if / else at compile time in C++?

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C++17 if constexpr

Oh yes, it has arrived:

main.cpp

#include <cassert>
#include <type_traits>

template<typename T>
class MyClass {
    public:
        MyClass() : myVar{0} {}
        void modifyIfNotConst() {
            if constexpr(!isconst) {
                myVar = 1;
            }
        }
        T myVar;

    protected:
        static constexpr bool isconst = std::is_const<T>::value;
};

int main() {
    MyClass<double> x;
    MyClass<const double> y;
    x.modifyIfNotConst();
    y.modifyIfNotConst();
    assert(x.myVar == 1);
    assert(y.myVar == 0);
    return 0;
}

GitHub upstream.

Compile and run:

g++-8 -std=c++17 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out

See also: Difference between “if constexpr()” Vs “if()”

This will be really cool together with C++20 “string literal template arguments”: Passing a string literal as a parameter to a C++ template class

Tested in Ubuntu 16.04, GCC 8.1.0.

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