__STDC_CONSTANT_MACROS和__STDC_CONSTANT_MACROS的作用

虽然是写C++出身，但还真不知道这两个宏是什么作用。查了一下，参见这里。

__STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS are a workaround to allow C++ programs to use stdint.h 
macros specified in the C99 standard that aren't in the C++ standard. The macros, such as UINT8_MAX, INT64_MIN, 
and INT32_C() may be defined already in C++ applications in other ways. To allow the user to decide 
if they want the macros defined as C99 does, many implementations require that __STDC_LIMIT_MACROS 
and __STDC_CONSTANT_MACROS be defined before stdint.h is included.

This isn't part of the C++ standard, but it has been adopted by more than one implementation.

简单的翻译一下，就是说两个宏__STDC_CONSTANT_MACROS和__STDC_CONSTANT_MACROS，是为在C++中使用C99定义的一些宏，它们定义在<stdint.h>中。这些宏，如：UINT8_MAX, INT64等。虽然这两个宏不是C++标准的内容，但是被不只一种实现采用。

当前在C++11和C11已经不再检测这个宏了，参见这里。