In C++17, the constexpr if statement was introduced to facilitate the conditionally compiled code within templates. This feature provides a more concise and readable way to write compile-time branching without using the traditional SFINAE (Substitution Failure Is Not An Error) technique.

Syntax

The syntax of constexpr if is as follows:

template <typename T>
void foo(T value)
{
    if constexpr (std::is_integral<T>::value)
    {
        // code for integral types
    }
    else if constexpr (std::is_floating_point<T>::value)
    {
        // code for floating point types
    }
    else
    {
        // code for other types
    }
}

In this example, the constexpr if statement checks the type T at compile time and conditionally compiles the corresponding block of code based on the type.

Benefits

Using constexpr if offers several benefits, such as:

1. Clarity: It improves code readability by placing the condition directly in the if statement, eliminating the need for helper structures or meta-programming tricks.

2. Compile-time Performance: By using constexpr if, the compiler can optimize the code by discarding unnecessary branches at compile time.

3. Simplicity: It simplifies template metaprogramming, making it easier to write and understand complex compile-time logic.

Example

Let’s consider an example of using constexpr if to conditionally handle different types of containers:

template <typename T>
void processContainer(const T& container)
{
    if constexpr (std::is_same<T, std::vector<typename T::value_type>>::value)
    {
        // Code for vector
    }
    else if constexpr (std::is_same<T, std::list<typename T::value_type>>::value)
    {
        // Code for list
    }
    else
    {
        // Generic code for other containers
    }
}

In this example, the constexpr if statement allows us to selectively execute code based on the type of the container provided.

Conclusion

constexpr if is a powerful feature that simplifies compile-time branching and enhances the expressiveness of C++ templates. By leveraging this feature, developers can write more readable and efficient code for handling different types and conditions at compile time.

References:

• C++ Reference: constexpr if
• CppCon 2017: The Nightmare of Initialization in C++