Is constexpr useful for overload

Question

Is there a way in c++ to get a different overload called based on the runtime/compile time constness of an input? My version(12) of MSVC can\'t do this using constexpr. Reading

Answer 1

To elaborate on my comment, you can try this to work around the limitation you're facing:

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#include 
using std::cout;
using std::endl;

template 
constexpr int Flip4() {
    return ((n & 0xFF) << 24) | ((n & 0xFF00) << 8) | ((n & 0xFF0000) >> 8) | ((n & 0xFF000000) >> 24);
}
inline int Flip4(int n) {
    return _byteswap_ulong(n);
}

int main() {
    constexpr int a = Flip4<0xabcd>();  // calc at compile time
    int b = Flip4(0xabcd);  // calc at runtime

    static_assert(a == -844431360, "");

    cout << "a: " << a << endl;
    cout << "b: " << b << endl;
}

EDIT: Don't lose hope! User-defined literals are here to the rescue :)

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#include 
using std::cout;
using std::endl;

// wraps a single integer (unsigned long long) in order to use it in a user-defined literal
// the type (unsigned long long) is a limitation of the standard: https://stackoverflow.com/a/16596909/865719
struct IntegerWrapper
{
    const unsigned long long value;
    constexpr explicit IntegerWrapper(unsigned long long val) : value{val} {}
};
// user-defined literal
constexpr IntegerWrapper operator "" _iw (const unsigned long long value) 
{ 
    return IntegerWrapper{value};
}

constexpr int Flip4(IntegerWrapper&& n) {
    return ((n.value & 0xFF) << 24) | ((n.value & 0xFF00) << 8) | ((n.value & 0xFF0000) >> 8) | ((n.value & 0xFF000000) >> 24);
}

inline int Flip4(int n) {
    return _byteswap_ulong(n);
}

int main() {

    constexpr int a = Flip4(0xabcd_iw);  // calc at compile time
    const     int b = Flip4(0xabcd);     // calc at runtime

    static_assert(a == -844431360, "");

    cout << "a: " << a << endl;
    cout << "b: " << b << endl;
}