mt19937
int getRandomNumber()
{
	// Initialize our mersenne twister with a random seed based on the clock
    std::mt19937 mersenne{static_cast<std::mt19937::result_type>(std::time(nullptr))};
    const int minNumber{1};
    const int maxNumber{100};
    std::uniform_int_distribution die{minNumber, maxNumber};
    return die(mersenne);
}

#include <cstdlib> // for rand() and srand()
// Generate a random number between min and max (inclusive)
// Assumes std::srand() has already been called
// Assumes max - min <= RAND_MAX
int getRandomNumber(int min, int max)
{
  static constexpr double fraction{ 1.0 / (RAND_MAX + 1.0) }; // static used for efficiency, so we only calculate this value once
  // evenly distribute the random number across our range
  return min + static_cast<int>((max - min + 1) * (std::rand() * fraction));
}
int main()
{
	std::srand(static_cast<unsigned int>(std::time(nullptr))); // set initial seed value to system clock
	std::rand(); // get rid of first result
	for (int i{ 0 }; i < 10; ++i)
	{
		Monster m{ Monster::getRandomMonster() };
		std::cout << "A " << m.getName() << " (" << m.getSymbol() << ") was created.\n";
	}
}

Arrays, Strings, Pointers, and References

指针p也是对象，它同样有地址&p和存储的值p，只不过，p存储的数据类型是数据的地址。

dynamic mamory

void doSomething()
{
    int *ptr{new int{}};
    ptr = nullptr; //mewmory leak
}

Pointers

箭头（->）：左边必须为指针；
点号（.）：左边必须为实体。

int x{5};
std::cout << typeid(&x).name() << '\n'; // prints “pi” (pointer to int)

When declaring a pointer variable, put the asterisk next to the variable name.
When declaring a function, put the asterisk of a pointer return value next to the type.

poinnter and array

Favor the pointer syntax (*) over the array syntax ([]) for array function parameters.
array[n] is the same as *(array + n)
Pass non-pointer, non-fundamental data type variables (such as structs) by (const) reference, unless you know that passing it by value is faster.
An array that decayed to a pointer cannot be used in a for-each loop.

Functions

Standard library functions may copy function objects (reminder: lambdas are function objects). If you want to provide lambdas with mutable captured variables, pass them by reference using std::ref.
Member functions can (and should) be made const if they do not modify the state of the class. Const class objects can only call const member functions.
Static member variables are shared among all objects of the class. Although they can be accessed from a class object, they can also be accessed directly via the scope resolution operator. Similarly, static member functions are member functions that have no *this pointer. They can only access static member variables.
C++中函数的形参的列表为void表面函数不接受任何参数，这与括号里面空着等效；
C中函数的形参的列表为void表面函数不接受任何参数，让括号里面空着意外着对是否接受参数保存沉默；

lambda

lambdas can only access specific kinds of identifiers: global identifiers, entities that are known at compile time, and entities with static storage duration
By default, variables are captured by const value.
To allow modifications of variables that were captured by value, we can mark the lambda as mutable.
To capture a variable by reference, we prepend an ampersand (&) to the variable name in the capture. Unlike variables that are captured by value, variables that are captured by reference are non-const, unless the variable they’re capturing is const.

Basic object-oriented programming

By default, all members of a class are private and all members of a struct are public. - - 对象是指一块能存储数据并具有某种类型的内存空间

classes-and-class-members

With normal non-member functions, a function can’t call a function that’s defined “below” it ,With member functions, this limitation doesn’t apply
Use the struct keyword for data-only structures. Use the class keyword for objects that have both data and functions.
Make member variables private, and member functions public, unless you have a good reason not to.
Getters should return by value or const reference.

constructors

Favor brace initialization to initialize class objects.
If you have constructors in your class and need a default constructor that does nothing, use = default.
Always initialize all member variables in your objects.
The default constructor is used if no initialization values are provided by the user. You should always provide at least one constructor for your classes.

constructor-member-initializer-lists

Member initializer lists allows you to initialize your member variables from within a constructor (rather than assigning the member variables values).
When the class’s constructor is executed, m_value1, m_value2, and m_value3 are created. Then the body of the constructor is run, where the member data variables are assigned values.
Use member initializer lists to initialize your class member variables instead of assignment.

Initializer list order

Don’t initialize member variables in such a way that they are dependent upon other member variables being initialized first (in other words, ensure your member variables will properly initialize even if the initialization ordering is different).
Initialize variables in the initializer list in the same order in which they are declared in your class. This isn’t strictly required so long as the prior recommendation has been followed, but your compiler may give you a warning if you don’t do so and you have all warnings turned on.

non-static-member-initialization

Favor use of non-static member initialization to give default values for your member variables.

