# Ifdef _ cplusplus and extern "C"

Often in CPPCodeSee this code in:

# Ifdef _ cplusplus
Extern "C "{
# Endif

// A piece of code

# Ifdef _ cplusplus
}
# Endif

What does this code mean? First, _ cplusplus is the reserved macro definition of the C ++ compiler. That is to say, the C ++ compiler considers this macro has been defined. If this macro is defined, it indicates that this is a piece of CPP Code.

That is to say,

The above code indicates:

If this is a piece of CPP Code, add extern "C" {And} to process the code;

Extern "C "{
# Endif

// A piece of code

}
# Endif // * extern "C "*/

If this is a piece of C code, skip extern "C" {And} and compile:

# Endif // * extern "C "*/

// A piece of code

# Endif //

To understand why extern "C" is used, you must start with the overload processing of functions in CPP. In C ++, in order to support the overload mechanism, the name of a function must be processed in the compiled assembly code, such as a function.

. In C, it is just a simple function name and no other information is added. that is to say, C ++ and C process the name of the generated function differently. the purpose is to implement mutual calls between C and C ++.

C. H implementation
# Ifndef _ C_H _
# DEFINE _ C_H _
# Ifdef _ cplusplus
Extern "C "{
# Endif

Void c_fun ();

# Ifdef _ cplusplus
}
# Endif

# Endif
-----------------------------------
C. c implementation
# Include "C. H"
Void c_fun ()
{
}
------------------------------------
Call c_test () in C. C in CPP. cpp ()
CPP. cpp implementation
# Include "C. H"
Int main ()
{
C_fun ()
}
Here, _ cplusplus is the reserved macro definition of the C ++ compiler. That is to say, the C ++ compiler considers this macro has been defined.
So the key is extern "C "{}
Extern "C" indicates that the stuff in the brackets of the C ++ compiler is compiled according to the OBJ file format of C. to connect, follow the naming rules of C.
======================================
So how does C call the cpp_fun () function in C ++?
Because there are C and C ++ first, you can only consider it from the C ++ code.
If a function or variable added to C ++ may be called by a file in C, it should be written in this way. It also uses extern "C "{}
But the header file must be added in the code, because it may be called in C ++.
--------------------------------------
Implementation of CPP. h

# Ifndef _ C_H _
# DEFINE _ C_H _
# Ifdef _ cplusplus
Extern "C "{
# Endif

Void cpp_fun ();

# Ifdef _ cplusplus
}
# Endif

.-------------------------------------------
CPP. cpp implementation
Extern "C" {// tell the C ++ compiler that the extension code is compiled according to the naming rules of C.
Void cpp_fun ()
{
.....
}
 
 
Summary
C and C ++ process functions differently. extern "C" is a means to enable C ++ to call library files written by C. If you want to prompt the compiler to use the C method to process functions, use extern "C" for example.

Ming.