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C++ High Performance

The if constexpr Statement

Learn how to use the if constexpr statement in template functions.

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if constexpr in template functions

The if constexpr statement allows template functions to evaluate different scopes in the same function at compile time. Take a look at the following example, where a function template called speak() tries to differentiate member functions depending on the type:

C++
struct Bear {
  auto roar() const { std::cout << "roar\n"; }
};
struct Duck {
  auto quack() const { std::cout << "quack\n"; }
};
template <typename Animal> 
auto speak(const Animal& a) {
  if (std::is_same_v<Animal, Bear>) {
    a.roar();
  } else if (std::is_same_v<Animal, Duck>) {
    a.quack();
  }
}

Let’s say we compile the following lines:

C++
auto bear = Bear{};
speak(bear);

The compiler will then generate a speak() ...