do I put [[maybe unused]] on function declarations or definitions?

Question

C++17 introduces the attribute [[maybe_unused]].
I assume this a standardized version of GCC and Clang's: __attribute__((unused)).

For unused functions that I don't want to see a warning from,
should I be specifying the attribute on

function declarations?

void maybe_used_function() [[maybe_unused]];

or function definitions?

void maybe_used_function() [[maybe_unused]] {
  /* impl */
}

Either one? Both?
Will the effect be the same on both the standardized and compiler specific attributes?
I can't find any clear documentation on placement behaviour, and what the common practice is.

---

When I place the attribute before the function body in a definition, GCC and clang give an error:

void function();
int main(){}
void function() __attribute__((unused)) {}  

warning: GCC does not allow 'unused' attribute in this position on a function definition [-Wgcc-compat] void function() __attribute__((unused)) {

---

However, the attribute can be placed in two other places without error:

__attribute__((unused)) void __attribute__((unused)) function() {}

Maybe one of these ways is how I'm expected to use the attribute on function definitions?

Answer 1

Neither. In

[[attr1]] void [[attr2]] f [[attr3]] () [[attr4]] {}

• attr1 and attr3 appertain (or apply) to f itself.
• attr2 appertains to the preceding type, void.
• attr4 appertains to f's type ("function of () returning void), not f.

You want maybe_unused to appertain to f, so you can put it in position 1 or 3, but not 2 or 4.

@ildjarn's answer covers the rest.

For GCC's __attribute__, you'll have to check its documentation.