无意中看到C++11中的新特性inline namespace, 先附上官方的解释
Inline namespace
The inline namespace mechanism is intended to support library evolution by providing a mechanism that support a form of versioning. Consider:
// file V99.h:
inline namespace V99 {
void f(int); // does something better than the V98 version
void f(double); // new feature
// ...
} // file V98.h:
namespace V98 {
void f(int); // does something
// ...
} // file Mine.h:
namespace Mine {
#include "V99.h"
#include "V98.h"
}
We here have a namespace Mine with both the latest release (V99) and the previous one (V98). If you want to be specific, you can:
#include "Mine.h"
using namespace Mine;
// ...
V98::f(); // old version
V99::f(); // new version
f(); // default version
The point is that the inline specifier makes the declarations from the nested namespace appear exactly as if they had been declared in the enclosing namespace.
This is a very ``static'' and implementer-oriented facility in that the inline specifier has to be placed by the designer of the namespaces -- thus making the choice for all users. It is not possible for a user of Mine to say ``I want the default to be V98 rather than V99.''
上面的说法大概可以这么理解，当你需要管理多个版本的类库的时候，可以用inline修饰namespace，来达到指定默认版本的目的，例如在以上的例子中 inline namespace V99 在编译的过程中会直接将里面的代码展开，就像如下的表示方式：
namespace Mine {
void f(int); // does something better than the V98 version
void f(double); // new feature // ...
// file V98.h:
namespace V98 {
void f(int); // does something
// ...
}
}所以V99里面的方法就相当于默认的方法一样，当我们不指定命名空间直接调用 f(1); 时，就是默认调用V99里面的f();而且这个默认是我们无法去改变的，也就是说我们想调用V98里面的f()，就必须写成这样V98::f(1); 。