if constexpr (C++17)

One-Liner

Selectively compiles a branch based on compile-time conditions within templates; discarded branches are not even syntactically checked—a powerful tool for compile-time polymorphism.

Header

None (language feature)

Core API Cheat Sheet

Syntax Form	Description
if constexpr ( condition )	Compiles the then branch if condition is true
if constexpr ( condition ) statement else statement	Compile-time binary selection
if constexpr chain	Multi-branch chain
if constexpr with Concepts	requires type trait checking
if constexpr with auto	(C++20) Concepts overloading is preferred instead

Minimal Example

template <typename T>
auto get_value(T t) {
    if constexpr (std::is_pointer_v<T>) {
        return *t; // Deduces return type to underlying type
    } else {
        return t;  // Deduces return type to T
    }
}

void usage() {
    int x = 10;
    get_value(x);   // Instantiates with T=int
    get_value(&x);  // Instantiates with T=int*
}

Embedded Applicability: High

• Zero runtime overhead: conditions are evaluated at compile time, and unmet branches generate no code.
• Replaces SFINAE and tag dispatching, significantly improving template metaprogramming readability.
• Ideal for selecting different code paths based on compile-time constants like hardware platforms or peripheral types.
• Available since C++17; 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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