How to determine if constexpr function is called in constant expression context?

std::is_constant_evaluated

We can use std::is_constant_evaluated function to determine if the constexpr function is called from constant expression context. For example,

#include <type_traits>
 
constexpr int max(int a, int b)
{
    if (std::is_constant_evaluated())
        return 10;
 
    return a < b ? b : a;
}
 
int result{max(1, 2)};
std::cout << result << "\n";
 
constexpr int result2{max(1, 2)};
std::cout << result2 << "\n";

For the first time, max is called in non-constant-expression context, so std::is_constant_evaluated returns false. We get result as 2. Next time, max is called in constant expression context, so std::is_constant_evaluated returns true and we get 10 as result.

We can use if consteval syntax for the same purpose as shown below:

constexpr int min(int a, int b)
{
    if consteval
    {
        return 10;
    }
 
    return a < b ? a : b;
}

Note

std::is_constant_evaluated and if consteval do not tell if the function is evaluated at compile time or runtime, they just tell if constexpr function is invoked in constant expression contexts.

References

1. https://www.learncpp.com/cpp-tutorial/constexpr-functions-part-3-and-consteval/