Runtime

Crossing the chasm

You often want the list of types you manipulate to produce effects at runtime. Yes, writing static assertions is nice and useful, but what about doing something at runtime which is based on a list of types generated at compile time?

In this tutorial we will see how you can execute code at runtime based on a list of types.

Printing the list of types

The brigand::for_each algorithm takes a list of types and execute a functor for each of them. The difference with other compile time algorithms is that brigand::for_each takes a regular runtime functor.

Let's see how we could print the list of types:

#include <iostream>

#include <brigand/algorithms/for_each.hpp>
#include <brigand/sequences/list.hpp>

using my_list = brigand::list<char, int, bool>;

struct f
{
    template <typename T>
    void operator()(brigand::type_<T>)
    {
        std::cout << typeid(T).name() << std::endl;
    }
};

int main(int, char **)
{
    // will print the types at runtime
    brigand::for_each<my_list>(f{});

    return 0;
}

As you can see, the lambda will receive a brigand::type_<T> type and not directly a T type. Without that indirection, it would not be possible to iterate on a list that contains the void type.

In C++ 14 you can even write:

#include <iostream>

#include <brigand/algorithms/for_each.hpp>
#include <brigand/sequences/list.hpp>

using my_list = brigand::list<char, int, bool>;

int main(int, char **)
{
    // will print the types at runtime
    brigand::for_each<brigand::list<char, int, bool>>([](auto x)
    {
        std::cout << typeid(decltype(x)::type).name() << std::endl;
    });

    return 0;
}

What does for_each return?

brigand::for_each returns the updated functor of the operation, not unlike std::for_each.

For example:

#include <brigand/algorithms/for_each.hpp>
#include <brigand/sequences/list.hpp>

struct f
{
    template <typename U>
    void operator(brigand::type_<U>)
    {
        acc += sizeof(U);
        std::cout << typeid(T).name() << std::endl;
    }

    int acc{0};
};

int main(int, char **)
{
    // will print the types at runtime
    // r contains the updated f
    auto r = brigand::for_each<my_list>(f{});

    std::cout << r.acc << std::endl;

    return 0;
}