-overlapping-and-delegating-constructors

If you have multiple constructors that have the same functionality, use delegating constructors to avoid duplicate code.

class class_c {
public:
    int max;
    int min;
    int middle;

    class_c(int my_max) {
        max = my_max > 0 ? my_max : 10;
    }
    class_c(int my_max, int my_min) : class_c(my_max) {
        min = my_min > 0 && my_min < max ? my_min : 1;
    }
    class_c(int my_max, int my_min, int my_middle) : class_c (my_max, my_min){
        middle = my_middle < max && my_middle > min ? my_middle : 5;
}
};
int main() {
    class_c c1{ 1, 3, 2 };
}

hidden-this-pointer

class Simple
{
private:
    int m_id;
 
public:
    Simple(int id)
    {
        setID(id);
    }
 
    void setID(int id) { m_id = id; }
    int getID() { return m_id; }
};
int main()
{
    Simple simple(1);
    simple.setID(2);
    std::cout << simple.getID() << '\n';
 
    return 0;
}

simple.setID(2);

void setId( Simple* const this,int id){this->m_id=id;}

class-code-and-header-files

For classes used in only one file that aren’t generally reusable, define them directly in the single .cpp file they’re used in.
For classes used in multiple files, or intended for general reuse, define them in a .h file that has the same name as the class.
Trivial member functions (trivial constructors or destructors, access functions, etc…) can be defined inside the class.
Non-trivial member functions should be defined in a .cpp file that has the same name as the class.

const-class-objects-and-member-functions

we can’t call non-const member functions on const objects
A const member function is a member function that guarantees it will not modify the object or call any non-const member functions (as they may modify the object).

friend-functions-and-classes

Anonymous objects

An anonymous object is essentially a value that has no name

nested-types-in-classe

Although enumerations are probably the most common type that is nested inside a class, C++ will let you define other types within a class, such as typedefs, type aliases, and even other classes!

Cents cents{ 5 }; // normal variable
Cents{ 7 }; // anonymous object

operator overloading

introduction-to-operator-overloading

First, almost any existing operator in C++ can be overloaded. The exceptions are: conditional (?:), sizeof, scope (::), member selector (.), member pointer selector (.*), typeid, and the casting operators.
you can only overload the operators that exist.
at least one of the operands in an overloaded operator must be a user-defined type.
it is not possible to change the number of operands an operator supports.
all operators keep their default precedence and associativity
If the meaning of an operator when applied to a custom class is not clear and intuitive, use a named function instead.

overloading-the-arithmetic-operators-using-friend-function

We’re multiplying temporary Fraction objects, but non-const references cannot bind to temporaries.
Prefer overloading operators as normal functions instead of friends if it’s possible to do so without adding additional functions.

// The non-const multiplication operator looks like this
Fraction operator*(Fraction &f1, Fraction &f2)
 
// This doesn't work anymore
Fraction f6{ Fraction{1, 2} * Fraction{2, 3} * Fraction{3, 4} };

overloading-operators-using-member-function

Not everything can be overloaded as a friend function

The assignment (=), subscript ([]), function call (()), and member selection (->) operators must be overloaded as member functions, because the language requires them to be.

When to use a normal, friend, or member function overload

When dealing with binary operators that don’t modify the left operand (e.g. operator+), the normal or friend function version is typically preferred. When dealing with binary operators that do modify the left operand (e.g. operator+=), the member function version is typically preferred. Unary operators are usually overloaded as member functions as well

The following rules of thumb can help you determine which form is best for a given situation:

If you’re overloading assignment (=), subscript ([]), function call (()), or member selection (->), do so as a member function.
If you’re overloading a unary operator, do so as a member function.
If you’re overloading a binary operator that does not modify its left operand (e.g. operator+), do so as a normal function (preferred) or friend function.
If you’re overloading a binary operator that modifies its left operand, but you can’t modify the definition of the left operand (e.g. operator<<, which has a left operand of type ostream), do so as a normal function (preferred) or friend function.
If you’re overloading a binary operator that modifies its left operand (e.g. operator+=), and you can modify the definition of the left operand, do so as a member functi

copy-constructor

Prior to C++17, copy elision is an optimization the compiler can make. As of C++17, some cases of copy elision (including the example above) have been made mandatory.

onverting-constructors-explicit-and-delete

There are two things to keep in mind: first, the parenthesis operator must be implemented as a member function. Second, in non-object-oriented C++, the () operator is used to call functions. In the case of classes, operator() is just a normal operator that calls a function (named operator()) like any other overloaded operator.
Consider making your constructors and user-defined conversion member functions explicit to prevent implicit conversion errors
explicit keyword makes constructor ineligible for implicit conversions
When a function has been deleted, any use of that function is considered a compile error.

Overloading the assignment operator

If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).
If a new object does not have to be created before the copying can occur, the assignment operator is used.
assignment operator must be overloaded as a member function.

shallow-vs-deep-copying

The default copy constructor and default assignment operators do shallow copies, which is fine for classes that contain no dynamically allocated variables.
Classes with dynamically allocated variables need to have a copy constructor and assignment operator that do a deep copy.
Favor using classes in the standard library over doing your own memory management.

Summary

If you’re overloading assignment (=), subscript ([]), function call (()), or member selection (->), do so as a member function.
If you’re overloading a unary operator, do so as a member function.
If you’re overloading a binary operator that modifies its left operand (e.g. operator+=), do so as a member function if you can.
If you’re overloading a binary operator that does not modify its left operand (e.g. operator+), do so as a normal function or friend functio

object relationship

If you can design a class using (composition, aggregation, association, or dependency), then you should use composition

Composition vs aggregation vs association summary

Property	Composition	Aggregation	Association
Relationship type	Whole/part	Whole/part	Otherwise unrelated
Members can belong to multiple classes	No	Yes	Yes
Members existence managed by class	Yes	No	No
Directionality	Unidirectional	Unidirectional	Unidirectional or bidirectional
Relationship verb	Part-of	Has-a	Uses-a

stdinitializer_list

Much like std::string_view, std::initializer_list is very lightweight and copies tend to be cheaper than an indirection.
If you provide list construction, it’s a good idea to provide list assignment as well.

std::vector<int> array(5); // Calls std::vector::vector(std::vector::size_type), 5 value-initialized elements: 0 0 0 0 0
std::vector<int> array{ 5 }; // Calls std::vector::vector(std::initializer_list<int>), 1 element: 5

Inheritance

inheritance-and-access-specifier

The protected access specifier allows the class the member belongs to, friends, and derived classes to access the member. However, protected members are not accessible from outside the class.
Different kinds of inheritance, and their impact on access.If you do not choose an inheritance type, C++ defaults to private inheritance (just like members default to private access if you do not specify otherwise).

