在命名空间中定义一个类
有没有比这个更简洁的方法来在命名空间中定义一个类:
Is there a more succinct way to define a class in a namespace than this:
namespace ns { class A {}; }
我希望像 class ns::A {}; 这样的东西可以工作，但可惜不行.
I was hoping something like class ns::A {}; would work, but alas not.
推荐答案
你已经接近了，你可以在命名空间中转发声明该类，然后如果需要在外部定义它:
You're close, you can forward declare the class in the namespace and then define it outside if you want:
namespace ns {
class A; // just tell the compiler to expect a class def
}
class ns::A {
// define here
};
你不能做的是在没有成员的命名空间中定义类，然后在命名空间之外再次定义类.这违反了单一定义规则(或类似的废话).
What you cannot do is define the class in the namespace without members and then define the class again outside of the namespace. That violates the One Definition Rule (or somesuch nonsense).
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