C++ Function already defined?

Question

At the top of a cpp file, I have

namespace PQL {
    class Synonym {
    ...
    public:
        ...
        int size();
    };
}

// removing the below chunk makes it work
int Synonym::size() {
    return ids.size();
}

Why does the bottom chunk make the code fail? I am creating the implementation of the function? Other functions defined a similar way works.

UPDATE:

Expired (dead) link

The error I got looks like:

Error 1 error LNK2005: "public: int __thiscall PQL::Synonym::size(void)" (?size@Synonym@PQL@@QAEHXZ) already defined in main.obj H:\Dropbox\Sch\CS3202\SPA_CPP\SPA\pql.obj

This chunk doesn't make the code fail. The problem is in the code that isn't shown. Provide the smallest example you can that shows the problem. — Pete Becker, Feb 22 '13 at 13:57
What's `ids`? If you haven't defined it before using, of course your code fails. — KBart, Feb 22 '13 at 13:59
You haven't informed us of why it fails. do you get compilation errors? does the program crash? Do nasal demons emerge? — default, Feb 22 '13 at 14:01
@Axel, I put everything in a CPP file to keep things simple, its just a test — Jiew Meng, Feb 22 '13 at 14:09
And do you include that CPP file in other source files? If yes, then that's the reason. — Axel, Feb 22 '13 at 14:19
As your code contains templates this looks. like a header file. Since this is a header file it should not contain `using namespace X;` lines. — Martin York, Feb 22 '13 at 16:40

score 4 · Answer 1 · answered Feb 22 '13 at 13:59

4

Because Synonym isn't a name in global scope.

Either use

int PQL::Synonym::size() {
    return ids.size();
}

or implement the method inside the namespace.

answered Feb 22 '13 at 13:59

Luchian Grigore

253,575
64
457
625

I am using `using namespace PQL` – Jiew Meng Feb 22 '13 at 14:09
@JiewMeng is the code in an implementation file? Are you including that file anywhere? Maybe in `main`? If you are - remove it. – Luchian Grigore Feb 22 '13 at 14:17

score 1 · Answer 2 · answered Feb 22 '13 at 14:22

From your comments, I put this together: You put everything in a single Cpp file and include that file in different other files. Each of those files compiles, and each of those files has an implementation of PQL::Synonym::size(). When linking, the linker sees all those definitions and doesn't know which one to choose.

Split your code into header and source files and just include the header in the other files.

score 1 · Accepted Answer · edited May 23 '17 at 12:04

Its because your code is in a header file and being included in multiple compilation units:

    inline int Synonym::size() {
//  ^^^^^^^
        return ids.size();
    }

Adding inline tells the linker that there may be multiple definitions.

Note: The keyword 'inline' has nothing to do with code inline-ing in modern compilers.

As a very important note.

Your header file contains:

 using namespace std;
 // and
 using namespace PQL;

This is a very bad idea. You are now forcing this on anybody that uses your code. I would never use your header file as it would contaminate my code and cause unforeseen problems. It is OK to do this in your own source files (when you know and understand the issues) but you should never force this on other developers.

See: Why is "using namespace std" considered bad practice?

C++ Function already defined?

3 Answers3

As a very important note.