// Inherit from Base publicly
class Pub: public Base
{
};
 
// Inherit from Base protectedly
class Pro: protected Base
{
};
 
// Inherit from Base privately
class Pri: private Base
{
};
 
class Def: Base // Defaults to private inheritance
{
};

Use public inheritance unless you have a specific reason to do otherwise.

Access specifier in base class	Access specifier when inherited publicly	Access specifier when inherited protectedly	Access specifier when inherited privately
Public	Public	Protected	Private
Protected	Protected	Protected	Private
Private	Inaccessible	Inaccessible	Inaccessible

Avoid multiple inheritance unless alternatives lead to more complexity.
Local variables are destroyed in the opposite order of definition.

virtual function

order in function const override final
A virtual function is a special type of function that, when called, resolves to the most-derived version of the function that exists between the base and derived class.
If a function is marked as virtual, all matching overrides are also considered virtual, even if they are not explicitly marked as such.
Rule: Never call virtual functions from constructors or destructors
Rule: Apply the override specifier to every intended override function you write.
Whenever you are dealing with inheritance, you should make any explicit destructors virtual.
- If you intend your class to be inherited from, make sure your destructor is virtual.
- If you do not intend your class to be inherited from, mark your class as final. This will prevent other classes from inheriting from it in the first place, without imposing any other use restrictions on the class itself.

Pure virtual functions

any class with one or more pure virtual functions becomes an abstract base class, which means that it can not be instantiated!
any derived class must define a body for this function, or that derived class will be considered an abstract base class as well.
A pure virtual function is useful when we have a function that we want to put in the base class, but only the derived classes know what it should return. A pure virtual function makes it so the base class can not be instantiated, and the derived classes are forced to define these functions before they can be instantiated. This helps ensure the derived classes do not forget to redefine functions that the base class was expecting them to.

A virtual function can be made pure virtual/abstract by adding “= 0” to the end of the virtual function prototype
A class contain a pure virtual function (whether with a body or not) oi an abstract class so it can’t be instantiated.
An interface class is a class that has no member variables, and where all of the functions are pure virtual!
An interface class is one with no member variables and all pure virtual functions. These are often named starting with a capital I.
Abstract classes still have virtual tables, as these can still be used if you have a pointer or reference to the abstract class. The virtual table entry for a pure virtual function will generally either contain a null pointer, or point to a generic function that prints an error (sometimes this function is named __purecall) if no override is provided.
To share a base class, simply insert the “virtual” keyword in the inheritance list of the derived class. This creates what is called a virtual base class, which means there is only one base object. The base object is shared between all objects in the inheritance tree and it is only constructed once.
The assigning of a Derived class object to a Base class object is called object slicing (or slicing for short).
Make sure your function parameters are references (or pointers) and try to avoid any kind of pass-by-value when it comes to derived classes.
Dynamic casting can be used to convert a pointer to a base class object into a pointer to a derived class object. This is called downcasting. A failed conversion will return a null pointer.
Although dynamic casts have a few different capabilities, by far the most common use for dynamic casting is for converting base-class pointers into derived-class pointers. This process is called downcasting.
Always ensure your dynamic casts actually succeeded by checking for a null pointer result.

class PoweredDevice
{
};
 
class Scanner: virtual public PoweredDevice
{
};
 
class Printer: virtual public PoweredDevice
{
};
 
class Copier: public Scanner, public Printer
{
};

Also note that there are several cases where downcasting using dynamic_cast will not work:

With protected or private inheritance.
For classes that do not declare or inherit any virtual functions (and thus don’t have a virtual table).
In certain cases involving virtual base classes (see this page for an example of some of these cases, and how to resolve them).

It turns out that downcasting can also be done with static_cast. The main difference is that static_cast does no runtime type checking to ensure that what you’re doing makes sense. This makes using static_cast faster, but more dangerous.
New programmers are sometimes confused about when to use static_cast vs dynamic_cast. The answer is quite simple: use static_cast unless you’re downcasting, in which case dynamic_cast is usually a better choice. However, you should also consider avoiding casting altogether and just using virtual functions.
In general, using a virtual function should be preferred over downcasting. However, there are times when downcasting is the better choice:

When you can not modify the base class to add a virtual function (e.g. because the base class is part of the standard library)
When you need access to something that is derived-class specific (e.g. an access function that only exists in the derived class)
When adding a virtual function to your base class doesn’t make sense (e.g. there is no appropriate value for the base class to return). Using a pure virtual function may be an option here if you don’t need to instantiate the base class.

The easiest way to overload operator<< for inherited classes is to write an overloaded operator<< for the most-base class, and then call a virtual member function to do the printing.

