为什么会出现此错误：C ++中'operator <<'不匹配？

Question

Please help me out on why I'm getting an error where I have mentioned the comment guys. I'm really stuck here! What other details should I give exactly?

Main.cpp

#include <iostream>
#include<person.h>
#include <sstream>

int main()
{
    cout << "Constructor Overloading Demo !!!" << endl;
    person vish;
    cout << vish.toString() << endl; /* I get an error for this statement though I feel this is correct */
    return 0;
}

person.cpp //constructor definition file

#include "person.h"
#include <sstream>

person::person()
{
    name = "vish";
    age = 25;
    //ctor
}

void person::toString()
{
    stringstream st;
    st << "Name: ";
    st << name;
    st << "& Age: ";
    st << age;
    prnt = st.str();
    cout << prnt << endl;
    //return 10;
}

person.h //constructor declaration

#ifndef PERSON_H
#define PERSON_H
#include<iostream>
using namespace std;
class person
{
    public:
        person();
       void toString();

        //virtual ~person();
    protected:
    private:
        string name;
        int age;
        string prnt;
};

#endif // PERSON_H

Answer 1

As name of method person::toString() suggests it should return string , not void (which means that method does not return anything):

class person { 
   ...
   std::string toString(); 
    //^--------- type should be changed

of course you need to change that method implementation accordingly:

std::string person::toString()
{
    stringstream st;
    st << "Name: ";
    st << name;
    st << "& Age: ";
    st << age;
    return st.str();
}

I realize that my function was returning void. but my question is: why does that matter ?

Your output to cout is this:

cout << vish.toString(); // endl is omitted for clarity

which is equal to one of:

cout.operator<<( vish.toString() );
operator<<( cout, vish.toString() ); 

whatever is available. This means that function or method operator<< needs result of call to toString() and it cannot accept void . If you just want to print from method toString() write:

vish.toString();

that would work fine (except method name would be still confusing, but compiler does not care).

Answer 2

operator<< expects a string to be passed to it. Your ToString function is returning void

string person::toString()
{
    stringstream st;
    st << "Name: ";
    st << name;
    st << "& Age: ";
    st << age;
    return st.str();

}

edit:

The statement

std::cout << vish.toString();

is identical to

std::cout.operator<<(vish.toString());

The prototype for the operator is

std::ostream& operator<<(std::string);
std::ostream& operator<<(int);
//...

The function expects a string or some other parameter as an argument. Your function is defined as a standalone print function. So it should either be modified as stated above or called by itself.

The function name 'toString()' implies that the function returns a string representation of the object that can be used for anything, not just printing, so it should return a string. If the function prints the object it should be called 'print' instead of 'toString'.