Error with pointers to member variables as parameters; why?

Question

Why do I keep on getting the following error in this code in Visual C++ 2010, and how do I fix it while maintaining the type inference capability for the member variable?

error C2825: 'Foo<T>::value_type': must be a class or namespace when followed by '::'

template<class T>
struct Foo
{
    typedef typename T::value_type value_type;

    template<class M>
    void foo(M value_type::*member) const;   // error
};
struct S { typedef int value_type; };

int main() { Foo<S> s; }

@CharlesBailey: lol oops... either I made a stupid mistake in the original code, or I reduced the test case wrong... let me find the problem and I'll post an update, thanks for pointing it out. — user541686, Aug 09 '12 at 06:01
Sorry everyone -- I misinterpreted what was happening. See [my comment](http://stackoverflow.com/questions/11877464/error-with-pointers-to-member-variables-as-parameters-why#comment15804094_11877528) on Nawaz's answer. — user541686, Aug 09 '12 at 06:06

score 3 · Accepted Answer · answered Aug 09 '12 at 05:58

3

The template parameter T turns out to be type S, therefore value_type turns out to be int (the nested-type in S). So how can you write value_type::*member? Note that it turns out to be int::*member which doesn't make sense. int is not a class type.

I think you meant T::*member instead of value_type::*member.

answered Aug 09 '12 at 05:58

Nawaz

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Oh fudge, either I made a stupid mistake in the original code, or I reduced the test case incorrectly... let me fix it; +1 thanks... – user541686 Aug 09 '12 at 05:59
1

Ohhhhhhh I figured it out -- this was a red herring. The actual problem was that I had a member function (with this weird parameter) and its template instantiation was eager instead of lazy (not sure if it's a bug or a feature, since the container was also a template...), so when I was chaining a couple of method calls, I thought the error was due to the second one, not the first one! The first one definitely had `value_type = int`, but the second one had `value_type = pair`. That makes a lot of sense, thanks! – user541686 Aug 09 '12 at 06:06

score 0 · Answer 2 · answered Aug 09 '12 at 06:15

0

value_type is not a member of structure S. Its just a typedef so you cant access it as you are doing.

answered Aug 09 '12 at 06:15

Yogesh

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Error with pointers to member variables as parameters; why?

2 Answers2