Can the conditional operator ( ? : ) in C++ be compile time?

Question

Can the ternary (conditional) operator be used as an analogous to constexpr if() , introduced in C++17?

I would like to add some conditionality to member variables initialization in a template. Would the following expression resolve at compile time or runtime? If so, is there any other operator that resolves at compile time such that template specialisation can be avoided?

template<int a>
struct hello {
    constexpr static int n = (a != 0) ? 10 : 20;
}

Answer 1

It depends on what you mean by "analogous to constexpr if() ". if constexpr requires that the condition is a constant expression. It also has certain privileges in template code to discard the branches not taken.

?: does not have that functionality.

However ?: can appear in constant expressions just fine, and it always could. It doesn't make an expression non-constant.

Answer 2

是的，它绝对可以，实际上它可以在引入if constexpr之前在 C++11中使用，甚至在常量表达式中的C++11 之前，例如您问题中的那个。