默认构造函数只有生成如果你不声明任何构造函数。假设如果你定义自己的构造函数，那么你还可以决定是否需要一个无参数的构造函数，如果这样定义的话。

The default constructor is only generated if you don't declare any constructors. It's assumed that if you're defining a constructor of your own, then you can also decide whether you want a no-args constructor, and if so define that too.

在C ++ 0x，会有一个明确的语法，说你想要的默认构造函数：

In C++0x, there will be an explicit syntax for saying you want the default constructor:

struct Foo {
    Foo() = default;
    ... other constructors ...
};