Linker error calling C-Function from Objective-C++

Question

I've got a weird linker issue. I have code that looks like so:

    double given_amount = self.modelController.levelCompleteRewardAmount;
    swrve_currency_given(swrve, (CFStringRef)@"currencyName", given_amount);

I have this code in two separate places: In an objective-c and an objective-c++ file. It compiles fine in objective-C land, but the swrve_currency_given() function causes the following in my WGController.mm file:

Undefined symbols for architecture armv7:
  "swrve_currency_given(Swrve*, __CFString const*, double)", referenced from:
      -[WGController giveTheUserSomeCashForPlayingThisLevel] in WGController.o
ld: symbol(s) not found for architecture armv7
collect2: ld returned 1 exit status

I'm not entirely sure if this error is related to the Obj-C vs. C++ thing, but it feels like it. My theory is that it perhaps thinks that it is a function on the Obj-C class? The 'swrve' code is 3rd party code, one .h and .c file and I'm importing like so:

#import "swrve.h"

Any help is appreciated! Thanks

Answer 1

You may need to surround the function prototype with:

#if defined __cplusplus
extern "C" {
#endif

void swrve_currency_given (...whatever goes here...);

#if defined __cplusplus
};
#endif

That tells the compiler that it's a C function and not a C++ function.

Answer 2

If you are using c function in c++ file. you should use extern "c"{}. In .h file

#ifdef __cplusplus
extern "C" {
#endif

swrve_currency_given(parameter1, parameter2, parameter3);// a c function


#ifdef __cplusplus
}
#endif  

extern "C" is meant to be recognized by a C++ compiler and to notify the compiler that the noted function is (or to be) compiled in C style.

If you're linking to a library that was compiled as C code. use

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

Take a look at When to use extern "C" in C++?