在c ++ 11模式下奇怪的g ++预处理器行为

Question

I have the problem that when g++ is run in c++11 mode, some proprocessor macros are not expanded correct. This causes me troubles during the compilation of programs using Qt.

$ g++ --version
g++ (GCC) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The following snipped exposes the problem:

$ cat foo.cpp
//#include <QtGui>
#define QTOSTRING_HELPER(s) #s
#define QTOSTRING(s) QTOSTRING_HELPER(s)
#ifndef QT_NO_DEBUG
# define QLOCATION "\0"__FILE__":"QTOSTRING(__LINE__)
# define METHOD(a)   qFlagLocation("0"#a QLOCATION)
# define SLOT(a)     qFlagLocation("1"#a QLOCATION)
# define SIGNAL(a)   qFlagLocation("2"#a QLOCATION)
#else
# define METHOD(a)   "0"#a
# define SLOT(a)     "1"#a
# define SIGNAL(a)   "2"#a
#endif

METHOD(grml)

Preprocesing it without c++11 does the right thing.

$ g++ -E foo.cpp
# 1 "foo.cpp"
# 1 "<command-line>"
# 1 "foo.cpp"
# 15 "foo.cpp"
qFlagLocation("0""grml" "\0""foo.cpp"":""15")

But in C++11 mode the QTOSTRING macro does not get expanded, causing a compile error at the source line.

$ g++ -std=c++11 -E foo.cpp
# 1 "foo.cpp"
# 1 "<command-line>"
# 1 "foo.cpp"
# 15 "foo.cpp"
qFlagLocation("0""grml" "\0"__FILE__":"QTOSTRING(15))

Is this behavior intended, and what can I do to enable the expansion?

Answer 1

This is a known problem and the new GCC behaviour is intentional as a result of a new C++11 feature, namely user-defined literals. You can insert a space before __FILE__ and QTOSTRING to ensure it will always be treated as a separate token and thus expanded.

QT bugreport here .