Template

If you feel that putting the Array.cpp code into the Array.h header makes the header too long/messy, an alternative is to rename Array.cpp to Array.inl (.inl stands for inline), and then include Array.inl from the bottom of the Array.h header. That yields the same result as putting all the code in the header, but helps keep things a little cleaner.
Another alternative is to use a three-file approach. The template class definition goes in the header. The template class member functions goes in the code file. Then you add a third file, which contains all of the instantiated classes you need:
Classes also support partial specialization, where only some of the templated parameters are specialized. Functions do not support partial specialization as of C++14.

Exception

throw type

throw -1; // throw a literal integer value
throw ENUM_INVALID_INDEX; // throw an enum value
throw "Can not take square root of negative number"; // throw a literal C-style (const char*) string
throw dX; // throw a double variable that was previously defined
throw MyException("Fatal Error"); // Throw an object of class MyException

Note that the compiler will not perform implicit conversions or promotions when matching exceptions with catch blocks! For example, a char exception will not match with an int catch block. An int exception will not match a float catch block. However, casts from a derived class to one of its parent classes will be performed.
Rule: Handlers for derived exception classes should be listed before those for base classes.
std::exception::what() this string is meant to be used for descriptive text only -- do not use it for comparisons, as it is not guaranteed to be the same across compilers.
Rule: When rethrowing the same exception, use the throw keyword by itself.
So when should I use exceptions?

The error being handled is likely to occur only infrequently. 
The error is serious and execution could not continue otherwise. 
The error cannot be handled at the place where it occurs. 
There isn’t a good alternative way to return an error code back to the caller.

The noexcept specifier defines a function as non-throwing.
Functions that are non-throwing by default:

default constructors
copy constructors
move constructors
destructors
copy assignment operators
move assignment operators

The following are potentially throwing by default:

Normal functions
User-defined constructors
Some operators, such as new

void foo() {throw -1;}
void boo() {};
void goo() noexcept {};
struct S{};
 
constexpr bool b1{ noexcept(5 + 3) }; // true; ints are non-throwing
constexpr bool b2{ noexcept(foo()) }; // false; foo() throws an exception
constexpr bool b3{ noexcept(boo()) }; // false; boo() is implicitly noexcept(false)
constexpr bool b4{ noexcept(goo()) }; // true; goo() is explicitly noexcept(true)
constexpr bool b5{ noexcept(S{}) };   // true; a struct's default constructor is noexcept by default

Use the noexcept specifier in specific cases where you want to express a no-fail or no-throw guarantee.

move semantics and smart pointers

C++11 adds a new type of reference called an r-value reference. An r-value reference is a reference that is designed to be initialized with an r-value (only). While an l-value reference is created using a single ampersand, an r-value reference is created using a double ampersand:

int x{ 5 };
int &lref{ x }; // l-value reference initialized with l-value x
int &&rref{ 5 }; // r-value reference initialized with r-value 5

First, r-value references extend the lifespan of the object they are initialized with to the lifespan of the r-value reference (l-value references to const objects can do this too). Second, non-const r-value references allow you to modify the r-value!
R-value references are more often used as function parameters. This is most useful for function overloads when you want to have different behavior for l-value and r-value arguments.

#include <iostream>

template <typename T>
class Auto_ptr4
{
	T *m_ptr;

public:
	Auto_ptr4(T *ptr = nullptr)
		: m_ptr(ptr)
	{
	}

	~Auto_ptr4()
	{
		delete m_ptr;
	}

	// Copy constructor -- no copying allowed!
	// Auto_ptr5(const Auto_ptr5& a) = delete;

	// Copy constructor
	// Do deep copy of a.m_ptr to m_ptr
	Auto_ptr4(const Auto_ptr4 &a)
	{
		m_ptr = new T;
		*m_ptr = *a.m_ptr;
		std::cerr << "call copy ()\n";
	}

	// Move constructor
	// Transfer ownership of a.m_ptr to m_ptr
	Auto_ptr4(Auto_ptr4 &&a) noexcept
		: m_ptr(a.m_ptr)
	{
		a.m_ptr = nullptr; // we'll talk more about this line below
		std::cerr << "call more ()";
	}
	// Copy assignment -- no copying allowed!
	// Auto_ptr5 &operator=(const Auto_ptr5 &a) = delete;

	// Copy assignment
	// Do deep copy of a.m_ptr to m_ptr
	Auto_ptr4 &operator=(const Auto_ptr4 &a)
	{
		// Self-assignment detection
		if (&a == this)
			return *this;

		// Release any resource we're holding
		delete m_ptr;

		// Copy the resource
		m_ptr = new T;
		*m_ptr = *a.m_ptr;
		std::cerr << "call copy =\n";

		return *this;
	}

	// Move assignment
	// Transfer ownership of a.m_ptr to m_ptr
	Auto_ptr4 &operator=(Auto_ptr4 &&a) noexcept
	{
		// Self-assignment detection
		if (&a == this)
			return *this;

		// Release any resource we're holding
		delete m_ptr;

		// Transfer ownership of a.m_ptr to m_ptr
		m_ptr = a.m_ptr;
		a.m_ptr = nullptr; // we'll talk more about this line below
		std::cerr << "call more =\n";

		return *this;
	}

	T &operator*() const { return *m_ptr; }
	T *operator->() const { return m_ptr; }
	bool isNull() const { return m_ptr == nullptr; }
};

class Resource
{
public:
	Resource() { std::cout << "Resource acquired\n"; }
	~Resource() { std::cout << "Resource destroyed\n"; }
};

