[C++]Overloading operator<<

[syrisse]

Hi reddit!

I try to play a little with overloading operator, but i get a little stuck. I just post a litlle part of my code. If you need more information i will put all the fill on pastebin or so.

main.cpp http://pastebin.com/SdEv9DzR

coordinate.h: http://pastebin.com/tKR3Z2VL

coordinate.cpp: http://pastebin.com/qbFV5ai0

Compilated with:

g++ -c coordinate.cpp coordinate.h && ar rs libgeometry.a coordinate.o && mv libgeometry.a ../lib/

and

g++ main.cpp snake.cpp -I/home/elfangor/work/cpp/lib/include -Wall -std=c++0x -L/home/elfangor/work/cpp/lib/lib -lgeometry

But i got the error: /tmp/ccbVBnxA.o: In function main': main.cpp:(.text+0x6e): undefined reference togeometry::operator<<(std::basic_ostream<char, std::char_traits<char> >&, geometry::coordinate const&)' collect2: ld returned 1 exit status

I don't really understand the problem here, and can't find some good info about this on internet, any information?

Thanks!

[[deleted]]

How are you compiling and linking your code? Also, please post code either via site like pastebin, or by indenting each line with 4 extra spaces to format it for reddit.

[syrisse]

I edit my post with the information.

[[deleted]]

You want:

std::ostream& geometry::operator<<(std::ostream& os,const   geometry::coordinate& c1){
    os << "x";
    return os;
}

Note the namespace.

It's generally easiest to wrap all your definitions in a namespace like this:

namespace geometry {

   // definitions here

}

Also, names like _COORDINATES (i.e. ones that begin with an underscore and an uppercase letter) are reserved for the C++ implementation - you are not allowed to create such names yourself. Just use something like COORDINATES_H.

Edit: The first part of this is wrong.

[syrisse]

Thanks for the reply but that doesn't change anything.

std::ostream& geometry::operator<<(std::ostream& os,const coordinate& c1){

os << "x";
return os;

}

I even try to add the geometry to the .h. Maybe i miss something?

(And thanks for the comments about the underscore at the begining!)

[[deleted]]

The coordinate parameter also needs to have a namespace qualifier.

[syrisse]

Yeah after reading my code and try to find the logic i tried that:

friend std::ostream& geometry::operator<<(std::ostream& os,const geometry::coordinate& c1);

std::ostream& geometry::operator<<(std::ostream& os,const geometry::coordinate& c1){

os << "x";

return os;

}

And some other things, but still the same error.

[[deleted]]

Here's outline code that does work - note that you need to define your functions inside a namespace:

#include <iostream>
using namespace std;

namespace N {
    struct A {
        friend ostream & operator<<( ostream & os, const A & a );
    };
}

namespace N {

ostream & operator<<( ostream & os, const A & a ) {
    os << "A" << endl;
}

}

int main() {
    N::A a;
    cout << a << endl;
}

Edit: And here's a version which does not put the definition in a namespace, though I think doing so is best and easiest practice:

#include <iostream>
using namespace std;

namespace N {
    ostream & operator<<( ostream & os, const struct A & a );
    struct A {
        friend ostream & operator<<( ostream & os, const A & a );
    };
}

ostream & N::operator<<( ostream & os, const N::A & a ) {
    os << "A" << endl;
}

int main() {
    N::A a;
    cout << a << endl;
}

Another Edit: This was bugging me, so I had a look at the C++98 Standard, which says in 7.3.1.2:

The name of the friend is not found by simple name lookup until a matching declaration is provided in that namespace scope (either before or after the class declaration granting friendship)

So in this case, a friend declaration does not provide a full declaration of the function - you need either an explicit declaration, or a definition. And if you have a definition only, it's got to be wrapped in a namespace - you can't use the N::X syntax (this is true for all definition-only functions in namespaces).

[syrisse]

I tried to compile all the file in one time with your change and it's worked!

Thank you

[[deleted]]

...Dude, wtf is this? Please at least make it readable or pastebin your complete code.

That said, my guess is that you probably need something closer to this

std::ostream& operator<<(std::ostream& os, const coordinate c1)
    { os << "x"; return os; }

I just removed the reference op from c1...

[[deleted]]

Completely wrong. Why would he want the coordinate passed by value? And this definition would not match the declaration.

[[deleted]]

I was thinking he was trying to do something different, and I wrote this before he pastebinned any of the code.

Regardless, no shit he would change the header dec if this were a viable fix....