Calling a template member function: Help me understand a code snippet from another StackOverflow post
With respect to Can a pointer to members bypass the member access level? , I would like to understand a piece of code in this question.
Paste the code snippet here.
#include <iostream>
template<typename Tag, typename Tag::type M>
struct Rob {
friend typename Tag::type get(Tag) {
return M;
}
};
// use
struct A {
A(int a):a(a) { }
private:
int a;
};
// tag used to access A::a
struct A_f {
typedef int A::*type;
friend type get(A_f);
};
template struct Rob<A_f, &A::a>;
int main() {
A a(42);
std::cout << "proof: " << a.*get(A_f()) << std::endl;
}
Among several questions, I single out one question here that can hack the rest.
I don't understand the following expression in a function main
:
a.*get(A_f())
I understand (I think) what is get(A_F())
returning a pointer to a.a
. However, I don't understand the function get()
. In what framework is it actually defined? How is he addressed in this structure?
I have two additional questions that can be answered if this question has been answered, but I put them here. First, in the definition, the Rob
keyword is friend
used before the function get
that is being defined and also declared. I thought the keyword friend
could only be used before a function declaration to indicate that a function defined elsewhere has access to the private / protected members of the class. Can someone please explain?
Second, I don't understand the line template struct Rob<A_f, &A::a>;
. I don't understand the use of the template
sans keyword <...>
and I don't understand why there is no template definition - it seems to be some kind of formal declaration. Can someone please explain? Thank.
However, I don't understand the function
get()
. In what framework is it actually defined? How is he addressed in this structure?
First of all: the function get()
is a friend of the class template A
and Rob
. He is not a member of any class. It's more of a free feature.
Now where exactly is this free function defined? Well, it is defined in the class template Rob
and at the same time it is declared as a friend
class template Rob
.
I thought the friend keyword could only be used before a function declaration to indicate that a function defined elsewhere has access to the private / protected members of the class. Can someone please explain?
Yes, it can be declared by a friend
class without defining it inside the class. It can also be defined within a class and at the same time can be declared by friend
that class. In your case, this is actually defined in the class itself.
Second, I don't understand the line
template struct Rob<A_f, &A::a>;
.
This is called an explicit instance of the class template with the given tempate arguments. You can see many other syntaxes to explicitly create a template here (well, if it's a function template):
- C ++ pattern specialization syntax