1. Introduction
The original intention of C ++ is to "a better C ", however, this does not mean that the compilation and connection methods used by global variables and functions similar to C in C ++ are the same as those in C. As a language to be compatible with C, C ++ retains the features of some procedural languages (known as "incomplete Ground Object "), therefore, it can define global variables and functions that do not belong to any class. However, C ++ is an object-oriented programming language after all. To support function overloading, C ++ treats global functions differently from C.
2. Start with the standard header file
An enterprise once gave the following interview question: why do standard header files have a structure similar to the following?
# Ifndef _ incvxworksh
# DEFINE _ incvxworksh
# Ifdef _ cplusplus
Extern "C "{
# Endif
/*...*/
# Ifdef _ cplusplus
}
# Endif
# Endif/* _ incvxworksh */
Analysis: Obviously, the compilation macro "# ifndef _ incvxworksh, # DEFINE _ incvxworksh, and # endif" in the header file is used to prevent the header file from being repeatedly included. So what is the role of the following code?
# Ifdef _ cplusplus
Extern "C "{
# Endif
# Ifdef _ cplusplus
}
# Endif
3. Deep encryption extern "C"
Extern "C" contains double meanings, which can be obtained literally: first, the target is "extern", and secondly, the target is "C style. Let's explain these two meanings in detail.
(
1) The function or variable specified by extern "C" is of the extern type.
Extern is a keyword in C/C ++ that indicates the range (visibility) of functions and global variables. This keyword tells the compiler, the declared functions and variables can be used in this module or other modules. Remember the following statement: extern int A; is just a declaration of a variable. It is not defining variable A, but does not allocate memory space for. Variable A can only be defined once as a global variable in all modules. Otherwise, a connection error occurs.
In general, the function and global variables referenced by this module to other modules are declared with the keyword extern in the module header file. For example, if Module B wants to reference the global variables and functions defined in module A, it only needs to include the header file of module. In this way, when Module B calls a function in module A, although Module B cannot find the function in the compilation phase, no error is reported; it will find this function from the target Code Compiled by module A in the connection phase.
The keyword corresponding to extern is static. The global variables and functions modified by it can only be used in this module. Therefore, a function or variable can only be used by this module and cannot be modified by extern "C.
(2) variables and functions modified by extern "C" are compiled and connected in C language.
3. 1. First, let's take a look at how C-like functions are compiled in C ++.As an object-oriented language, C ++ supports function overloading, while Procedural Language C does not. The name of the function in the symbol library after being compiled by C ++ is different from that in the C language. For example, assume that the prototype of a function is void Foo (int x, int y). After the function is compiled by the C compiler, its name in the symbol library is _ Foo, the C ++ compiler generates names such as _ foo_int_int (different compilers may generate different names, but all adopt the same mechanism, the new name is called "mangled name "). A name such as _ foo_int_int contains the function name, number of function parameters, and type information. c ++ relies on this mechanism to implement function overloading. For example, in C ++, the void Function
Foo (int x, int y) and void Foo (int x, float y) are generated in different ways. The latter is _ foo_int_float.
Similarly, variables in C ++ support both local variables and class member variables and global variables. The class member variables of the program written by the user may have the same name as the global variables, which are distinguished. In essence, the compiler uses a unique name for the variables in the class when compiling, similar to the function processing. This name is different from the global variable name with the same name in the user program.
. Connection method when the extern "C" statement is not added
Suppose in C ++, the header file of module A is as follows:
// Module A header file modulea. h
# Ifndef module_a_h
# Define module_a_h
Int Foo (int x, int y );
# Endif
Reference this function in Module B:
// Module B implements the file moduleb. cpp
# Include "modulea. H"
Foo (2, 3 );
In fact, in the connection phase, the connector looks for symbols such as _ foo_int_int from the target file modulea. OBJ generated by module!
3. The compilation and Connection Methods After the extern "C" clause is added
After the extern "C" statement is added, the header file of module A is changed:
// Module A header file modulea. h
# Ifndef module_a_h
# Define module_a_h
Extern "C" int Foo (int x, int y );
# Endif
In the implementation file of Module B, Foo (2, 3) is still called. The result is: (1) When module A compiles the foo target code, its name is not specially processed, the C language is used. (2) when the connector looks for the Foo (2, 3) call for the target code of Module B, it looks for the unmodified symbol name _ Foo.
If the function in module A declares that foo is of the extern "C" type, and Module B contains the extern int Foo (INT X, int y ), module B cannot find the function in module A, and vice versa.
Therefore, we can use a single sentence to summarize the true purpose of the extern "C" statement (the birth of any syntax feature in any language is not random and comes from the needs of the real world. When thinking about a problem, we should not just focus on how the language is made, but also ask why it is doing so and what the motivation is, so that we can better understand many problems ):Implement mixed programming of C ++, C and other languages. Understand the motivation for setting up extern "C" in C ++. Next we will analyze the general usage skills of extern "C" in detail.
3.4.extern "C"
(1)Reference functions and variables in C language in C ++, and include the C Language header file(For example, cexample. h:
Extern "C"
{
# Include "cexample. H"
}In the header file of the C language, the external function can only be specified as the extern type. The C language does not support the extern "C" declaration. when the c file contains extern "C", a compilation syntax error occurs.
/* C header file: cexample. H */
# Ifndef c_example_h
# Define c_example_h extern int add (int x, int y); // Note: It can be written as extern "C" int add (INT, INT );
# Endif
/* C language implementation file: cexample. C */
# Include "cexample. H"
Int add (int x, int y)
{
Return X + Y;
}
// C ++ implementation file, call Add: cppfile. cpp
Extern "C" // required
{
# Include "cexample. H"// Note: this is not correct. If it cannot be compiled in this way, replace it with extern "C" int add (INT, INT). You can use}
Int main (INT argc, char * argv [])
{Add (2, 3 );
Return 0;
} If C ++ calls a. dll file written in C language, when it includes the header file of. dll or declares an interface function, add extern "C "{}.
(2)When C references functions and variables in C ++, the header file of C ++ needs to add extern "C ", however, you cannot directly reference this header file that declares extern "C" in C. You should only declare the extern "C" function defined in C ++ as the extern type..
// C ++ header file cppexample. h
# Ifndef cpp_example_h
# Define cpp_example_h
Extern "C" int add (int x, int y );
# Endif
// C ++ implementation file cppexample. cpp
# Include "cppexample. H"
Int add (int x, int y)
{
Return X + Y;
}
/* C implementation file cfile. c
/* Compilation errors: # include "cexample. H "*/
Extern int add (int x, int y );
Int main (INT argc, char * argv [])
{
Add (2, 3 );
Return 0;
}
If you thoroughly understand the role of extern "C" described in section 3rd in the compilation and connection phases, you can really understand the usage of referencing c Functions and C ++ functions from C ++ described in this section.
