constexpr if

constexpr if (C++17)

constexpr if enables us to compile source code conditionally.

if constexpr (cond) statement1;
else statement2;

• The expression cond has to be a constant expression.
• The unused code has to be valid.
• Thanks to constexpr if, functions can have different return types.

The following code snippet shows a function, which returns an int or a double.

template <typename T>
auto getAnswer(T t){
  static_assert(std::is_arithmetic_v<T>); // arithmetic
  if constexpr (std::is_integral_v<T>)    // integral
    return 42;
  else                                    
    return 42.0;        // floating point
}

constexpr if is a replacement for tag dispatch and SFINAE. SFINAE stands for Substitution Failure Is Not An Error.

To study further about tag dispatching, click here.

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In the next lesson, we’ll look at an example of constexpr if.