C++函数、类、类的成员函数作为 类模板的友元 实例

//program 18.5.1.cpp 函数、类、类的成员函数作为 类模板的友元 #include<iostream> using namespace std; template <class T> class Tmpl; template <class T> void Func1(Tmpl<T> &x) { cout << "this is a global function :Func1()" << endl<<"\t"; x.print(); } /*模板类B*/ template <class T> class B { public: void Func(Tmpl<T>& x); }; /*类模板Tmpl*/ template <class T> class Tmpl { T a; public: friend void Func1();//全局友元函数 friend class A;//友元类A template<class T> friend void B<T>::Func(Tmpl<T>&);//类B的成员函数作为 类模板Tmpl的友元; Tmpl(T a_=0):a(a_)//构造函数 { cout << "the Tmpl<T>::constructor was called!" << endl; } void print() { cout << "a=" << a << endl; } }; class A { Tmpl<double> obj_in_A; public: void printA() { cout << "this is a function of class A" << endl; } A(double a_):obj_in_A(a_) { //obj_in_A.a = a_; cout << "从友元类A(普通类A)中访问Tmpl<T>的私有成员a:" <<endl; cout << "a=" << obj_in_A.a << endl; } }; template<class T>//不可少 void B<T>:: Func(Tmpl<T>& x) { cout << "this is a function of class B:" << endl; cout << "从友元函数(类模板B<T>的函数Func(Tmpl<T>& x))中访问Tmpl<T>的私有成员a:" << endl; cout << "x.a=" << x.a<< endl; } int main() { Tmpl<int> i(7); i.print(); Func1(i);//全局友元函数 Tmpl<double> f(7.889); f.print(); A obj1(3.14);//类A中有Tmpl<double>类的成员对象,会先调用Tmpl<double>的构造函数. B<double> obj2; obj2.Func(f); }