整理自 Thinking in C++。
The multiple-declaration problem
比如很多的 lib 都有 #include <iostream>，我引用这些库，再加上我自己可能也要用 <iostream>，所以实际上你暗地里不知道已经 #include 了多少次 <iostream>。那么这样做有什么后果呢？
The compiler considers the redeclaration of a structure (this includes both structs and classes) to be an error. In fact, both C and C++ allow you to redeclare a function, as long as the two declarations match, but neither will allow the redeclaration of a structure. In C++ this rule is especially important because if the compiler allowed you to redeclare a structure and the two declarations differed, which one would it use?
那我们怎么解决这个问题呢？很简单，在 header 里 #define 一个 “inclusion flag” 就好了，比如：
//: C04:Simple.h
// Simple header that prevents re-definition
#ifndef SIMPLE_H
#define SIMPLE_H
struct Simple {
int i,j,k;
initialize() { i = j = k = 0; }
};
#endif // SIMPLE_H //
第一次 #include，declare 成功；第二次再 #include，因为 SIMPLE_H 已经 #define 了，#ifndef SIMPLE_H 直接判为 false，直接 pass。
这个 inclusion flag 直接和 header 的名字相关，所以基本不会重复。
Never put using in headers
You’ll virtually never see a using directive in a header file (at least, not outside of a scope). If you put a using directive (outside of a scope) in a header file, it means that this loss of “namespace protection” will occur with any file that includes this header, which often means other header files. 换言之就是 using 的作用会连锁传播，影响太大。