fameta-counter

Compile time counter that works with all major modern compilers, compatible with C++11 and above.

TL;DR

If you're in a hurry, click here for the How To

Intro

Back in 2015, Filip Roséen discovered yet another C++ twist that was unforeseen by its inventors. Venturing in these dark corners of the language doesn't come without consequence, as compiler vendors haven't felt compelled to stay compatible with whatever was the mechanism that allowed Roséen's work to actually perform as expected. The fact that the C++ Core language Working Group decided that this mechanism is "archane" hasn't made things better, but since they only stated that «it should be made ill-formed» gives us hope that it might not, after all, go.

Meanwhile others have leveraged the technique to make compilers do all sort of seemingly magic things: unconstexpr from DaemonSnake is pretty nifty, even though it only works with g++7 and presumably leaves the standard too far behind, eating the dust.

But...

Compile-time counters are a pretty nice thing to have, and the fact that most compiler vendors implemented a __COUNTER__ macro into their preprocessor, even though the standard doesn't mandate it, gives credit to this idea. But __COUNTER__ is not enough: it's just a macro, and as such has no knowledge about the context it's being used in and assumptions about its value can be messed up by the code that gets #include'd into our own.

Other kind of counters, more language-aware, have been implemented, like CopperSpice has done, but they come with their own limitations, among which stringent limits to the counter maximum value or the fact that incrementing the counter can only be done in a namespace scope (i.e. not within a function body or a class definition).

The solution

Filip Roséen's work doesn't work any longer with recent-ish compilers. I did some investigations and came up with some ideas as of why this is happenning (read about it on StackOverflow), and on the basis of those ideas I built fameta::counter.

How fameta::counter works

fameta::counter is as easy to use as Roséen's one, but it does need a bit of help from the user, which makes the usage of the counter more verbose, but the preprocessor makes it perfectly manageable.

The main trick that makes fameta::counter work is that a unique value or type has to be used as the template parameter of the next<Unique>() method each time the next counter value has to be retrieved. This makes the compiler always instantiate a new set of methods to access and modify the counter, which avoids the "caching problem" that seems to affect Roséen's solution.

__LINE__ and __COUNTER__

Where do we get such a unique value from? The preprocessor comes to our help: the __LINE__ macro, which increments its value each time it's used, with the only caveats to remember that it expands to the same value each time it's used on a given source line and that it's reset at the beginning of each file being included in the current translation unit.

This means that __LINE__ might not work in two cases:

1. if it's usage is hidden within other macros that expand on multiple next<__LINE__>() calls on the same line;
2. if the counter's scope is the whole translation unit and it's incremented in two or more different files and at least two instances of the next<__LINE__>() calls in two different files happen to expand on the same line number in those files.

In both the above cases, the uniqueness prerequisite would be invalidated.

If those scenarios present themselves, a simple workaround, is to use the __COUNTER__ macro, which changes value even if used multiple times on the same line and has a unique value across the whole translation unit. __COUNTER__ is not defined by the standard, but all the major compiler vendors support it.

So, to recap, instead of doing counter::next() as Roséen's code would have allowed us to do, all we need to do is counter.next<__COUNTER__>() or, in the rare event that the compiler doesn't provide the __COUNTER__ macro, counter.next<__LINE__>(), which isn't that bad after all, is it?

A moment of your attention, please
It 's been noted that Anthony Williams on Stack Overflow proposed a solution that avoids having to pass a unique value to each call into the counter, just like Filip Roséen's solution. However, his solution doesn't seem portable across compilers and has a few quirks also with the compilers that make it work. And I can't fully grasp how it works, where it does. Shame on me.

One more short moment
DaemonSnake has produced a version of his uncostexpr library that makes use of the new C++20 facilities: uncostexpr-cpp20. His library does lots more than providing a counter, but fameta-counter works also with just C++11. Also read his announcement on reddit.

Example of usage

#include <fameta/counter.hpp>

// Utility template to generate a unique type that makes the whole counter unique
template <int I>
struct U;

// First template parameter is the unique type, 
// the second template parameter is the first value we want fameta::counter::next() to return.
constexpr fameta::counter<U<1>, 100> C;

// As if int array[] = { 100, 101, 102 };
constexpr int array[] = {
    C.next<__COUNTER__>(), 
    C.next<__COUNTER__>(),
    C.next<__COUNTER__>()
};

static_assert(array[0] == 100 && array[1] == 101 && array[2] == 102, "oops");

// We can instantiate as many counters as we want, as long as we abide by the contract.
// Third template parameter is the step at which the the counter has to be incremented.
constexpr fameta::counter<U<2>, 100, 10> C2;

// As if int array2[] = { 100, 110, 120 };
constexpr int array2[] = {
    C2.next<__COUNTER__>(),
    C2.next<__COUNTER__>(),
    C2.next<__COUNTER__>()
};

static_assert(array2[0] == 100 && array2[1] == 110 && array2[2] == 120, "oops");

// Counters can also be decremented: just set a negative step
constexpr fameta::counter<U<3>, 1000, -10> C3;
 
// As if int array3[] = { 1000, 990, 980 };
constexpr int array3[] = {
    C3.next<__COUNTER__>(),
    C3.next<__COUNTER__>(),
    C3.next<__COUNTER__>()
};

static_assert(array3[0] == 1000 && array3[1] == 990 && array3[2] == 980, "oops");

// We could also use __LINE__ and it would still work in **MOST** cases.
// By default, the start value is set to 0 and the step to 1.
constexpr fameta::counter<U<4>> C4;
 
// As if int array3[] = { 0, 1, 2};
constexpr int array4[] = {
    C4.next<__LINE__>(),
    C4.next<__LINE__>(),
    C4.next<__LINE__>()
};

static_assert(array4[0] == 0 && array4[1] == 1 && array4[2] == 2, "oops");

// To simulate the case in which two counters shared the same lines in two different files within the same translation unit,
// we will use hardcoded numbers.
constexpr fameta::counter<U<5>> C5;
 
// As if int array5[] = { 0, 1, 2};
constexpr int array5[] = {
    C5.next<1010>(),
    C5.next<1020>(),
    C5.next<1030>()
};

static_assert(array5[0] == 0 && array5[1] == 1 && array5[2] == 2, "oops");

constexpr fameta::counter<U<5>> C6;
 
// What do you expect? I guess: int array6[] = { 0, 1, 2};
// But instead we get: { 3, 2, 4 }. Why? 
// Because C6 and C5 share the same type fameta::counter<U<5>>, 
// hence C5.next<1030>() and C6.next<1030>() return the same value,
// whilst C6.next<1001>() and C6.next<1040>() increment the value

constexpr int array6[] = {
    C6.next<1001>(),
    C6.next<1030>(),
    C6.next<1040>()
};

static_assert(array6[0] == 3 && array6[1] == 2 && array6[2] == 4, "oops");

// We can also use CRTP to make the counter unique
struct C7: fameta::counter<C7>{};

constexpr int array7[] = {
    C7::next<1001>(),
    C7::next<1030>(),
    C7::next<1040>()
};

static_assert(array7[0] == 0 && array7[1] == 1 && array7[2] == 2, "oops");

See it working on godbolt: https://godbolt.org/z/YaYaqbfxc