calling .cpp function in .m and .mm file, error

Question

i am trying to integrate my own small cpp project into a big objective-c project. I have compiled my own project into a .dylib file and added into the big project, and .h file is included. But in order to follow the legacy i have to call my function from a .m file in the big project.

i know that .m file can not call cpp functions so what i have tried is to include my cpp .h file in another .mm file, then in my .m file i include the header file to the .mm file. here is what i did:

A.mm (i created this file together with the header)

#import "A.h"
#include "mycppfunc.h"
int ParseFile(char * filename)
{
    ....(using cpp functions)
    return 0;
}

A.h

int ParseFile(char* filename);

B.mm (originally in the big project)

#import "B.h"
#import "A.h"
.... (original codes in B.mm)

B.h

#import "C.h"
....

C.m (originally in the big project, i have to call my own functions from here due to legacy)

#import "C.h"
#import "B.h"
....
- (IBAction) call_my_proj: (id) sender
{
    ....
    ParseFile("myfile.txt");
}

C.h

.... (this file is un-changed)

When i compile, i got this error:

Implicit declaration of function 'ParseFile' is invalid in C99

Can anyone help me explain where it went wrong? Can it be due to the circular reference to different header file (but i can not change it since it affects other peoples code). how can i get rid of this error? any help is highly appreciated! thanks.

Are you including A.h anywhere (e.g. from C.h or C.m)? That's the only place that ParseFile is defined. — Nathan S.
– Nathan S., Commented Jan 13, 2014 at 5:17

user1118321 · Accepted Answer · 2014-01-13 05:49:29Z

1

You need to #include "A.h" in C.m. You are calling ParseFile() in C.m.

answered Jan 13, 2014 at 5:49

user1118321

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2 Comments

user3188926 Over a year ago

thanks. but once i included A.h in C.m i got the linker error

user3188926 Over a year ago

Undefined symbols for architecture i386: "_ParseFile", referenced from: -[...this is where i call the ParseFile function] in C.o ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Community · Accepted Answer · 2017-05-23 12:13:07Z

0

First, you need to include A.h from C.h or C.m.

But, because the functions defined in A.mm are being compiled with a (Obj)C++ compiler, you might need to make them callable from C. You do this using C linkage. Then ParseFile() should be callable from the C.m file. For instance, in the header file:

extern "C"
{
   int ParseFile(const char *);
}

edited May 23, 2017 at 12:13

CommunityBot

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answered Jan 13, 2014 at 7:03

Nathan S.

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3 Comments

user3188926 Over a year ago

Thanks Nathan, this seems to make some progress. i added #ifdef __cplusplus extern "C" { #endif int ParseFile(char * filename); #ifdef __cplusplus } #endif

user3188926 Over a year ago

Thanks Nathan, this seems to make some progress. i added #ifdef __cplusplus extern "C" { #endif int ParseFile(char * filename); #ifdef __cplusplus } #endif and when i compile i don't get the same error anymore. however, it seems to mess up the namespace in a cpp file (D.cpp) which is called by A.mm (D.h is included in A.mm). i got error "redefinition of '<namespace name>' as a different kind of symbol", and some class names become "undeclared"---i got "use of undeclared identifier xxxx " error. Do you have any idea why it became so?

Nathan S. Over a year ago

I would first look at the namespaces being used in D.cpp/D.h to see if there is a naming conflict with ParseFile or some other function.

CodenameLambda1 · Accepted Answer · 2014-01-13 06:37:51Z

0

Rename extension of all your .m files to .mm. This way you can directly call cpp functions in your objective-C classes without any wrappers.

answered Jan 13, 2014 at 6:37

CodenameLambda1

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1 Comment

user3188926 Over a year ago

thanks Lambda1 but i can not change the extension as it affects the code written by other people. what can i do instead?

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