Type of the first parameter of a member function in C++11

Question

I have written a metafunction to retrieve the type of the first parameter of a member function, which of course receives one or more parameters. The code I have written is as follow:

template <typename...> struct parameter;

template < typename O, typename A, typename R, typename... Args>
struct parameter <R (O::*)(A, Args...)  > {
     using first_param = A;
};

I use this meta function as follow:

using mem_fn = void(mem_type::*)(std::vector<int>);
using f_pm = parameter<mem_fn>::first_param;

and it compiles and works. But when I have a class:

struct mem_type{

    void update(std::vector<int>) {

    }
};

and use my metafunction as follow:

using mem_fn = decltype(mem_type::update);
using f_pm = parameter<mem_fn>::first_param;

the code does not compiles and visual studio 2013 gives: error C2027: use of undefined type parameter<mem_fn>.

Does anyone knows the reason for this error?

Skiffle

Does using mem_fn = decltype(&mem_type::update); work? I've seen similar issues in the past and being more explicit on the address has helped. Here to match your member function pointer specialisation. – Didi

MSVC using support is flaky. Try typedef? Oh, and &mem_type::update might also help. Bah, @Didi beat me to the 2nd one. mem_type::update isn't a valid way to get a pointer to it: unlike functions, member functions to not auto-decay the same way. – Bush

Your specialization matches a pointer-to-member-function, so decltype(&mem_type::update). – Marelya

@Snps No it doesn't. – Marelya

It works fine here using g++ (Debian 4.7.2-5) 4.7.2 (if you supply the ampersand decltype(&mem_type::update). I suspect VS2013 could be flaky with the use of using – Myrmecophagous

Answer 1

First, an id-expression naming a nonstatic member function (mem_type::update in this case) can't be used as an unevaluated operand (such as the operand of decltype). §5.1.1 [expr.prim.general]/p13 (footnote omitted):

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

• as part of a class member access (5.2.5) in which the object expression refers to the member’s class or a class derived from that class, or
• to form a pointer to member (5.3.1), or
• if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

§7.1.6.2 [dcl.type.simple]/p4:

The operand of the decltype specifier is an unevaluated operand (Clause 5).

And even if update were a regular function, decltype would produce a function type rather than a function pointer type, and your specialization matches a pointer-to-member-function type.

You need to created a pointer-to-member-function with & - i.e., decltype(&mem_type::update).

Marelya

Exactly (nice answer) – Myrmecophagous

Good answer. Is there a better way to accomplish the task of getting the first parameter of a member function? – Skiffle

@RaulAlonso You actually need 11 extra specializations (in addition to the one you have) to handle the various combinations of cv- and ref-qualifiers, but I'm not aware of any other way to do it. Also, the base template parameter shouldn't be variadic. – Marelya