Weird define in C++ preprocessor

Question

I've come across this

#define DsHook(a,b,c) if (!c##_) {  INT_PTR* p=b+*(INT_PTR**)a;  VirtualProtect(&c##_,4,PAGE_EXECUTE_READWRITE,&no); *(INT_PTR*)&c##_=*p;  VirtualProtect(p,4,PAGE_EXECUTE_READWRITE,&no);  *p=(INT_PTR)c; }

and everything is clear except the "c##_" word, what does that mean?

Answer 1

It means to "glue" together, so c and _ get "glued together" to form c_. This glueing happens after argument replacement in the macro. See my example:

#define glue(a,b) a##_##b

const char *hello_world = "Hello, World!";

int main(int arg, char *argv[]) {
    printf("%s\n", glue(hello,world)); // prints Hello, World!
    return 0;
}

Answer 2

It is called a token-pasting operator. Example:

// preprocessor_token_pasting.cpp
#include <stdio.h>
#define paster( n ) printf( "token" #n " = %d", token##n )
int token9 = 9;

int main()
{
   paster(9);
}

Output

token9 = 9

Answer 3

That's concatenation that appends an underscore to the name passed as c. So when you use

DsHook(a,b,Something)

that part turns into

if (!Something_) 

Answer 4

After the preprocessor, your macro will be expanded as:

if (!c_) {  INT_PTR* p=b+*(INT_PTR**)a;  VirtualProtect(&c_,4,PAGE_EXECUTE_READWRITE,&no); *(INT_PTR*)&c_=*p;  VirtualProtect(p,4,PAGE_EXECUTE_READWRITE,&no);  *p=(INT_PTR)c; }

The ## directive concatenates the value of c which you pass as a macro parameter to _

Answer 5

Simple one:

#define Check(a) if(c##x == 0) { }

At call site:

int varx; // Note the x
Check(var);

Would expand as:

if(varx == 0) { }

Answer 6

It is called Token Concatenation and it is used to concatenate tokens during the preprocessing For example the following code will print out the values of the values of c, c_, c_spam:

#include<stdio.h>

#define DsHook(a,b,c) if (!c##_) \
    {printf("c=%d c_ = %d and c_spam = %d\n",\
    c, c##_,c##_spam);}

int main(){
    int a,b,c=3;
    int c_ = 0, c_spam = 4;

    DsHook(a,b,c);

    return 0;
}

Output:

c=3 c_ = 0 and c_spam = 4