使用基类中现有类型的声明与在子类中创建类型别名 [英] Using-declaration of an existing type from base class vs creating a type alias inside child class

问题描述

我想提供子类中基类对现有类型的访问.

I would like to provide access to an existing type from base class inside child class.

我发现了两种不同的方式:

I found two different ways :

struct A {
    typedef int mytype;
};

struct B {
    typedef double mytype;
};

我可以使用using声明包含"类型:

I can "include" the type with a using declaration :

struct C : A, B {
    using typename A::mytype;
};

或者我可以创建一个类型别名:

Or I can create a type alias :

struct C : A, B {
    typedef A::mytype mytype;
    using mytype = A::mytype; //C++11
};

1. 有什么区别吗?
2. 每种语法的优缺点是什么?
3. 哪个是最常用/推荐的?

谢谢.

相关问题:使用-声明现有名称空间类型与创建类型别名

推荐答案

有区别.考虑一下，如果将结构A和B定义为:

There is a difference. Consider what would happen if your structs A and B were defined as:

struct A {
protected:
    int mytype;
};

struct B {
protected:
    double mytype;
};

在这种情况下

struct C : A, B {
    using typename A::mytype;  // Would compile, but is mytype a type or
                               // an exposed member of the base class?
    //using mytype = A::mytype;  // Would not compile
};

在您的情况下，我建议您使用using mytype = A::mytype;，因为它不太含糊.

In your case I would recommend using using mytype = A::mytype; as it is less ambiguous.

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