Call function outside of current namespace in C++

Question

I'm trying to call a function defined in a C file from my CPP code and I think I am having issues getting the correct namespace. When compiling I get the error: "Undefined reference to 'Get'".

My C header:

// c.h
#ifndef C_H
#define C_H

#ifdef __cplusplus
    extern "C" {
#endif

typedef enum
{
    VAL_A1,
    VAL_A2
} TYPE_A;

typedef enum
{
    VAL_B1,
    VAL_B2
} TYPE_B;

typedef enum
{
    VAL_C1,
    VAL_C2
} TYPE_C;

typedef struct
{
    TYPE_B b;
    TYPE_C c;
} TYPE_D;

TYPE_A Get(TYPE_B b, TYPE_D *d);

#ifdef __cplusplus
    }
#endif

#endif

And my CPP file:

// main.cpp
...

extern "C" {
    #include "c.h"
}

...

namespace MyNamespace
{
    ...

    MyClass::MyFunc()
    {
        TYPE_D d;
        // None of these calls will compile
        // Get(VAL_B1, &d);
        // ::Get(VAL_B1, &d);
    }

    ...
}

I have tried calling without namespace reference and also with the "root" namespace using "::" with no luck. Any help is appreciated. I've read through this which seems to clarify it but I don't really understand it:

using C++ with namespace in C

Answer 1

"Undefined reference" means that the function has been declared (in the header), but not defined. You'll need to define the function in a source file somewhere (presumably the C file you refer to), and make sure that is linked when you build the program.

Answer 2

First, let's note what that error means. An undefined reference at the linker stage means that the compiler is unable to find the instance of something. In this case, the implementation of a function.

Let's look at your code.. There are a few things missing that we need to add to make it compilable:

• A definition for Get() .
• main()
• The class definition for MyClass .

Once we added those three fixes, the code compiles without error.

extern "C" { extern "C" {
    typedef enum {
        VAL_A1,
        VAL_A2
    } TYPE_A;

    typedef enum {
        VAL_B1,
        VAL_B2
    } TYPE_B;

    typedef enum {
        VAL_C1,
        VAL_C2
    } TYPE_C;

    typedef struct {
        TYPE_B b;
        TYPE_C c;
    } TYPE_D;

    TYPE_A Get(TYPE_B b, TYPE_D *d) {
        return VAL_A1;
    }
}}

namespace MyNamespace {
    struct MyClass {
        void MyFunc();
    };

    void MyClass::MyFunc() {
        TYPE_D d;

        Get(VAL_B1, &d);
        ::Get(VAL_B1, &d);
    }
}

int main() {}

Answer 3

The definition of Get (not shown in the question) also needs to be enclosed in extern "C" .

The main difference between C and C++ functions, in practice, is the way they are named in the executable format. C++ functions get "name mangling" treatment by the linker but C functions do not. The linker will see the C++ definition of Get and it will have no idea of its relation to the C declaration, even if they have the same signature.