Auto_ptr4<Resource> generateResource()
{
	Auto_ptr4<Resource> res{new Resource};
	return res; // this return value will invoke the move constructor
}

int main()
{
	Auto_ptr4<Resource> mainres;
	mainres = generateResource(); // this assignment will invoke the move assignment
	int *a{new int[11111]};
	delete[] a;
	return 0;
}

However, if we construct an object or do an assignment where the argument is an r-value, then we know that r-value is just a temporary object of some kind. Instead of copying it (which can be expensive), we can simply transfer its resources (which is cheap) to the object we’re constructing or assigning. This is safe to do because the temporary will be destroyed at the end of the expression anyway, so we know it will never be used again!

void fun(const int &lref) // l-value arguments will select this function
{
	std::cout << "l-value reference to const\n";
}
 
void fun(int &&rref) // r-value arguments will select this function
{
	std::cout << "r-value reference\n";
}
 
int main()
{
	int x{ 5 };
	fun(x); // l-value argument calls l-value version of function
	fun(5); // r-value argument calls r-value version of function
 
	return 0;
}

In C++11, std::move is a standard library function that serves a single purpose -- to convert its argument into an r-value. We can pass an l-value to std::move, and it will return an r-value reference. std::move is defined in the utility header.

#include <iostream>
#include <string>
#include <utility> // for std::move

template <class T>
void myswapMove(T &a, T &b)
{
    T tmp{std::move(a)}; // invokes move constructor
    a = std::move(b);    // invokes move assignment
    b = std::move(tmp);  // invokes move assignment
}

int main()
{
    std::string x{"abc"};
    std::string y{"de"};

    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';

    myswapMove(x, y);

    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';

    return 0;
}

std::unique_ptr has an overloaded operator* and operator-> that can be used to return the resource being managed. Operator* returns a reference to the managed resource, and operator-> returns a pointer
Rule: Favor std::array, std::vector, or std::string over a smart pointer managing a fixed array, dynamic array, or C-style string
use std::make_unique() instead of creating std::unique_ptr and using new yourself
std::unique_ptr can be safely returned from a function by value:

std::unique_ptr<Resource> createResource()
{
     return std::make_unique<Resource>();
}
int main()
{
    auto ptr{ createResource() };
 
    // do whatever
 
    return 0;
}

std::shared_ptr
std::shared_ptr uses two pointers internally. One pointer points at the resource being managed. The other points at a “control block”, which is a dynamically allocated object that tracks of a bunch of stuff, including how many std::shared_ptr are pointing at the resource.
std::make_shared() is simpler and safer(there’s no way to directly create two std::shared_ptr pointing to the same resource using this method)
std::weak_ptr is the smart pointer class used when you need one or more objects with the ability to view and access a resource managed by a std::shared_ptr, but unlike std::shared_ptr, std::weak_ptr is not considered when determining whether the resource should be destroyed.

#include <iostream>
#include <memory> // for std::shared_ptr and std::weak_ptr
#include <string>

class Person
{
	std::string m_name;
	std::weak_ptr<Person> m_partner; // note: This is now a std::weak_ptr

public:

	Person(const std::string &name): m_name(name)
	{
		std::cout << m_name << " created\n";
	}
	~Person()
	{
		std::cout << m_name << " destroyed\n";
	}

	friend bool partnerUp(std::shared_ptr<Person> &p1, std::shared_ptr<Person> &p2)
	{
		if (!p1 || !p2)
			return false;

		p1->m_partner = p2;
		p2->m_partner = p1;

		std::cout << p1->m_name << " is now partnered with " << p2->m_name << '\n';

		return true;
	}
};

int main()
{
	auto lucy { std::make_shared<Person>("Lucy") };
	auto ricky { std::make_shared<Person>("Ricky") };

	partnerUp(lucy, ricky);

	return 0;
}

The Standard Template Library

accumulate

Defined in header <numeric>

#include <iostream>
#include <vector>
#include <numeric>
#include <string>
#include <functional>
 
int main()
{
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
 
    int sum = std::accumulate(v.begin(), v.end(), 0);  //55
    //   int product = std::accumulate(v.begin(), v.end(), 1, std::puls<int>());   //55
    int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<int>());  //3628800
 
    auto dash_fold = [](std::string a, int b) {
                         return std::move(a) + '-' + std::to_string(b);
                     };
 
    std::string s = std::accumulate(std::next(v.begin()), v.end(),
                                    std::to_string(v[0]), // start with first element
                                    dash_fold);    //1-2-3-4-5-6-7-8-9-10
 
    // Right fold using reverse iterators
    std::string rs = std::accumulate(std::next(v.rbegin()), v.rend(),
                                     std::to_string(v.back()), // start with last element
                                     dash_fold);  //10-9-8-7-6-5-4-3-2-1
 
    std::cout << "sum: " << sum << '\n'
              << "product: " << product << '\n'
              << "dash-separated string: " << s << '\n'
              << "dash-separated string (right-folded): " << rs << '\n';
}

