References to function object types

[Christopher Schmidt]

The current implementation of boost::result_of doesn't permit Fn in boost::result_of<Fn(Args...)> to be a reference to a class type. I would like to see that changed. Allowing reference-qualified function object types would be in useful with c++11's explicit reference qualification of member functions (n3090 / 13.3.1/4), and this change would bring boost::result_of more in line with c++11's std::result_of, which does allow "reference[s] to function object type[s]" (n3090 / 20.7.6.6)

[Christopher Schmidt]

"The current implementation of boost::result_of permits Fn in boost::result_of<Fn(Args...)>"...

oh, there is a mistake in there. Of course I meant that the current implementation of boost::result_of does not permit Fn in boost::result_of<Fn(Args...)>"...

[Michel Morin]

Boost's decltype-based result_of already accepts references to function objects. Users can explicitly enable decltype-based result_of by defining BOOST_RESULT_OF_USE_DECLTYPE. Or, in the trunk code, it is automatically enabled when a compiler supports N3276 decltype (e.g. clang-3.1).

For example, on clang-3.1 with the following functor:

struct F
{
    short operator()(int) &  { return 0; }
    float operator()(int) && { return 0; }
};

• boost::result_of<F& (int)>::type evaluates to short.
• boost::result_of<F&&(int)>::type evaluates to float.

Is this enough for you? Or, do you want this feature in tr1-style result_of?

[anonymous]

We could update the old TR1 protocol to handle references to function objects, but that would take some refactoring. It could be done at the same time as refactoring for #6731... But updating TR1 result_of is lower priority.

• Daniel Walker