Link indication: extern "C" C and C ++ Mixed Programming Problems

Analyze the following code:

/* = Sum. H = */

# Ifndef sum_h
# Define sum_h
# Include <stdio. h>

Int sum (int A, int B );
# Endif;

/* = Sum. c = */

# Include "sum. h"
Int sum (int A, int B)
{
Int c = A + B;
Return C;
}

/* = Main. cpp = */

# Include "sum. h"

Void Mian (){

Cout <sum (1, 2) <Endl;

}

The above three files are called and compiled successfully, but the following problems occur during execution:

OBJ: Error lnk2001: the external symbol "int _ cdecl sum (INT, INT)" that cannot be parsed )"(? Sum @ yahhh @ Z)
E:/programming/grapic/test/debug/test.exe: Fatal error lnk1120: 1 external command that cannot be parsed

Where is the problem? Sum. C is called in Main. cpp, that is to say, C is called in C ++. If no corresponding processing is done, a link error will occur.

Extern "C" indicates that the internal Symbol names generated by Compilation Use the c Convention. C ++ supports function overloading, but C does not. The compilation rules of the two are different. 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 may be _ 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.

So if C ++ code is called in C and how can C code be called in C ++?

Extern "C" indicates that the internal Symbol names generated by Compilation Use the c Convention.

1. How to call C in C ++?

C ++ calls C and extern "c". The function is to let the C ++ connector use the C method to find the symbol used to call the function.

The pen questions raised at the beginning of this article can be modified as follows:

/* = Sum. H = */

# Ifndef sum_h
# Define sum_h
# Include <stdio. h>

Int sum (int A, int B );
# Endif;

/* = Sum. c = */

# Include "sum. h"
Int sum (int A, int B)
{
Int c = A + B;
Return C;
}

/* = Main. cpp = */

Extern "C"
{
# Include "sum. h"
}

Void Mian (){

Cout <sum (1, 2) <Endl;

}

Execution successful

I believe this is almost clear.

2. How to call C ++ in C?

Reference C ++ functions in C (C calls C ++, the use of extern "C" tells the compiler to compile and encapsulate the functions defined by extern "C" in the CPP file in the way of C, of course, the C ++ syntax in the interface function is still compiled in C ++ mode)

Run:Test1.obj: Error lnk2019: the external symbol _ sum that cannot be parsed. This symbol is referenced in function _ main.
E:/programming/grapic/test/debug/test.exe: Fatal error lnk1120: 1 external command that cannot be parsed

/* ======= Sum. h ============= */

# Ifndef sum_h
# Define sum_h
# Include <stdio. h>

Int sum (int A, int B );
# Endif;

/* = Sum. cpp = */

# Include "sum. h"
Extern "C"
{
Int sum (int A, int B)
{
Int c = A + B;
Return C;
}
}

/* = Main. c = */

# Include "sum. h"

Void Mian (){

Cout <sum (1, 2) <Endl;

}

3. Standard and standard writing

Generally, we place the function declaration in the header file. When our function may be used by C or C ++, we cannot determine who calls it, this makes it uncertain whether to declare the function in extern "c". Therefore, we can add

# Ifdef _ cplusplus

Extern "C"

{

# Endif

// Function declaration

# Ifdef _ cplusplus

}

# Endif

The above statement form can be used comprehensively.

When C references functions and variables in C ++, C ++ functions or variables must be declared in extern "C, however, extern "C" cannot be used in C; otherwise, an error occurs during compilation. (Error: Error c2059: syntax error: 'string'. This error has been found online for a long time. The Chinese website has not found any direct explanation for the cause, the reason is that extern "C" is the keyword in C ++. If it is not C, all errors will occur.

/* = Sum. H = */

# Ifndef sum_h
# Define sum_h
# Include <stdio. h>

Int sum (int A, int B );
# Endif;

/* = Sum. cpp = */

# Include "sum. h"
Int sum (int A, int B)
{
Int c = A + B;
Return C;
}

/* = Main. c = */

# Include "sum. h"

Void Mian (){

Cout <sum (1, 2) <Endl;

}