Array

int a1[5] = { 1, 2, 3 };  // other element will be set as the default value，也就是该数组的第4，5个元素的值为0
int size = sizeof(a1) / sizeof(*a1);  // sizeof(a1)的值为20，sizeof(*a1)为4，*a1为数组第一个元素，值为1，*(a1+2)为数组第三个元素，值为3
for (int& item : a1) {cout << " " << item;} // Iterate all Elements
sort(a1, a1 + size); //sort

vector

	vector<int> v2(v1.begin(), v1.end());vector<int> v3(v2);// 2. make a copy
	// 2. cast an array to a vector
	int a[5] = { 0, 1, 2, 3, 4 };
	vector<int> v4(a, *(&a + 1));  //如果v4.rezise(3)，会把后面的两个值丢掉，此时的值为0,1,2
	// 5. iterate the vector
	cout << "[Version 1] The contents of v4 are:";
	for (int i = 0; i < v4.size(); ++i) {
		cout << " " << v4[i];
	}
	cout << endl;
	cout << "[Version 2] The contents of v4 are:";
	for (int& item : v4) {
		cout << " " << item;
	}
	cout << endl;
	cout << "[Version 3] The contents of v4 are:";
	for (auto item = v4.begin(); item != v4.end(); ++item) {
		cout << " " << *item;//typeid(*item).name()的值为int

	}

unordered_map

unordered_map<int, int> m;
cout<<[0]// 默认值为1

String s='asdas';
s[0]//数据类型为char

string

Input and Output

*打印格式*	*含义*
%hd	输出short类型
%d	输出int类型
%ld	输出long类型
%lld	输出long long类型
%hu	输出unsigned short类型
%u	输出unsigned int类型
%lu	输出unsigned long类型
%llu	输出unsigned long long类型

输入/输出函数家族

家族名	目的	可用于所有流	只用于stdin和stdout
getchar	字符输入	fgetc、getc	getchar
putchar	字符输出	fputc、putc	putchar
gets	文本行输入	fgets	gets
puts	文本行输出	fputs	puts
scanf	格式化输入	fscanf	scanf
printf	格式化输出	fprintf	printf

input-with-istream

setw (in the iomanip.h header) can be used to limit the number of characters read in from a stream.

#include <iomanip.h>
char buf[10];
std::cin >> std::setw(10) >> buf;

get() function, which simply gets a character from the input stream but not discard whitespace

char ch;
while (std::cin.get(ch))
    std::cout << ch;

char strBuf[11];
std::cin.get(strBuf, 11);
std::cout << strBuf << '\n';

getline() that works exactly like get() but reads the newline as well

char strBuf[11];
// Read up to 10 characters
std::cin.getline(strBuf, 11);
std::cout << strBuf << '\n';

// Read up to 10 more characters
std::cin.getline(strBuf, 11);
std::cout << strBuf << '\n';

use gcount() to know how many character were extracted by the last call of getline()

char strBuf[100];
std::cin.getline(strBuf, 100);
std::cout << strBuf << '\n';
std::cout << std::cin.gcount() << " characters were read" << std::endl;

A special version of getline() for std::string

std::string strBuf;
std::getline(std::cin, strBuf);
std::cout << strBuf << '\n';

ignore() discards the first character in the stream.
ignore(int nCount) discards the first nCount characters.
peek() allows you to read a character from the stream without removing it from the stream.
unget() returns the last character read back into the stream so it can be read again by the next call.
putback(char ch) allows you to put a character of your choice back into the stream to be read by the next call.

Output with ostream and ios

To switch a flag on, use the setf() function, with the appropriate flag as a parameter.use unsetf() To turn a flag off

std::cout.setf(std::ios::showpos); // turn on the std::ios::showpos flag
std::cout << 27 << '\n';
std::cout.unsetf(std::ios::showpos); // turn off the std::ios::showpos flag
std::cout << 28 << '\n';

std::cout.unsetf(std::ios::dec); // turn off decimal output
std::cout.setf(std::ios::hex); // turn on hexadecimal output
std::cout << 27 << '\n';

when using this form of setf(), all of the flags belonging to the group are turned off, and only the flag passed in is turned on. For example:

// Turn on std::ios::hex as the only std::ios::basefield flag
std::cout.setf(std::ios::hex, std::ios::basefield);
std::cout << 27 << '\n';

manipulators : change the formatting options

std::cout << std::hex << 27 << '\n'; // print 27 in hex
std::cout << 28 << '\n'; // we're still in hex
std::cout << std::dec << 29 << '\n'; // back to decimal

Useful formatters

Group	Flag	Meaning
	std::ios::boolalpha	If set, booleans print “true” or “false”. If not set, booleans print 0 or 1

Manipulator	Meaning
std::boolalpha	Booleans print “true” or “false”
std::noboolalpha	Booleans print 0 or 1 (default)

Group	Flag	Meaning
	std::ios::showpos	If set, prefix positive numbers with a +

Manipulator	Meaning
std::showpos	Prefixes positive numbers with a +
std::noshowpos	Doesn’t prefix positive numbers with a +

Group	Flag	Meaning
	std::ios::uppercase	If set, uses upper case letters

Manipulator	Meaning
std::uppercase	Uses upper case letters
std::nouppercase	Uses lower case letters

Group	Flag	Meaning
std::ios::basefield	std::ios::dec	Prints values in decimal (default)
std::ios::basefield	std::ios::hex	Prints values in hexadecimal
std::ios::basefield	std::ios::oct	Prints values in octal
std::ios::basefield	(none)	Prints values according to leading characters of value

Manipulator	Meaning
std::dec	Prints values in decimal
std::hex	Prints values in hexadecimal
std::oct	Prints values in octal

