Why do we need this

[ssharish2005]

Found this code in one of the header file of some project. I dont really understand why do we use this.

#ifdef __cplusplus
extern "C" {
#endif

Does any one know where and why do need to include this in the header file. How does it help. Just curoius to know

Thank you

ssharish2005

[matsp]

Because the syntax and meaning of some things are slightly different between C and C++, extern "C" tells the compiler that the following definition(s) are "C" rather than "C++", so that it can use the "old" syntax and semantics. One example is:
int foo(); in C++ means that foo takes no arguments.
int foo(); in C means that foo is undefined how many arguments it takes.

--
Mats

[ssharish2005]

int foo(); in C means that foo is undefined how many arguments it takes.

What happens when this is the case

int foo(); 

int main()
{
    foo();
    
    return 0;
}

int foo()
{
    printf("I am foo");
}

In this case the foo is defines. Would that still interprert foo undefined? Or where u just speaking about the prototype.

ssharish2005

[matsp]

What happens when this is the case

int foo(); 

int main()
{
    foo();
    
    return 0;
}

int foo()
{
    printf("I am foo");
}

In this case the foo is defines. Would that still interprert foo undefined? Or where u just speaking about the prototype.

ssharish2005

In C with the above code would be OK to call foo(3) or foo("blah", 47, 4.5), just as well as foo(). In C++, only foo() is accepted by the compiler.

Of course, in C, the compiler may moan about missing prototypes and such if you do this - and rightly so, I would say - but it doesn't stop old/clever code from abusing this rule - a particular example would be using "home-made variable number of arguments", something like this:

int sumit();

int main() {
   printf("sum = ", sumit(2, 19, 23));
   printf("sum = ", sumit(3, 19, 23, 42));
}

int sumit(int n, int a, int b, int c, int d, int e) 
{
    int sum = 0;
    switch(n) {
        case 5:
           sum += e;
        case 4:
           sum += d;
        case 3:
           sum += c;
        case 2:
           sum += b;
        case 1:
           sum += a;
           break;
        default: 
           .... do some error handling ... 
    }
    return sum;
}

[I haven't tested that code, but I believe it works.

--
Mats

[ssharish2005]

Thanks Map, i got it. And code works fine except this

 printf("sum = %d ", sumit(2, 19, 23));
 printf("sum = %d ", sumit(3, 19, 23, 42));

Thanks a lot

ssharish2005

[Dave_Sinkula]

Found this code in one of the header file of some project. I dont really understand why do we use this.

http://www.parashift.com/c++-faq-lit....html#faq-32.4

[matsp]

Thanks Map, i got it. And code works fine except this

 printf("sum = %d ", sumit(2, 19, 23));
 printf("sum = %d ", sumit(3, 19, 23, 42));

Thanks a lot

ssharish2005

"It's always the simple things that trip you up" :-)

--
Mats

[whiteflags]

I would like to stress that due to the undefined nature of a function such as bar(), that it's not really something that you should use extensively as a design. The normal rules about getting a compile without errors and warnings should always apply.

bar isn't type strict, and doesn't really have a place in C.

void bar();

... 

bar(42);

void bar(const char *s)
{
  puts(s);
}

Variable argument lists are useful, but there is a better way to do it: use the stuff in <varargs.h>

[ssharish2005]

"It's always the simple things that trip you up" :-)

--
Mats

Hey, i dint ment to pick you up there. heyy i am sorry

ssharish2005

[ssharish2005]

And I also think this wouldn't be really necessary when we save the file with .c extension. But perhaps in the header file which dosn't differentiate may be it would be necessary.

And is it true to include this to call a C++ function in C

#ifdef __cplusplus
extern "C++" {
#endif

function prototypes

#ifdef __cplusplus
}
#endif

ssharish2005

[brewbuck]

Found this code in one of the header file of some project. I dont really understand why do we use this.

C++ names are type-mangled. C names are not. extern "C" tells the compiler that the following name should not be type-mangled for linkage purposes. It allows C++ code to call C code.

[tanmay_solanki]

ITS TO RUN C++ style fuctions to c compiler.

[QuantumPete]

ITS TO RUN C++ style fuctions to c compiler.

You can't run C++ functions in a C compiler. C++ is a superset of C. You can compile C with C++ compiler, but not the other way round.

QuantumPete