运算符重载代码编译错误，模板参数推导/替换失败

Question

I'm trying to apply some concepts of operator loading that I've learnt in the following C++ class.

#include<iostream>
using namespace std;

class Point
{
private:
    int x, y;
public:
    Point(int, int);
    Point operator+(const Point&);
    friend istream& operator>>(istream& in, Point&);
    friend ostream& operator<<(ostream& out, Point&);
    Point operator=(const Point&);
};

Point::Point(int x = 0, int y = 0)
    :x(x), y(y){}

Point Point::operator+(const Point& p)
{
    int r1, r2;
    r1 = x + p.x;
    r2 = y + p.y;
    return Point(r1, r2);
}

Point Point::operator=(const Point& p)
{
    this->x = p.x;
    this->y = p.y;
    return *this;
}

istream& operator>>(istream& in, Point& p)
{
    char openParenthesis, closeParenthesis, comma;
    cout << "Enter data in the format (1,2): ";
    in >> openParenthesis >> p.x >> comma >> p.y >> closeParenthesis;
    return in;
}

ostream& operator<<(ostream& out, Point& p)
{
    out << "(" << p.x << "," << p.y << ")";
    return out;
}

int main()
{
    Point a, b;
    cin >> a >> b;

    cout << "a + b is: " << a + b << endl;
    return 0;
}

The code compiles and runs fine on Visual Studio. But when I try to compile it on Linux with gcc, it throws a long list of errors along the lines of:

In file included from /usr/include/c++/4.8/iostream:39:0, from optr_overload.cpp:1: /usr/include/c++/4.8/ostream:471:5: note: template std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT) operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c) ^ /usr/include/c++/4.8/ostream:471:5: note: template argument deduction/substitution failed: optr_overload.cpp:53:30: note:
deduced conflicting types for parameter '_CharT' ('char' and 'Point') cout << "a + b is: " << a + b << endl;

I understand that the problem lies with the line where I passed "a + b" to the overloaded binary stream insertion operator which only receives reference to one Point object as the argument. But I've no idea how to fix the code other than assigning "a + b" to a third object and pass that single object as the argument to "<<". Could someone explain to me what exactly needs to be done in order for gcc to compile my code, preferably without involving the use of an extra placeholder object.

Answer 1

The value computed with a + b is a temporary object and therefore cannot be passed as a Point& to operator<< ; the language only allows temporaries to be passed as const Point& instead. Just change he declaration of the output operator to accept a const Point& .

Allowing passing a temporary result as a non-const reference was a known bug in old versions of VC++.

Answer 2

You've got almost everything right, but your ostream& operator<<() should take Point by a const reference:

friend ostream& operator<<(ostream& out, const Point&);

This should fix your issue.

(And don't worry about Point::x and Point::y being private, you've already declared your operator<< as a friend.)