What is "Class::*"

Question

I am learning SFINAE(Substitution failure is not) I found an example of it in a site,

template<typename T>
class is_class {
   typedef char yes[1];
   typedef char no [2];
   template<typename C> static yes& test(int C::*); // What is C::*?
   template<typename C> static no&  test(...);
   public:
   static bool const value = sizeof(test<T>(0)) == sizeof(yes);
};

I found a new signature, int C::* at a line 5. At first I thought it is operator* but I suppose it is not true. Please tell me what is it.

Similar question: http://stackoverflow.com/questions/670734/c-pointer-to-class-data-member — Havenard, Jun 23 '15 at 04:57
I am sorry asking a similar question. I tried to search it by keyword "::*" but I could not find it. And thank you for letting me know the link to the similar question. — mora, Jun 23 '15 at 05:10

score 6 · Accepted Answer · answered Jun 23 '15 at 04:51

6

int C::* is a pointer to a member of class C whose type is int.

Example:

struct C
{
   C () : a(0), b(0) {}
   int a;
   int b;
};

int main()
{
   int C::*member1 = &C::a;
   int C::*member2 = &C::b;
   C c1;
   c1.*member1 = 10;  // Sets the value of c1.a to 10
   c1.*member2 = 20;  // Sets the value of c1.b to 20
}

answered Jun 23 '15 at 04:51

R Sahu

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Thank you very much, R Sahu. It was new for me. Please let me ask you more. Is it a special keyword like "this" keyword? Does it exists anytime even when the class does not have member? Thank you very much again. – mora Jun 23 '15 at 05:03
The type exists even if a class does not have a member but you can't initialize it to point to a member. For example, you can use `float C::*member3 = nullptr;`. Since `C` does not have a member of type `float`, `member3` cannot point to a member of `C`. – R Sahu Jun 23 '15 at 05:10
@mora Its not a keyword, its language syntax, its a mechanic that allows you to specify a pointer to a member of a class. What makes it special is that it is not a pointer to a member of a specific object, but a pointer that is good to the same member of any objects of that same class. I honestly cant think of any practical use for this mechanic, but the language offers it if needed. – Havenard Jun 23 '15 at 05:16
@Havenard, see [this SO answer](http://stackoverflow.com/questions/30987112/implementing-access-to-vector-of-stucts-using-a-vector-as-indices/30987413#30987413) for an example usage of this language construct. – R Sahu Jun 23 '15 at 05:18
1

@RSahu Thats a nice example. – Havenard Jun 23 '15 at 05:21
Thank you very much R Sahu and Havenard. – mora Jun 23 '15 at 05:22

What is "Class::*"

1 Answers1