c++ - C++ 中 C 库的范围 - <X.h> 与 <cX>

Question

The C++ Programming Language : Special Edition 在第 431 页指出...

For every header < X.h > defining part of the C standard library in the global namespace and also in namespace std, there is a header < cX > defining the same names in the std namespace only.

然而，当我在 风格中使用 C 头文件时，我不需要限定 namespace 。例如……

#include <cmath>
void f() {
  double var = sqrt( 17 );
}

这会很好地编译。尽管书中说使用 header 仅在 std 命名空间中定义名称，但您可以在不限定命名空间的情况下使用这些名称。我在这里缺少什么？

附言使用 GNU.GCC 编译器

Answer 1

Stephan T. Lavavej，MSVC 团队的一员，在他的一篇博文 (http://blogs.msdn.com/vcblog/archive/2008/08/28/the-mallocator.aspx#8904359) 的评论中解决了这种情况的现实情况(以及对标准的一些改进):

> also, <cstddef>, <cstdlib>, and std::size_t etc should be used!

I used to be very careful about that. C++98 had a splendid dream wherein <cfoo> would declare everything within namespace std, and <foo.h> would include <cfoo> and then drag everything into the global namespace with using-declarations. (This is D.5 [depr.c.headers].)

This was ignored by lots of implementers (some of which had very little control over the C Standard Library headers). So, C++0x has been changed to match reality. As of the N2723 Working Paper, http://open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2723.pdf , now <cfoo> is guaranteed to declare everything within namespace std, and may or may not declare things within the global namespace. <foo.h> is the opposite: it is guaranteed to declare everything within the global namespace, and may or may not declare things within namespace std.

In reality and in C++0x, including <cfoo> is no safeguard against everything getting declared in the global namespace anyways. That's why I'm ceasing to bother with <cfoo>.

This was Library Issue 456, http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#456 .

(C++0x still deprecates the <foo.h> headers from the C Standard Library, which is hilarious.)

我 100% 同意 Lavavej，除了我从未尝试过非常小心地使用 <cfoo> 样式的 header ，即使是在我第一次开始使用 C++ 时也是如此——标准的 C header 太根深蒂固了——而且从来没有真正的使用它们的世界问题(显然使用 <cfoo> 样式 header 从来没有任何现实世界的好处)。