friend function problem

[loveless]

hi everyone
i had a problem when implementing the friend function

why i can't compile the program when the friend function is written in the header files?
but i can successfully compile the program if i write the class function in the main program?

the header file (test.h) is the following code:

class test
{
    public:
    int a, b;
    test();
    test(int x, int y);

    friend ostream& operator<<(ostream out, test &x);
};

test::test()
{
    a = b = 0;
}

test::test(int x, int y)
{
    a = x;
    b = y;
}

ostream& operator<<(ostream out, test &x)
{
    out << "a = " << x.a << endl;
    out << "b = " << x.b << endl;

    return out;
}

the main program is as follow:

#include<iostream>
#include"test.h"

using namespace std;

int main()
{
    test a(3, 5);
    test b(4, 8);

    cout << a << b << endl;
}

can someone tell me if i am doing some mistake when implementing the friend function?
or is that i can't implement the friend function in the header file?

thanks

[kbw]

ostream& operator<<(ostream out, test &x) should be
ostream& operator<<(ostream& out, const test &x)

If you only have one source file, it won't make a difference. If you have more than one, you'd expect it to compile but fail at link time with multiple definitions of ostream& operator<<(ostream& out, const test &x).

[loveless]

just now i've tried to add the const but my compiler still can't compile it correctly?
why does it still can't compile?
is there something wrong with the source code??

[kbw]

Did you change the friend declaration too?
friend ostream& operator<<(ostream& out, const test &x);

[loveless]

ya
i've changed the function prototype and the function

class test
{
    public:
    int a, b;
    test();
    test(int x, int y);

    friend ostream& operator<<(ostream out, const test &x);
};

test::test()
{
    a = b = 0;
}

test::test(int x, int y)
{
    a = x;
    b = y;
}

ostream& operator<<(ostream out, const test &x)
{
    out << "a = " << x.a << endl;
    out << "b = " << x.b << endl;

    return out;
}

[jsmith]

You must pass the stream by reference, not by value. streams are not copyable

ostream& operator<<( ostream& out, const test& x ) {
    return out << "a = " << x.a << std::endl << "b = " << x.b" << std::endl;
} 

[loveless]

oo i see i go try it out

[loveless]

=.=''' i still got the same problem

my compiler said:

line 8 : 'ostream' is neither function nor member function; cannot be declared friend
line 8 : expected ';' before '&' token
line 22: expected constructor, destructor, or type conversion before '&' token
line 23: missing terminating " character
line 11: no match for 'operator<<' in 'std::cout << a'

[lloydchristmas759]

Add #include <ostream> at the beginning of your test.h file, and use std::ostream instead of ostream.

[loveless]

??then do i need to delete the friend ostream& operator<<(ostream out, const test &x); in the class member?

[van]

You should put the specification of the class in .h file and the implementation in .cpp file.

[van]

Don't delet. Friend function is not a memeber of a class, but its declaration must be included in the class specification.

[loveless]

=.=''' i've tried to implement the .h file into the .cpp file but it still shows the same error
but if i tried to implement the class into the main program it compiles and works fine

#include<iostream>

using namespace std;

class test
{
    public:
    int a, b;
    test();
    test(int x, int y);

    friend ostream& operator<<( ostream& out, const test& x );
};

test::test()
{
    a = b = 0;
}

test::test(int x, int y)
{
    a = x;
    b = y;
}

ostream& operator<<( ostream& out, const test& x ) {
    return out << "a = " << x.a << std::endl << "b = " << x.b << std::endl;
}


int main()
{
    test a(3, 5);
    test b(4, 8);

    cout << a << b << endl;
}

//this works fine
//but if i separate the class to the .h file my compiler will said error 

[van]

have you done this way?

.h file

class test
{
    public:
    int a, b;
    test();
    test(int x, int y);

    friend ostream& operator<<( ostream& out, const test& x );
};

.cpp file

test::test()
{
    a = b = 0;
}

test::test(int x, int y)
{
    a = x;
    b = y;
}

ostream& operator<<( ostream& out, const test& x ) {
    return out << "a = " << x.a << std::endl << "b = " << x.b << std::endl;
}

and also don't forget to include necessary libraries.

[loveless]

ya i've tried already but my compiler is still saying the same error

by the way, the compiler i'm using is code block

[lloydchristmas759]

i've tried to implement the .h file into the .cpp file but it still shows the same error
but if i tried to implement the class into the main program it compiles and works fine

Which is why I told you to replace ostream by std::ostream in your .h file....


To Van: Please don't confuse people. What you propose can obviously not compile as is.

[loveless]

o i see
now i can compile it successfully
special thanks lloydchristmas
and thanks to everyone that has help me to understand how to use friend function

//test.h file
class test
{
    public:
    int a, b;
    test();
    test(int x, int y);

    friend std::ostream& operator<<( std::ostream& out, const test& x );
};

test::test()
{
    a = b = 0;
}

test::test(int x, int y)
{
    a = x;
    b = y;
}

std::ostream& operator<<( std::ostream& out, const test& x )
{
    out << "a = " << x.a << std::endl << "b = " << x.b << std::endl;

    return out;
}

//main program
#include<iostream>
#include"test.h"

using namespace std;

int main()
{
    test a(3, 3);
    test b(4, 8);

    cout << a << b << endl;
}

[jysonia]

Another way to do is to add following lines in hpp file(new_file.hpp):
#include <iostream>
using namespace std ;

<CLASS DECLARATION>

CPP FILE will look like :-
#include <iostream>
#include "new_file.hpp"
<CLASS DEFINITION>
main() {
.....
}

Thanks
JySonia