Should C++ attributes be in the header or the implementation or both?

Question

I have looked around quite a bit and have not found whether or not C++ attributes should be in the header or the implementation or both. For example:

file.h

[[nodiscard]] std::future<int> get_data();

file.cpp

[[nodiscard]] std::future<int> get_data() { return ...; }

Should both have the attribute or only one? The examples provided by cppreference are all functions with no forward declaration.

Answer 1

To be effective, the attribute needs to be applied to the declaration (i.e., in the header). If a call is being compiled, and the declaration the compiler has seen lacks the attribute, the compiler normally won't be able to issue a diagnostic based on the attribute (since it hasn't seen it).

§[dcl.attr.nodiscard]/1:

The attribute-token nodiscard may be applied to the declarator-id in a function declaration or to the declaration of a class or enumeration.

The same basic notion applies to most other attributes as well, but a few belong in the implementation--most obviously the [[fallthrough]] attribute, which goes at the end of a case in a switch statement, so it has to be located where the switch statement itself is.

The [[noreturn]] attribute is a little bit more specific. It directly requires that if a function is declared more than once, the first declaration the compiler sees for that function must have the [[noreturn]] attribute. If the compiler sees a [[noreturn]] attribute on a function that has previously been declared without it, the code is ill-formed (but no diagnostic is required if the two declarations were in separate translation units).