Can Lambdas Be Used in Constant Expressions in C ?

Can Lambdas Utilize constexpr?

In earlier versions of C , the inclusion of lambda functions in constant expressions was restricted, as outlined in [expr.const]/(2.6). However, N4487, included in the working draft N4582, introduces a change that allows lambdas to be used in these expressions.

Syntax for Constexpr Lambdas

The proposed change in N4487 provides guidelines for the correct syntax when using lambdas in constant expressions:

• Certain lambda expressions are permitted within constant expressions.
• Operations performed on specific closure objects are allowed within constant expressions.
• Closure types are considered literal types if each of their data members is also a literal type.
• If the constexpr specifier is not provided within the lambda declarator, the generated function call operator will be considered constexpr if it meets the requirements of a constexpr function. This behavior aligns with the implicit constexpr inference for constructors and assignment operator functions.

Workarounds for Earlier C Versions

For C versions prior to the implementation of N4487, alternative approaches are necessary to use lambdas in constant expressions. One workaround involves utilizing template metaprogramming techniques to achieve the desired functionality.

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