CODEWITHSUNDEEP
c++cmacrosgnupreprocessor
Aizat Khir
Aizat Khir

Reputation: 41

How to concatenate a variable string with literal string in C macro?

Im creating a macro that accept 2 strings: variable string and literal string. How do I combine both of them to a function which accept single variable?

I know that we can combine 2 literal string in a macro.

#define LOGPRINT(x,y) func(x y)
#define LOGPRINTSINGLE(x) func(x)

func(char *fmt)
{
   printf(fmt);
}

Below code works fine:

char * test = "hey "; 

LOGPRINT("hello ", "world!");
LOGPRINTSINGLE(hey);

But below code fails. 

LOGPRINT(test, "world!");

How do I combine variable string test to literal string "world" in the macro? Expected result is "hey world" is passed to the func().

**Edit/Note: Rule is I only allowed to change code on this side and not the caller and func().

Upvotes: 3

Views: 1894

Answers (2)

flott
flott

Reputation: 231

Maybe not the most beautiful or safe macro in the world, but it works :-)

#include <stdio.h>
#include <string.h>

#define LOGPRINT(x,y)                       \
{                                           \
char dest[strlen(x) + strlen(y) + 1];       \
memcpy(dest, x, strlen(x));                 \
memcpy(dest + strlen(x), y, strlen(y));     \
dest[strlen(x) + strlen(y)] = 0;            \
func(dest);                                 \
}
#define LOGPRINTSINGLE(x) func(x)

void func(char *fmt)
{
   printf(fmt);
}


int main()
{
    char * test = "hey "; 

    LOGPRINT("hello ", "world!");
    printf("\n");
    LOGPRINTSINGLE(test);
    printf("\n");
    LOGPRINT(test, "world!");

    return 0;
}

Output:

hello world!
hey
hey world!

Upvotes: 2

Subangkar KrS
Subangkar KrS

Reputation: 363

Pretty simple using c++ string if you include "string" STL

change #define LOGPRINT(x,y) func(x y)

to #define LOGPRINT(x,y) func((std::string(x) + std::string(y)).data())

or #define LOGPRINT(x,y) func(std::string(x).append(y).data())

Upvotes: 1

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