I have the following template class and template function which intends to access the class' private data member:
#include <iostream>
template<class T>
class MyVar
{
int x;
};
template<class T>
void printVar(const MyVar<T>& var)
{
std::cout << var.x << std::endl;
}
template<class T>
void scanVar(MyVar<T>& var)
{
std::cin >> var.x;
}
struct Foo {};
int main(void)
{
MyVar<Foo> a;
scanVar(a);
printVar(a);
return 0;
}
To declare the two functions as MyVar<T>'s friend functions, I've tried the following ways inside the declaration of template<class T> class MyVar to declare friendship. None of them works. How should I do?
template<class T> friend void printVar(const MyVar&);
template<class T> friend void scanVar(MyVar&);
// compilation error
template<class T> friend void printVar(const MyVar<T>&);
template<class T> friend void scanVar(MyVar<T>&);
// compilation error
friend void printVar(const MyVar<T>&);
friend void scanVar(MyVar<T>&);
// link error
friend void printVar(const MyVar&);
friend void scanVar(MyVar&);
// link error too
The simplest option is to define the friend within the class:
template<class T>
class MyVar
{
int x;
friend void printVar(const MyVar & var) {
std::cout << var.x << std::endl;
}
friend void scanVar(MyVar & var) {
std::cin >> var.x;
}
};
The downside is that the functions can only be called through argument-dependent lookup. That's not a problem in your example, but might be a problem if they don't have a suitable argument, or you want to specify the name without calling it.
If you want a separate definition, then the template will have to be declared before the class definition (so it's available for a friend declaration), but defined afterwards (so it can access the class members). The class will also have to be declared before the function. This is a bit messy, so I'll only show one of the two functions:
template <typename T> class MyVar;
template <typename T> void printVar(const MyVar<T> & var);
template<class T>
class MyVar
{
int x;
friend void printVar<T>(const MyVar<T> & var);
};
template <typename T> void printVar(const MyVar<T> & var) {
std::cout << var.x << std::endl;
}