Convert digit to string in macro

Question

Possible duplicate:
Convert preprocessor token to string

#define num 1234 

I want to define "const char *" based on num, in the example it would be:

 #define num_str "1234" 

Can I write a macro to achieve this? NB: 1234 will be changed.

Thanks.

Answer 1

Yes you can, but macro substitution may look a little strange. A replacement with two macros exists for some reason, and if you think about it for a while, it will become clear why this is necessary.

 #define STRINGIZER_(exp) #exp #define STRINGIZER(exp) STRINGIZER_(exp) #define NUM 1234 int main(int argc, char *argv[]) { const char *p = STRINGIZER(NUM); printf("%s\n",p); return EXIT_SUCCESS; } 

Launch:

 1234 

The reason for the double replacement: at first glance, you might think that this will solve the problem:

 #define STRINGIZER(exp) #exp #define NUM 1234 int main(int argc, char *argv[]) { const char *p = STRINGIZER(NUM); printf("%s\n",p); return EXIT_SUCCESS; } 

But it gives:

 NUM 

what we are not trying to do. If you want the actual extension of the NUM macro to be the first, then this is what you need to do: force extension. Forcing the preprocessor to be replaced first through an intermediate extension (as I will show at the top of this answer), the macro of the passed first expands and then builds.

Sidebar: This method is especially useful for generating wide-format wide-char preprocessor macros that otherwise save regular lines. For example, the __FILE__ macro. Suppose you need a wide char version (a string added with 'L'). At first you might think that this would work:

 #define WIDESTR(str) L##str 

but expanding this with __FILE__ , as in:

 const wchar *p = WIDESTR(__FILE__); 

will result in a compiler error: "Undefined identifier: L__FILE__"

So how can we solve this? Similarly, we did the above.

 #define WIDESTR_(str) L##str #define WIDESTR(str) WIDESTR_(str) int main(int argc, char *argv[]) { const wchar_t* p = WIDESTR(__FILE__); wcout << p << endl; return EXIT_SUCCESS; } 

On my system, this produces:

 /Users/craig/tmp/main/main/test.cpp 

In closing ...

As a consolation prize, we combine everything in this answer into one giant corn, which we assume when we do this:

 int main() { const wchar_t *p = WIDESTR(STRINGIZE(NUM)); wcout << p << endl; return EXIST_SUCCESS; } 

Answer 2

How will the string be used in the rest of your code? Should it be a string macro? The standard way to create strings from other values ​​is to use sprintf () from string.h

 #define num 1234 sprintf(string,"%d",num);