Constexpr
constexpr is a C++ or C language specifier that allows entities or expressions to satisfy compile-time constant-evaluation requirements.
Learn C++ by Example
constexpr is a C++ specifier for variables and functions that permits evaluation at compile time when their initialization or arguments satisfy the required constant-expression rules.
- How it is used
- Uses compile-time evaluation to check properties early, reduce runtime work, and make suitable calculations testable.
- How deep it goes
- Explains variables, function arguments, static_assert, and evolving language support, but defers detailed constant-expression rules.
- Examples
- Demonstrates prime checks, triangle-number vectors, shuffled slot-machine reels, and input parsing with optional results.
- What you can do after
- Evaluate and test suitable C++ functions at compile time, while retaining runtime calls when inputs are unavailable earlier.
Modern C, Third Edition
constexpr is a C23 language construct for declaring entities whose values or initialization satisfy compile-time constant-expression requirements.
- How it is used
- Uses constexpr to provide named or unnamed constants for compile-time expressions and file-scope initialization.
- How deep it goes
- Explains read-only guarantees, exact type fitting, integer constant-expression use, compound literals, and possible storage optimization.
- Examples
- Demonstrates π, an invalid unsigned conversion, a corvid-name array compound literal, and constexpr operands in enumeration expressions.
- What you can do after
- Evaluate/understand which C23 objects and expressions qualify as compile-time constants and diagnose value-changing initializations.
100 C++ Mistakes and How to Avoid Them
constexpr is a C++ specifier that permits a variable or function to participate in compile-time constant evaluation when its requirements are satisfied.
- How it is used
- Uses compile-time constant evaluation to replace potentially unsafe macro definitions.
- How deep it goes
- Defines compile-time evaluation and type guarantees but does not detail constexpr function requirements or implementation internals.
- Examples
- Uses a PI constant and contrasts macro-based code with C++ constructs that involve compiler checking.
- What you can do after
- Apply constexpr to express compile-time values while exposing type mismatches to the compiler.
Read how 100 C++ Mistakes and How to Avoid Them explains constexpr →
Related concepts
- read-only objectproduces · A read-only object is an object that a program is not permitted to modify through its applicable access or type qualification.
- compound literaluses · A compound literal is a C expression consisting of a parenthesized type name followed by an initializer that creates an unnamed object.
- static_assertuses · A static assertion is a compile-time check requiring a constant condition to be true and otherwise producing a diagnostic.
- const qualifierproduces · A language feature that marks a value, binding, or access path as constant, restricting reassignment or modification according to the language’s rules.
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