Printing space using macro and # operator in C
I worked with macros and wrote one like this:
#define STR(name) #name
I meant STR()
scribbling whatever was given to him as an argument and it looks like it worked.
printf( STR(Hello) )
gave the result as expected:
Hello
So did
printf( STR(Hello world) );
printf( STR(String) STR(ise) );
who gave
Hello world
Stringise
But when I tried to use STR()
only space to print it just didn't work.
printf( STR(Hello) STR( ) STR(World) ); //There’s a space between the parenthesis of the second STR
Output output:
HelloWorld
STR( )
Ignored here .
Why is this? Is there a way to use it while keeping the space-only macros as an argument?
I'm just wondering if this is possible.
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It is impossible for a building to fit into one space. The semantics of the operator are #
detailed in C11 6.10.3.2p2 :
If the substitution list is immediately preceded by a
#
preprocessing token , then both are replaced with a single character-string preprocessor character, which contains the spelling of the preprocessing token sequence for the corresponding argument. Each occurrence of a space between the argument preprocessing tokens becomes a space character in the character string literal. A space before the first preprocessing token and after the last preprocessing token that constitutes this argument is removed. [...] character string literal, the corresponding empty arguments""
. ...].
Thus, since the space is not a preprocessing token, and the trailing and trailing space is removed, it is not possible for a string operator to create a resulting string literal that contains only one space. As you noticed, STR( )
will pass an empty argument to the macro, and this will bind to ""
; Similarly
STR( Hello World )
will be expanded to "Hello World"
; that is, each occurrence of a space will become a single spatial symbol, and the previous and trailing white space will be removed.
However, while it is impossible to create a tiny space, you can achieve the desired result. The preprocessor concatenates sequential string literals into one, so it "Hello" " " "World"
will be converted to "Hello world"; so
printf(STR(Hello) " " STR(World));
after expanding macro exchange to
printf("Hello" " " "World");
and then -
printf("Hello World");
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