if constexpr (C++17)

In a Nutshell

Inside a template, selectively compile a branch based on a compile-time condition—the discarded branch does not even need to pass syntax checking. A powerful tool for compile-time polymorphism.

Header

None (language feature)

Core API Cheat Sheet

Syntax Form	Description
if constexpr (cond) { ... }	Compiles the then branch if cond is true
if constexpr (cond) { ... } else { ... }	Compiles exactly one of the two branches
if constexpr (cond1) { ... } else if constexpr (cond2) { ... } else { ... }	Multi-branch chain
if constexpr combined with concepts	if constexpr (std::integral\<T\>) concept constraint check (std::integral is a concept)
if constexpr combined with requires	(C++20) Concepts-based overloading is generally preferred instead

Minimal Example

// Standard: C++17
#include <iostream>
#include <type_traits>

template <typename T>
auto print_type(const T& val) {
    if constexpr (std::is_integral_v<T>) {
        std::cout << "integral: " << val << "\n";
    } else if constexpr (std::is_floating_point_v<T>) {
        std::cout << "float: " << val << "\n";
    } else {
        std::cout << "other\n";
    }
}

int main() {
    print_type(42);     // integral: 42
    print_type(3.14);   // float: 3.14
    print_type("hi");   // other
}

Embedded Applicability: High

• Zero runtime overhead: the condition is evaluated at compile time, and branches that do not match generate no code at all
• Replaces SFINAE and tag dispatch, greatly improving the readability of template metaprogramming
• Well suited for choosing different code paths based on compile-time constants such as hardware platform or peripheral type
• Available as of C++17; already supported by GCC 7+ and ARM Clang 6+

Compiler Support

GCC	Clang	MSVC
7	3.9	19.1

See Also

• cppreference: if constexpr

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