Can I compute values that require a special function during compilation of C++?

Question

I appreciate I am being somewhat vague about what is exactly my issue, but I think that the fundamental question is clear. Please bear with me for a moment.

In brief, I have a static constexpr array of points which are used to find certain bounds I need to use. These bounds depend only on the array, so they can be precomputed. However, we want to be able to change these points and it is a pain to go and change every value every time we try to test something.

For example, let's say that I have the following setup:

The static constexpr array is

static constexpr double CHECK_POINTS[7] = { -1.5, -1.0, -0.5, 0.0, -0.5, 1.0, 1.5 };

and then in a function I'm calling, I have the following block of code:

std::vector<double> bounds = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0};
for(int i=0; i<bounds.size(); i++)
{
    bounds[i] = std::exp(CHECK_POINTS[i]);
}

Clearly, the values of bounds can be computed during compilation. Is there anyway I can make gcc do that?

EDIT: The vector in my code block is not essential, an array will do.

@appleapple just that? It doesn't need any special treatment? — tst, Aug 12 '22 at 15:28
There are some rules in `constexpr` functions, not every syntax is legal. But in C++17, you can have advanced power including loops. If the compiler accepts to compile, it should work. — prapin, Aug 12 '22 at 15:30
The rules are explained here : https://en.cppreference.com/w/cpp/language/constant_expression — Pepijn Kramer, Aug 12 '22 at 15:30
@appleapple [`std::exp` isn't required to be `constexpr`](https://en.cppreference.com/w/cpp/numeric/math/exp). In fact [most](https://stackoverflow.com/q/42189190/995714) [functions](https://stackoverflow.com/q/17347935/995714) in [`cmath`](https://stackoverflow.com/q/21303498/995714) [aren't `constexpr`](https://stackoverflow.com/q/50477974/995714) and the proposal to make them `constexpr` hasn't been approved yet although some implementations do make them constexpr themselves — phuclv, Aug 12 '22 at 15:31
@phuclv I believe that's not problem. I'm more concerned constexpr construct of vector. — apple apple, Aug 12 '22 at 15:32
@JasonLiam it isn't simple like that. You can't call a non constexpr function like `std::exp` in a constexpr function — phuclv, Aug 12 '22 at 15:32
@appleapple I wouldn't care about that first. It's easy to construct a fixed list of bounds but only after you have a constexpr way to calculate the values — phuclv, Aug 12 '22 at 15:33
@phuclv it's fine with array https://godbolt.org/z/be57xsGjf — apple apple, Aug 12 '22 at 15:35
@appleapple as I said, that's a [gcc extension](https://stackoverflow.com/q/70523217/995714). It [wont' work in other compilers](https://godbolt.org/z/j4jY9saG4). And FYI [there's `constexpr std::vector` in C++20](https://stackoverflow.com/a/52801072/995714) so that won't be a problem. The only issue here is the constexpr std::exp — phuclv, Aug 12 '22 at 15:38
@phuclv yes, but the `constexpr std::vector` doesn't seems to works here anyway https://godbolt.org/z/E8fY33nYs — apple apple, Aug 12 '22 at 15:43

apple apple · Accepted Answer · 2022-08-15T15:02:27.177

constexpt std::vector does not work here, but you can use std::array.
std::exp is not constexpr so you need to find constexpr alternatives
- it would work in gcc as an extension.

static constexpr double CHECK_POINTS[7] = { -1.5, -1.0, -0.5, 0.0, -0.5, 1.0, 1.5 };

static constexpr auto vec = [](){
    std::array bounds = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0};
    for(int i=0; i<bounds.size(); i++)
    {
        bounds[i] = std::exp(CHECK_POINTS[i]);
    }
    return bounds;
}();

it's would compile fine with gcc https://godbolt.org/z/x5a9q9M1d

(constexpr std::exp is an gcc extension, thanks to @phuclv to point out)

Can I compute values that require a special function during compilation of C++?

1 Answers1