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constructorc++14constexprassignment-operatorc++17
Vincent
Vincent

Reputation: 60451

Constexpr implicitly declared functions

For a class of type T, the following members can be generated by the compiler, depending on the class:

In C++14, and in C++17, what are the rules that lead to the generation of constexpr versions of these functions by the compiler?

Upvotes: 5

Views: 870

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474186

The rule is simple: if the generated definition satisfies the requirements of a constexpr function, then it will be a constexpr function. For example, from C++17, [class.ctor]/7:

If that user-written default constructor would satisfy the requirements of a constexpr constructor (10.1.5), the implicitly-defined default constructor is constexpr.

The wording around implicit default constructors is describes in terms of what a "user-written default constructor" would look like. So "that user-written default constructor" means "what the compiler generates".

Similar wording exists for the copy/move constructors.

The wording is slightly more complex for the assignment operators, but it boils down to the same thing. The type must be a literal type and the assignment operators selected to do the copy/move for each subobject (non-static data member and base class) must be constexpr.

Upvotes: 5

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