Recommendation 19: understand how to use C in C ++

Chapter 1 changes from C to C ++

The father of C ++ originally designed the language as "a better C". Therefore, C ++ is generally considered as a super set of C, but should not be mistaken for it, "This means that C ++ is compatible with everything in C language ". As a language compatible with C, C ++ retains the features of some procedural languages, and most of the C code can be compiled in C ++ easily, however, there are still a few differences, resulting in some effective C Code cannot be compiled in C ++.

Therefore, from C to C ++, we need to make some changes due to these differences, and we should be familiar with these differences, use the rich original C library to better serve the current C ++ project.

Recommendation 19: understand how to use C in C ++

First, analyze the following code snippet:

 
 

  
  
// Demo.h  
  
  
#ifndef SRC_DEMO_H  
  
  
#define SRC_DEMO_H  
  
  
extern "C"  
  
  
{  
  
  
...  // do something  
  
  
}  
  
  
#endif  // SRC_DEMO_H 
 
 

Obviously, the compilation macro "# ifndef SRC_DEMO_H, # define SRC_DEMO_H, and # endif" in the header file is used to prevent the header file from being repeatedly referenced (see recommendation 9 for details ). So what are the special functions of extern "C? Keep this question for the moment.

C ++ is called "C with classes", "a better C", or "a super set of C", but it is not compatible with C language, the "Datong" between the two cannot completely erase the "slight differences ". The most common difference is that C allows implicit conversion from a void pointer to another type of pointer, but C ++ explicitly prohibits this behavior for security considerations. For example, the following code is valid in C:

 
 

  
  
// Implicitly convert from void * to double *
  
  
Double * pDouble = malloc (nCount * sizeof (double ));
 
 

However, to make it run correctly in C ++, explicit conversion is required:

 
 

  
  
double *pDouble = (double *)malloc(nCount * sizeof(double)); 
 
 

In addition, there are some other portable problems, such as new and class are keywords in C ++, but in C, they can be used as variable names.

To use a large number of ready-made C libraries in C ++, you must put them in extern "C" {/* code. At this point, you may be able to understand the real functions of the Macros in the code snippets listed in this suggestion. Of course, readers with strong curiosity may have a new question: why is it easy to add extern "C" {/* code? This is a problem. Next we will analyze the real reasons behind this phenomenon: C and C ++ have different compilation and link methods. The C compiler does not include the type information of the function when compiling the function, but only the name of the function symbol. The C ++ compiler carries the type information of the function during compilation to implement function overloading. Assume that the prototype of a function is:

 
 

  
  
int Function(int a, float b); 
 
 

The C compiler compiles the Function into a symbol similar to _ Function (this symbol is generally called a mangled name). Once the C linker finds this symbol, it can be connected successfully and called. The C-compiled linker does not verify its parameter type information, but assumes that the information is correct, which is the disadvantage of the C-compiled linker. In C ++, the compiler checks the parameter type information, the above function prototype will be compiled into symbols such as _ Function_int_float (this mechanism also provides necessary support for the implementation of function overloading ). During the connection process, the linker searches _
Function_int_float.

Resolving these conflicts is the most direct cause and motivation for setting the extern "C" syntax. The Function of extern "C" is to tell the C ++ linker to look for the symbol used to call the Function, and let the compiler look for _ Function instead of _ Function_int_float.

To implement the code that calls C in C ++, you can use the following methods:

(1) modify the header file of C code. When it contains C ++ code, add extern "C" to the Declaration ". The Code is as follows:

 
 

  
  
/* C header file: CDemo. h */
  
  
# Ifndef C_SRC_DEMO_H
  
  
# Define C_SRC_DEMO_H
  
  
Extern "C" int Function (int x, int y );
  
  
# Endif // C_SRC_DEMO_H
  
  
 
  
  
/* C language implementation file: CDemo. c */
  
  
# Include "CDemo. h"
  
  
Int Function (int x, int y)
  
  
{
  
  
... // Processing code
  
  
}
  
  
 
  
  
// C ++ calls the file
  
  
# Include "CDemo. h"
  
  
Int main ()
  
  
{
  
  
Function (2, 3 );
  
  
Return 0;
  
  
}
 
 

(2) re-declare the C function in the C ++ code, and add extern "C" to the Declaration ". The Code is as follows:

 
 

  
  
/* C header file: CDemo. h */
  
  
# Ifndef C_SRC_DEMO_H
  
  
# Define C_SRC_DEMO_H
  
  
Extern int Function (int x, int y );
  
  
# Endif // C_SRC_DEMO_H
  
  
 
  
  
/* C language implementation file: CDemo. c */
  
  
# Include "CDemo. h"
  
  
Int Function (int x, int y)
  
  
{
  
  
... // Processing code
  
  
}
  
  
 
  
  
// C ++ calls the file
  
  
# Include "CDemo. h"
  
  
Extern "C" int Function (int x, int y );
  
  
 
  
  
Int main ()
  
  
{
  
  
Function (2, 3 );
  
  
Return 0;
  
  
}
 
 

(3) When the C header file is included, add extern "C ". The Code is as follows:

 
 

  
  
/* C header file: CDemo. h */
  
  
# Ifndef C_SRC_DEMO_H
  
  
# Define C_SRC_DEMO_H
  
  
Extern int Function (int x, int y );
  
  
# Endif // C_SRC_DEMO_H
  
  
 
  
  
/* C language implementation file: CDemo. c */
  
  
# Include "CDemo. h"
  
  
Int Function (int x, int y)
  
  
{
  
  
... // Processing code
  
  
}
  
  
 
  
  
// C ++ calls the file
  
  
Extern "C "{
  
  
# Include "CDemo. h"
  
  
}
  
  
 
  
  
Int main ()
  
  
{
  
  
Function (2, 3 );
  
  
Return 0;
  
  
}
 
 

In use, remember: extern "C" must be added to the C ++ code file to work.

Remember:

If you want to use a large number of ready-made C libraries in C ++ to implement mixed programming between C ++ and C, you must understand what extern "C" is like, understand how to use extern "C.