为什么不能定义这些宏名称？

Question

If I make a define at the command like -Dfirst and -Dsecond I get a bunch of errors:

In file included from main.cpp:1:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/iostream:39:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/ostream:38:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/ios:40:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/char_traits.h:39:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_algobase.h:64:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_pair.h:101:11: error: expected member name or ';' after declaration specifiers
      _T1 first;                 /// @c first is a copy of the first object
      ~~~ ^
<command line>:1:15: note: expanded from here
#define first 1
              ^
...
              ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.

But if I use capital FIRST and SECOND it's fine.

Answer 1

If you define first and second before including anything, that includes std::pair inside <algorithm> , it will screw your code, because the variables inside std::pair are called first and second , and your macro is defined to that, so it expands the names to the expression they are defined to, making your code malformed.

Basically, its the same as doing

#define A 1

class A{
};

This will also not compile, because A gets converted to 1, and 1 is not valid class name

Answer 2

如您所见，first和second在整个STL中都使用，因此，通过将它们定义为没有，您将使其变得不可解析