Using macro with string fails on VC 2015

c++ macros visual-studio-2015 user-defined-literals
Ajay picture Ajay · Jul 31, 2015 · Viewed 14k times · Source

Why does this fail to compile?

char programDate[] = "("__DATE__")";

But this compiles fine (see space):

char programDate[] = "(" __DATE__")";

I do know VC2015 now supports literal-operators. But shouldn't that be in compilation phase? __DATE__ should have been processed by the pre-processor. What is going on here?

I thought of some mix-match issue with Unicode/non-Unicode build - but it doesn't help. It's not just issue with pre-defined macros, but with user defined also:

#define MACRO "abc"
char data[] = "("MACRO")";

EDIT:

Error C3688 invalid literal suffix '__DATE__'; literal operator or literal operator template 'operator ""__DATE__' not found

Answer

M.M picture M.M · Jul 31, 2015

Since C++11, user-defined literals exist and are part of preprocessing. The grammar is:

preprocessing-token:
    user-defined-string-literal
    // other stuff...

user-defined-string-literal:
    string_literal ud-suffix

ud-suffix:
    identifier

So "("__DATE__ matches preprocessing-token, but "(" __DATE__ doesn't (that is two separate preprocessing tokens).

Macro replacement happens after tokenization. Since there is no token __DATE__ in your first example, there is no replacement.

Related questions

What does "#pragma comment" mean?

What does #pragma comment mean in the following? #pragma comment(lib, "kernel32") #pragma comment(lib, "user32")

Read More
What is the worst real-world macros/pre-processor abuse you've ever come across?

What is the worst real-world macros/pre-processor abuse you've ever come across (please no contrived IOCCC answers *haha*)? Please add a short snippet or story if it is really entertaining. The goal is to teach something instead of always telling …

Read More
How to identify platform/compiler from preprocessor macros?

I'm writing a cross-platform code, which should compile at linux, windows, Mac OS. On windows, I must support visual studio and mingw. There are some pieces of platform-specific code, which I should place in #ifdef .. #endif environment. For example, here …

Read More