"Use of undeclared identifier" in Objective-C++ when importing from another source file - objective-c++

Question

I have declared a functions in a header file like this (pseudo code):
// funcs.h
#import <Foundation/Foundation.h>
NSString* func1();
void func2();
Then I have the implementations in
//funcs.mm
#import "funcs.h"
NSString* func1()
{
// do something and use C++ functions
}
void func2()
{
// do more
}
When I include the header in a third file
// AppDelegate.mm
#import "funcs.h"
....
string1 = func1();
func2();
....
Whenever I try to use these functions I get "Use of undeclared identifier" errors.
What am I doing wrong? Isn't function declaration in files in Objective-C++ the same as in normal C?

Answer 1

A .mm file is Objective-C++. Plain functions in a .mm file are not C functions, but C++ functions. Objective-C is the same as C, but you are not using Objective-C, you are using Objective-C++.
I would strongly recommend that you only use .mm files for interfacing with C++. For example, your AppDelegate should be a .m file and not .mm.
You need to write C functions like this:
extern "C" {
NSString* func1()
{
// do something and use C++ functions
}
}
Also, in the .h file you should declare them like this:
#if __cplusplus
extern "C" {
#endif
NSString* func1(void);
#if __cplusplus
}
#endif
The macro __cplusplus might be misspelled, google for it.
extern "C" tells the C++ or Objective-C++ compiler that a function should have C linkage.