constexpr (C++11)

In a Nutshell

Tells the compiler "this value or function can be evaluated at compile time," shifting runtime computations to the compile phase and achieving zero-overhead complex logic.

Header

None (language keyword)

Core API Quick Reference

Operation	Signature	Description
Compile-time variable	constexpr T var = expr;	Requires expr to be a constant expression; the variable is implicitly const
Compile-time function	constexpr T func(params);	If arguments are constants, it evaluates at compile time; otherwise, it degrades to a normal function
Compile-time construction	constexpr T::T(params);	Allows constructing literal type objects in constant expressions
Compile-time destruction	constexpr T::~T();	(C++20) Allows destroying objects in constant expressions
Feature test macro	__cpp_constexpr	Detects the current compiler's level of constexpr support

Minimal Example

// Standard: C++14
#include <iostream>

constexpr int factorial(int n) {
    int res = 1;
    while (n > 1) res *= n--;
    return res;
}

int main() {
    constexpr int val = factorial(5); // 编译期计算
    std::cout << val << '\n';         // 输出: 120
    int k = 4;
    std::cout << factorial(k) << '\n';// 运行期计算: 24
}

Embedded Applicability: High

• Moves computations like table lookups, CRC checks, and protocol parsing to compile time, saving Flash/RAM space
• Compile-time computed values can be used directly as template parameters (e.g., array sizes), meeting the static configuration needs of bare-metal environments
• Offers better readability and debugging experience compared to C macros and template metaprogramming
• Note that C++11 has many restrictions (single return statement); we recommend embedded projects use at least the C++14 standard

Compiler Support

GCC	Clang	MSVC
4.6	3.1	19.0

See Also

• cppreference: constexpr specifier

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