为 >> 实现重载并且<<模板类中的运算符 [英] Implementing overloads for >> and << operators in a template class

问题描述

我正在尝试编写用于重载运算符>>"和<<"的函数定义在类定义之外，尽管在同一个文件中，如您所见.我收到以下错误:

I'm trying to write the function definition for overloading the operators ">>" and "<<" outside of the class definition, though in the same file, as you can see. I get the following errors:

1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class MyClass<int> &)" (??5@YAAAV?$basic_istream@DU?$char_traits@D@std@@@std@@AAV01@AAV?$MyClass@H@@@Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class MyClass<int>)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@V?$MyClass@H@@@Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class MyClass<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > &)" (??5@YAAAV?$basic_istream@DU?$char_traits@D@std@@@std@@AAV01@AAV?$MyClass@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@@Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class MyClass<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@V?$MyClass@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@@Z) referenced in function _main

我和这个人一起在河上游来游去.奇怪的是，如果我将它们的定义写在类定义中，它就可以完美运行.

I've been up and down the river with this one. The strange thing is that if I write their definitions inside the class definition, it works flawlessly.

#include <iostream>
#include <string>
using namespace std;

template <class MYTYPE>
class MyClass {
    MYTYPE *myVector;
    int dim;
    string name;
public:
    MyClass(int, string);
    MyClass() {};
    friend istream& operator>>(istream&, MyClass<MYTYPE>&);
    friend ostream& operator<<(ostream&, MyClass<MYTYPE>);
};

template <class MYTYPE>
MyClass<MYTYPE>::MyClass(int x, string y) {
    dim = x;
    name = y;
    myVector = new MYTYPE[dim];
}


template <class MYTYPE>
istream& operator>>(istream& X, MyClass<MYTYPE>& a){
    cout<<"Reading vector: "<<a.name<<endl;
    for(int indice = 0; indice < a.dim; indice++){
        cout<<a.name<<'['<<indice<<"]= ";
        X >> a.myVector[indice];
    }
    return X;
}

template <class MYTYPE>
ostream& operator<<(ostream& X, MyClass<MYTYPE> a){
    X<<"Vector: "<<a.name<<endl;
    for(int indice = 0; indice < a.dim; indice++)
        X<<a.myVector[indice]<<' ';
        X<<endl;
    return X;
}


int main() {
    MyClass<int> object(4, "Ints vector");
    MyClass<string> object2(5, "String vector");
    cin >> object;
    cin >> object2;
    cout << object;
    cout << object2;
    system("pause");
    return 0;
}

推荐答案

调高编译器的警告级别有帮助.通过将 -Wall 与 g++ 一起使用，我会在链接器错误之前收到以下警告.

Turning up the warning level of the compiler helps. By using -Wall with g++, I get the following warnings before the linker error.

socc.cc:13:58: warning: friend declaration ‘std::istream& operator>>(std::istream&, MyClass<MYTYPE>&                    )’ declares a non-template function [-Wnon-template-friend]
     friend istream& operator>>(istream&, MyClass<MYTYPE>&);
                                                          ^
socc.cc:13:58: note: (if this is not what you intended, make sure the function template has already                     been declared and add <> after the function name here)
socc.cc:14:57: warning: friend declaration ‘std::ostream& operator<<(std::ostream&, MyClass<MYTYPE>                     ’ declares a non-template function [-Wnon-template-friend]
     friend ostream& operator<<(ostream&, MyClass<MYTYPE>);

您需要为 operator>> 和 operator<< 函数使用函数模板.您可以在类定义之前声明它们:

You need to use function templates for the operator>> and operator<< functions. You can declare them before the definition of the class with:

// Forward the class template.
template <class MYTYPE> class MyClass;

// Declare the function templates.
template <class MYTYPE> 
std::istream& operator>>(std::istream&, MyClass<MYTYPE>&);

template <class MYTYPE> 
std::ostream& operator<<(st::ostream&, MyClass<MYTYPE>);

然后，您必须使用带有适当模板参数的 friend 声明.

Then, you'll have to use the friend declaration with the appropriate template parameter.

// This makes sure that operator>><int> is not a friend of MyClass<double>
// Only operator>><double> is a friend of MyClass<double>
friend std::istream& operator>><MYTYPE>(std::istream&, MyClass<MYTYPE>&);
friend std::ostream& operator<<<MYTYPE>(std::ostream&, MyClass<MYTYPE>);

这是为我构建的代码的更新版本.我还没试过运行它.

Here's an updated version of your code that builds for me. I haven't tried to run it.

#include <iostream>
#include <string>
using namespace std;

template <class MYTYPE> class MyClass;

template <class MYTYPE> 
std::istream& operator>>(std::istream&, MyClass<MYTYPE>&);

template <class MYTYPE> 
std::ostream& operator<<(std::ostream&, MyClass<MYTYPE>);

template <class MYTYPE>
class MyClass {
    MYTYPE *myVector;
    int dim;
    string name;
public:
    MyClass(int, string);
    MyClass() {};

    friend std::istream& operator>><MYTYPE>(std::istream&, MyClass<MYTYPE>&);
    friend std::ostream& operator<<<MYTYPE>(std::ostream&, MyClass<MYTYPE>);
};

template <class MYTYPE>
MyClass<MYTYPE>::MyClass(int x, string y) {
    dim = x;
    name = y;
    myVector = new MYTYPE[dim];
}


template <class MYTYPE>
std::istream& operator>>(std::istream& X, MyClass<MYTYPE>& a){
    cout<<"Reading vector: "<<a.name<<endl;
    for(int indice = 0; indice < a.dim; indice++){
        cout<<a.name<<'['<<indice<<"]= ";
        X >> a.myVector[indice];
    }
    return X;
}

template <class MYTYPE>
std::ostream& operator<<(std::ostream& X, MyClass<MYTYPE> a){
    X<<"Vector: "<<a.name<<endl;
    for(int indice = 0; indice < a.dim; indice++)
        X<<a.myVector[indice]<<' ';
        X<<endl;
    return X;
}


int main() {
    MyClass<int> object(4, "Ints vector");
    MyClass<string> object2(5, "String vector");
    cin >> object;
    cin >> object2;
    cout << object;
    cout << object2;
    system("pause");
    return 0;
}

这篇关于为 &gt;&gt; 实现重载并且&lt;&lt;模板类中的运算符的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！