Using cin >> and cout << to populate fields of a class C++

问题

I have a class in MyClass.h defined like this:

#ifndef MyClass_h
#define MyClass_h

#include <iostream>
#include <stdio.h>
#include <string>

using namespace std;

class MyClass {
public:
    string input
    void ReadFrom(istream &is);
    void WriteTo(ostream &os) const;
};
#endif /* MyClass_h */

MyClass.cpp looks like this:

#include <stdio.h>
#include <string>
#include <iostream>
#include "MyClass.h"

using namespace std;

void MyClass::ReadFrom(istream &is) {
    // put data into member 'input'
}

void MyClass::WriteTo(ostream &os) const {
    // output data in member 'input'
}

istream& operator >>(istream &is, MyClass &cls) {
    cls.ReadFrom(is);
    return is;
}

ostream& operator <<(ostream &os, const MyClass &cls) {
    cls.WriteTo(os);
    return os;
}

main.cpp looks like this:

#include <stdio.h>
#include <string>
#include <iostream>
#include "MyClass.h"

using namespace std;

int main(int argc, const char * argv[]) {
   MyClass c;
   std::cout << "Enter some input" << endl;
   std::cin >> c;
   return 0;
}

What I am trying to accomplish is to override the >> and << operators so that std::cin can simply read data into the class member(s), and then std::cout can spit out all of the data in those members.

I do not want to use friend functions.

Right now, I am getting an error around the std::cin >> c; line that says:

Invalid operands to binary expression ('istream' (aka 'basic_istream<char>') and 'MyClass')

回答1:

The compiler fails to see the operator overloads in your main.cpp translation unit, because the overloads are not found in that file, nor are they found in any of the #include files. You need to declare both overloads in your MyClass.h file instead, after the MyClass declaration:

MyClass.h:

#ifndef MyClass_h
#define MyClass_h

#include <iostream>
#include <stdio.h>
#include <string>

using namespace std;

class MyClass {
public:
    string input
    void ReadFrom(istream &is);
    void WriteTo(ostream &os) const;
};

istream& operator >>(istream &is, MyClass &cls);    
ostream& operator <<(ostream &os, const MyClass &cls);

#endif /* MyClass_h */

You can leave the definitions as-is in your MyClass.cpp file.

来源：https://stackoverflow.com/questions/42821684/using-cin-and-cout-to-populate-fields-of-a-class-c