001.14.19: The Double Expansion Trap

pravjot

"We wrote a macro

#define STR(x) #x and called STR(__LINE__),

expecting to print the line number, but you got the string "__LINE__".

Walk me through the exact preprocessor rules causing this and how to rewrite the macro to fix it."

mayanktuteja3069

we need two levels of macro to print the line number in it

because

# -Convert the macro argument into a string literal without expanding it first so

#define STR(x) #x //Argument stays literal

#define XSTR(x) STR(x) // this layers forces expansion before stringize

printf("%s\n", XSTR(__LINE__)); // it prints the line number