Constraints and Concepts (C++20)

One-Liner

A mechanism for specifying semantic requirements on template parameters (such as "hashable" or "iterator"), catching incorrect types early at compile time and producing readable error messages.

Header

#include <concepts>

Core API Cheat Sheet

Operation	Signature	Description
Concept definition	template<...> concept Name = constraint-expression;	Defines a named set of constraints
requires expression	requires { /* 表达式 */ }	Checks whether an expression is valid
Nested requirement	{ expr } -> std::convertible_to<T>;	Requires an expression to be valid and its result convertible to T
Abbreviated function template	void f(Concept auto param)	Uses concept constraints directly in the parameter list
requires clause	template<typename T> requires Concept<T> void f(T);	Appends constraints after a template declaration
Trailing requires	template<typename T> void f(T) requires Concept<T>;	Appends constraints after a function parameter list
Logical AND	Concept1 && Concept2	Combines multiple constraints (conjunction)
Logical OR	Concept1 || Concept2	Combines multiple constraints (disjunction)

Minimal Example

#include <concepts>
#include <iostream>

template<typename T>
concept Addable = requires(T a, T b) { a + b; };

template<Addable T>
T add(T a, T b) { return a + b; }

int main() {
    std::cout << add(1, 2) << '\n';     // OK: int 满足 Addable
    // add("a", "b");                   // Error: const char* 不满足 Addable
}

Embedded Applicability: High

• A purely compile-time feature with zero runtime overhead, suitable for resource-constrained environments
• Constraint-driven design catches type errors at compile time, preventing undefined behavior (UB) on the target board
• Standard library concepts (such as std::integral, std::same_as) can directly constrain the interfaces of hardware register wrapper types
• Error messages are significantly shortened, greatly accelerating the development and debugging cycle of low-level template libraries

Compiler Support

GCC	Clang	MSVC
10.0	10.0	19.28

See Also

• cppreference: Constraints and concepts

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