Constraints and Concepts (C++20)

In a Nutshell

A mechanism for specifying semantic requirements on template parameters (such as "hashable" or "iterator"), which catches wrong types as early as possible at compile time and produces readable error messages.

Header File

#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 { /* expression */ }	Checks whether the expressions are valid
Nested requirement	{ expr } -> std::convertible_to<T>;	Requires the expression to be valid and its result convertible to T
Abbreviated template parameter	void f(Concept auto param)	Uses a concept constraint directly in the parameter list
requires clause	template<typename T> requires Concept<T> void f(T);	Appends a constraint after the template declaration
Trailing requires	template<typename T> void f(T) requires Concept<T>;	Appends a constraint after the 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 satisfies Addable
    // add("a", "b");                   // Error: const char* does not satisfy Addable
}

Embedded Applicability: High

• A pure compile-time feature with zero runtime overhead, well suited to resource-constrained environments
• Constraint-driven design intercepts type errors at compile time, avoiding undefined behavior triggered on the target board
• Standard library concepts (such as std::integral, std::same_as) can be used directly to constrain the interfaces of hardware register wrapper types
• Error messages shrink dramatically, noticeably speeding up the develop-and-debug 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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