Precision, notation, and decimal points

Group	Flag	Meaning
std::ios::floatfield	std::ios::fixed	Uses decimal notation for floating-point numbers
std::ios::floatfield	std::ios::scientific	Uses scientific notation for floating-point numbers
std::ios::floatfield	(none)	Uses fixed for numbers with few digits, scientific otherwise
std::ios::floatfield	std::ios::showpoint	Always show a decimal point and trailing 0’s for floating-point values

Manipulator	Meaning
std::fixed	Use decimal notation for values
std::scientific	Use scientific notation for values
std::showpoint	Show a decimal point and trailing 0’s for floating-point values
std::noshowpoint	Don’t show a decimal point and trailing 0’s for floating-point values
std::setprecision(int)	Sets the precision of floating-point numbers (defined in iomanip.h)

Member function	Meaning
std::precision()	Returns the current precision of floating-point numbers
std::precision(int)	Sets the precision of floating-point numbers and returns old precision

Option	Precision	12345.0	0.12345
Normal	3	1.23e+004	0.123
4	1.235e+004	0.1235
5	12345	0.12345
6	12345	0.12345
Showpoint	3	1.23e+004	0.123
4	1.235e+004	0.1235
5	12345.	0.12345
6	12345.0	0.123450
Fixed	3	12345.000	0.123
4	12345.0000	0.1235
5	12345.00000	0.12345
6	12345.000000	0.123450
Scientific	3	1.235e+004	1.235e-001
4	1.2345e+004	1.2345e-001
5	1.23450e+004	1.23450e-001
6	1.234500e+004	1.234500e-001

Width, fill characters, and justification

Group	Flag	Meaning
std::ios::adjustfield	std::ios::internal	Left-justifies the sign of the number, and right-justifies the value
std::ios::adjustfield	std::ios::left	Left-justifies the sign and value
std::ios::adjustfield	std::ios::right	Right-justifies the sign and value (default)

Manipulator	Meaning
std::internal	Left-justifies the sign of the number, and right-justifies the value
std::left	Left-justifies the sign and value
std::right	Right-justifies the sign and value
std::setfill(char)	Sets the parameter as the fill character (defined in iomanip.h)
std::setw(int)	Sets the field width for input and output to the parameter (defined in iomanip.h)

Member function	Meaning
std::fill()	Returns the current fill character
std::fill(char)	Sets the fill character and returns the old fill character
std::width()	Returns the current field width
std::width(int)	Sets the current field width and returns old field width

std::cout.fill('*');
std::cout << -12345 << '\n'; // print default value with no field width
std::cout << std::setw(10) << -12345 << '\n'; // print default with field width
std::cout << std::setw(10) << left << -12345 << '\n'; // print left justified
std::cout << std::setw(10) << right << -12345 << '\n'; // print right justified
std::cout << std::setw(10) << internal << -12345 << '\n'; // print internally justified
/*
-12345
****-12345
-12345****
****-12345
-****12345
*/

Stream classes for strings

stringstream

std::stringstream os;
os << "12345 67.89"; // insert a string of numbers into the stream
 
std::string strValue;
os >> strValue;
 
std::string strValue2;
os >> strValue2;
 
// print the numbers separated by a dash
std::cout << strValue << " - " << strValue2 << '\n';

Conversion between strings and numbers

converting numbers into a string:

std::stringstream os;
 
int nValue{ 12345 };
double dValue{ 67.89 };
os << nValue << ' ' << dValue;
 
std::string strValue1, strValue2;
os >> strValue1 >> strValue2;
 
std::cout << strValue1 << ' ' << strValue2 << '\n';

convert a numerical string to a number:

std::stringstream os;
os << "12345 67.89"; // insert a string of numbers into the stream
int nValue;
double dValue;
 
os >> nValue >> dValue;
 
cout << nValue << ' ' << dValue << '\n';

Clearing a stringstream for reuse

std::stringstream os;
os << "Hello ";
 
os.str(std::string{}); // erase the buffer
 
os << "World!";
cout << os.str();

std::stringstream os;
os << "Hello ";
 
os.str(""); // erase the buffer
os.clear(); // reset error flags
 
os << "World!";
std::cout << os.str();

Stream states and input validation

Stream states

The ios_base class contains several state flags that are used to signal various conditions that may occur when using streams:

Flag	Meaning
goodbit	Everything is okay
badbit	Some kind of fatal error occurred (e.g. the program tried to read past the end of a file)
eofbit	The stream has reached the end of a file
failbit	A non-fatal error occurred (eg. the user entered letters when the program was expecting an integer)
ios also provides a number of member functions in order to conveniently access these states:
Member function	Meaning
-------------------	--------------------------------------------------------------------------
good()	Returns true if the goodbit is set (the stream is ok)
bad()	Returns true if the badbit is set (a fatal error occurred)
eof()	Returns true if the eofbit is set (the stream is at the end of a file)
fail()	Returns true if the failbit is set (a non-fatal error occurred)
clear()	Clears all flags and restores the stream to the goodbit state
clear(state)	Clears all flags and sets the state flag passed in
rdstate()	Returns the currently set flags
setstate(state)	Sets the state flag passed in

Input validation

C++ provides a number of useful functions that we can use to determine whether specific characters are numbers or letters. The following functions live in the cctype header:

