Question

如果我对-Dfirst和-Dsecond这样的命令进行定义，我会收到一堆错误：

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.

但是，如果我使用大写FIRST和SECOND就可以了。

Answer 1

如果您在包含std::pair <algorithm>之内的任何内容之前定义第一个和第二个，那么它将会破坏您的代码，因为std::pair中的变量称为first并且second，并且您的宏被定义为，因此它将名称扩展为它们定义的表达式，从而使您的代码格式错误。

基本上，它和做

一样

#define A 1

class A{
};

这也不会编译，因为A转换为1，而1不是有效的类名

Answer 2

正如您所看到的，第一个和第二个在整个STL中使用，因此通过将它们定义为无，您已将其渲染为不可解析

为什么我不能定义这些宏名称？

2 个答案: