Constraints and Concepts (C++20)

In a Nutshell

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

Header File

<concepts>

Core API Cheat Sheet

Operation	Signature	Description
Concept definition	template <...> concept Name = ...;	Defines a named set of constraints
requires expression	requires { expression; }	Checks if an expression is valid
Nested requirement	requires expression;	Requires expression validity and result convertible to T
Abbreviated function template	void func(C auto& x)	Uses concept constraints directly in parameter list
requires clause	template<...> requires ...	Appends constraints after template declaration
Trailing requires	void func(...) requires ...	Appends constraints after function parameter list
Logical AND	C1 && C2	Combines multiple constraints (conjunction)
Logical OR	C1 || C2	Combines multiple constraints (disjunction)

Minimal Example

Expand (23 lines)Collapse

#include <concepts>
#include <vector>
#include <print>

// Define a concept: 'T' must be an integral type
template<typename T>
concept Integral = std::is_integral_v<T>;

// Use concept to constrain function template
// Only accepts types satisfying the Integral concept
auto add(Integral auto a, Integral auto b) {
    return a + b;
}

int main() {
    // OK: int satisfies Integral
    std::println("{}", add(1, 2));

    // Compile Error: double does not satisfy Integral
    // std::println("{}", add(1.0, 2.0));

    return 0;
}

Embedded Applicability: High

• Pure compile-time feature with zero runtime overhead, suitable for resource-constrained environments.
• Constraint-driven design intercepts type errors at compile time, avoiding undefined behavior on the target board.
• Standard library concepts (such as std::integral, std::floating_point) can directly constrain interfaces of hardware register wrapper types.
• Significantly shortens error messages, 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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