String concatenation using preprocessor

Question

is it possible to concatenate strings during preprocessing?

I found this example

#define H "Hello "
#define W "World!"
#define HW H W

printf(HW); // Prints "Hello World!"

However it does not work for me - prints out "Hello" when I use gcc -std=c99

UPD This example looks like working now. However, is it a normal feature of c preprocessor?

Answer 1

Concatenation of adjacent string litterals isn't a feature of the preprocessor, it is a feature of the core languages (both C and C++). You could write:

printf("Hello "
       " world\n");

Answer 2

You can indeed concatenate tokens in the preprocessor, but be careful because it's tricky. The key is the ## operator. If you were to throw this at the top of your code:

#define myexample(x,y,z) int example_##x##_##y##_##z## = x##y##z 

then basically, what this does, is that during preprocessing, it will take any call to that macro, such as the following:

myexample(1,2,3);

and it will literally turn into

int example_1_2_3 = 123;

This allows you a ton of flexibility while coding if you use it correctly, but it doesn't exactly apply how you are trying to use it. With a little massaging, you could get it to work though.

One possible solution for your example might be:

#define H "Hello "
#define W "World!"
#define concat_and_print(a, b) cout << a << b << endl

and then do something like

concat_and_print(H,W);

Answer 3

From gcc online docs:

The '##' preprocessing operator performs token pasting. When a macro is expanded, the two tokens on either side of each '##' operator are combined into a single token, which then replaces the '##' and the two original tokens in the macro expansion.

Consider a C program that interprets named commands. There probably needs to be a table of commands, perhaps an array of structures declared as follows:

 struct command
 {
   char *name;
   void (*function) (void);
 };

 struct command commands[] =
 {
   { "quit", quit_command },
   { "help", help_command },
   ...
 };

It would be cleaner not to have to give each command name twice, once in the string constant and once in the function name. A macro which takes the name of a command as an argument can make this unnecessary. The string constant can be created with stringification, and the function name by concatenating the argument with _command. Here is how it is done:

 #define COMMAND(NAME)  { #NAME, NAME ## _command }

 struct command commands[] =
 {
   COMMAND (quit),
   COMMAND (help),
   ...
 };

Answer 4

I just thought I would add an answer that cites the source as to why this works.

The C99 standard §5.1.1.2 defines translation phases for C code. Subsection 6 states:

6. Adjacent string literal tokens are concatenated.

Similarly, in the C++ standards (ISO 14882) §2.1 defines the Phases of translation. Here Subsection 6 states:

6 Adjacent ordinary string literal tokens are concatenated. Adjacent wide string literal tokens are concatenated.

This is why you can concatenate strings simply by placing them adjacent to one another:

printf("string"" one\n");

>> ./a.out
>> string one

The preprocessing part of the question is simply the usage of the #define preprocessing directive which does the substitution from identifier (H) to string ("Hello ").