Member function templates and static members

Yes, you can declare a static member variable inside a member function template. However, there are a few important things to note about how these interact.

First, each instantiation of the member function template gets its own copy of the static variable. This means that if you instantiate the member function template with different types, each instantiation will have a separate static variable.

Here's an example:

1#include <iostream>
2
3class MyClass {
4 public:
5  template <typename T>
6  void func() {
7    static int count = 0;
8    count++;
9    std::cout << "func<" << typeid(T).name()
10      << ">: " << count << std::endl;
11  }
12};
13
14int main() {
15  MyClass obj;
16  obj.func<int>();     // prints: func<int>: 1
17  obj.func<int>();     // prints: func<int>: 2
18  obj.func<double>();  // prints: func<double>: 1
19  obj.func<int>();     // prints: func<int>: 3
20}

1func<int>: 1
2func<int>: 2
3func<double>: 1
4func<int>: 3

In this code, MyClass::func declares a static variable count. However, because func is a member function template, each instantiation of func gets its own count. So func<int> has one count, and func<double> has a separate count.

It's also worth noting that if you want to define the static variable outside the class (which is necessary if it's a class type or an array), you need to provide the full template declaration:

1template<>
2int MyClass::func<int>::count = 0;

This can get quite complex, especially if the class itself is also a template.

In general, it's often simpler and clearer to avoid static variables inside member function templates unless you specifically need a separate static variable for each instantiation. If you do need per-instantiation static data, consider making the entire class a template instead, and putting the static variable at the class level.