Function	Meaning
std::isalnum(int)	Returns non-zero if the parameter is a letter or a digit
std::isalpha(int)	Returns non-zero if the parameter is a letter
std::iscntrl(int)	Returns non-zero if the parameter is a control character
std::isdigit(int)	Returns non-zero if the parameter is a digit
std::isgraph(int)	Returns non-zero if the parameter is printable character that is not whitespace
std::isprint(int)	Returns non-zero if the parameter is printable character (including whitespace)
std::ispunct(int)	Returns non-zero if the parameter is neither alphanumeric nor whitespace
std::isspace(int)	Returns non-zero if the parameter is whitespace
std::isxdigit(int)	Returns non-zero if the parameter is a hexadecimal digit (0-9, a-f, A-F)

#include <algorithm> // std::all_of
#include <cctype> // std::isalpha, std::isspace
#include <iostream>
#include <string>
#include <string_view>
 
bool isValidName(std::string_view name)
{
  return std::ranges::all_of(name, [](char ch) {
    return (std::isalpha(ch) || std::isspace(ch));
  });
 
  // Before C++20, without ranges
  // return std::all_of(name.begin(), name.end(), [](char ch) {
  //    return (std::isalpha(ch) || std::isspace(ch));
  // });
}
 
int main()
{
  std::string name{};
 
  do
  {
    std::cout << "Enter your name: ";
    std::getline(std::cin, name); // get the entire line, including spaces
  } while (!isValidName(name));
 
  std::cout << "Hello " << name << "!\n";
}

Basic file I/O

File modes#

Ios file mode	Meaning
app	Opens the file in append mode
ate	Seeks to the end of the file before reading/writing
binary	Opens the file in binary mode (instead of text mode)
in	Opens the file in read mode (default for ifstream)
out	Opens the file in write mode (default for ofstream)
trunc	Erases the file if it already exists

std::ofstream outf{ "Sample.dat", std::ios::app };

// If we couldn't open the output file stream for writing
if (!outf)
{
    // Print an error and exit
    std::cerr << "Uh oh, Sample.dat could not be opened for writing!\n";
    return 1;
}

outf << "This is line 3" << '\n';
outf << "This is line 4" << '\n';

Explicitly opening files using open()

std::ofstream outf{ "Sample.dat" };
outf << "This is line 1" << '\n';
outf << "This is line 2" << '\n';
outf.close(); // explicitly close the file
 
// Oops, we forgot something
outf.open("Sample.dat", std::ios::app);
outf << "This is line 3\n";
outf.close();

Random file I/O

The seekg() and seekp() functions take two parameters. The first parameter is an offset that determines how many bytes to move the file pointer. The second parameter is an Ios flag that specifies what the offset parameter should be offset from.

Ios seek flag	Meaning
beg	The offset is relative to the beginning of the file (default)
cur	The offset is relative to the current location of the file pointer
end	The offset is relative to the end of the file

inf.seekg(14, ios::cur); // move forward 14 bytes
inf.seekg(-18, ios::cur); // move backwards 18 bytes
inf.seekg(22, ios::beg); // move to 22nd byte in file
inf.seekg(24); // move to 24th byte in file
inf.seekg(-28, ios::end); // move to the 28th byte before end of the file

this can be used to determine the size of a file:

std::ifstream inf("Sample.dat");
inf.seekg(0, std::ios::end); // move to end of file
std::cout << inf.tellg();

Name		Name	Last commit message	Last commit date
Latest commit History 120 Commits
.vscode		.vscode
C++		C++
C		C
datascture		datascture
img		img
leetcode		leetcode
thread		thread
.gitignore		.gitignore
LICENSE		LICENSE
README.md		README.md
TODO.md		TODO.md
hello.cpp		hello.cpp
install.sh		install.sh
main.cpp		main.cpp

License

tallsong/cpp

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C++ 与C的区别

need to know

Fundamental Data Types

char

Now that fixed-width integers have been added to C++, the best practice for integers in C++ is as follows:

Floating point number

const

Debug

literals

Operators

static

static member variable

static member functions

ObjectScopeandConversions

Arrays, Strings, Pointers, and References

dynamic mamory

Pointers

poinnter and array

Functions

lambda

Basic object-oriented programming

classes-and-class-members

constructors

constructor-member-initializer-lists

Initializer list order

non-static-member-initialization

-overlapping-and-delegating-constructors

hidden-this-pointer

class-code-and-header-files

const-class-objects-and-member-functions

friend-functions-and-classes

Anonymous objects

nested-types-in-classe

operator overloading

introduction-to-operator-overloading

overloading-the-arithmetic-operators-using-friend-function

overloading-operators-using-member-function

Not everything can be overloaded as a friend function

When to use a normal, friend, or member function overload

The following rules of thumb can help you determine which form is best for a given situation:

copy-constructor

onverting-constructors-explicit-and-delete

Overloading the assignment operator

shallow-vs-deep-copying

Summary

object relationship

Composition vs aggregation vs association summary

stdinitializer_list

Inheritance

inheritance-and-access-specifier

virtual function

Pure virtual functions

Template

Exception

move semantics and smart pointers

The Standard Template Library

accumulate

Array

vector

unordered_map

string

Input and Output

input-with-istream

Output with ostream and ios

Useful formatters

Precision, notation, and decimal points

Width, fill characters, and justification

Stream classes for strings

stringstream

Conversion between strings and numbers

Clearing a stringstream for reuse

Stream states and input validation

Stream states

Input validation