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

Issue

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.

Solution

• 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)

Answered By - apple apple

Answer Checked By - Terry (WPSolving Volunteer)