几个 C++ 类模板的类内外声明实现实例

#include<iostream>
#include<string>

using namespace std;

template<class T1, class T2>
class Demo {
public:
    Demo(T1 v, T2 a);

    void show();

private:
    T1 var;
    T2 arg;
};

template<class T1, class T2>
Demo<T1, T2>::Demo(T1 v, T2 a) {
    this->var = v;
    this->arg = a;
}

template<class T1, class T2>
void Demo<T1, T2>::show() { // 即使未用到，也需要声明模板的参数列表。
    cout << "Var:" << this->var << "\nArg:" << this->arg << endl;
}

int main() {
    Demo<string, int> d("demo", 21);
    d.show();
}

#include<iostream>
#include<string>

using namespace std;

template<class T1, class T2>
class Demo {
    friend void show(Demo<T1, T2> d) {
        cout << "Var:" << d.var << "\nArg:" << d.arg << endl;
    }

public:
    Demo(T1 v, T2 a) {
        this->var = v;
        this->arg = a;
    };
private:
    T1 var;
    T2 arg;
};

int main() {
    Demo<string, int> d("demo", 21);
    show(d);
}

#include<iostream>
#include<string>

using namespace std;

template<class T1, class T2>
class Demo; // 需提前声明相关的类

template<class T1, class T2>
void show(Demo<T1, T2> d) {
    cout << "Var:" << d.var << "\nArg:" << d.arg << endl;
}

template<class T1, class T2>
class Demo {
    friend void show<>(Demo<T1, T2> d);

public:
    Demo(T1 v, T2 a) {
        this->var = v;
        this->arg = a;
    }

private:
    T1 var;
    T2 arg;
};

int main() {
    Demo<string, int> d("demo", 21);
    show(d);
}