c++11 - How to use static_assert for constexpr function arguments in C++?

Question

I have several brief constexpr functions in my libraries that perform some simple calculations. I use them both in run-time and compile-time contexts.

I would like to perform some assertions in the body of these functions, however assert(...) is not valid in a constexpr function and static_assert(...) can not be used to check function parameters.

Example:

constexpr int getClamped(int mValue, int mMin, int mMax) noexcept
{
    assert(mMin <= mMax); // does not compile!
    return mValue < mMin ? mMin : (mValue > mMax ? mMax : mValue);
}

Is there a way to check whether the function is being executed in a run-time or compile-time constant and execute the assert only if it's being executed at run-time?

constexpr int getClamped(int mValue, int mMin, int mMax) noexcept
{
    assert_if_runtime(mMin <= mMax); 
    return mValue < mMin ? mMin : (mValue > mMax ? mMax : mValue);
}

Answer 1

Throwing an exception might be useful as the compiler will ignore the run-time part when it knows at compile-time that the exception is not thrown.

#include <cassert>

constexpr int getClamped(int mValue, int mMin, int mMax)
{
    return ( mMin <= mMax ) ? 
           ( mValue < mMin ? mMin : (mValue > mMax ? mMax : mValue) ) :
           throw "mMin must be less than or equal to mMax";
}

int main( int argc, char** argv )
{
    // These two work:
    static_assert( getClamped( 42, 0, 100 ) == 42, "CT" );
    assert( getClamped( argc, 0, 100 ) == argc );

    // Fails at compile-time:
    // static_assert( getClamped( 42, 100, 0 ) == 42, "CT" );

    // Fails at run-time:
    // assert( getClamped( argc, 100, 0 ) == argc );
}

Live example