How to make preprocessor generate a string for __LINE__ keyword?

Question

__FILE__ is replaced with \"MyFile.cpp\" by C++ preprocessor. I want __LINE__ to be replaced with \"256\" string not with 256 integer. Without using m

Answer 1

You need the double expansion trick:

#define S(x) #x
#define S_(x) S(x)
#define S__LINE__ S_(__LINE__)

/* use S__LINE__ instead of __LINE__ */

Addendum, years later: It is a good idea to go a little out of one's way to avoid operations that may allocate memory in exception-handling paths. Given the above, you should be able to write

throw std::runtime_error("exception at " __FILE__ " " S__LINE__);

which will do the string concatenation at compile time instead of runtime. It will still construct a std::string (implicitly) at runtime, but that's unavoidable.