C++20 Concepts & Constraints: Writing Readable, Safe Generic Code

C++20 Concepts & Constraints: Writing Readable, Safe Generic Code

Table of Contents

• The Problem: Template Error Wall of Text
• Four Constraint Syntax Forms
• Using Built-In Standard Library Concepts
• requires Expressions: Compound Requirements
• Defining Custom Concepts
• Combining Concepts with && and ||
• Concept-Based Overloading: Subsumption Rules
• Concepts as Template Parameter Constraints
• std::ranges Concepts for Collections
• Frequently Asked Questions
• Key Takeaway

The Problem: Template Error Wall of Text

Four Constraint Syntax Forms

All four forms are equivalent — choose the most readable for each context:

Using Built-In Standard Library Concepts

The <concepts> header provides a comprehensive set of ready-made constraints:

requires Expressions: Compound Requirements

requires expressions check validity of expressions — they test syntax, not runtime behavior:

Defining Custom Concepts

Combining Concepts with && and ||

Concept-Based Overloading: Subsumption Rules

When multiple constrained overloads are valid, the compiler picks the most constrained one:

Frequently Asked Questions

Do concepts impact runtime performance? No — concepts are purely compile-time. They add zero runtime overhead. The generated machine code for a constrained template is identical to an unconstrained one; constraints only affect which instantiation is chosen and what errors are produced.

Can I use concepts to check for non-type properties, like a specific value? No — concepts check syntactic and semantic properties of types (do they have this function? does it return this type?). They cannot check runtime values or non-type properties directly. For value-based compile-time checks, use if constexpr with static_assert, or template non-type parameters with value constraints.

Should I use concepts everywhere, or only at interfaces? Use concepts on every public template function/class interface — they serve as documentation and provide better error messages. Inside implementation details, if constexpr + type traits are still appropriate for compile-time branching. Don't add concepts to private helper functions that aren't part of any public API.

Key Takeaway

C++20 Concepts complete the template programming story. Before: templates were "duck typing" — if it compiles, it works; if not, good luck reading the error. After: templates are precisely specified contracts. The requires clause says exactly what properties T must have, the compiler verifies this at the call site, and error messages name the violated constraint. Combined with standard library concepts (<concepts>, <ranges>), you can constrain almost any generic API without writing a single custom concept.

Read next: Meta-programming: constexpr and Compile-Time Logic →

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