英文:
Derived class from template parameter
问题
Multiple definitions of Derived class is not allowed, so the following is a compile time error.
多次定义`Derived`类是不允许的，因此以下是编译时错误。
Sameways, why isn't the following, a compile time error?
Wouldn't there be 2 versions of Derived class, one inheriting from A1 while the other from A2?
同样地，为什么以下代码不是编译时错误？
难道不会有两个版本的`Derived`类，一个继承自`A1`，另一个继承自`A2`？
class A1
{ };
class A2
{ };
template<class T>
class Derived: public T
{ };
int main()
{
Derived<A1> *ptr1 = new Derived<A1>;
Derived<A2> *ptr2 = new Derived<A2>;
}
英文:
Multiple definitions of Derived class is not allowed, so the following is a compile time error.
class A1
{ };
class A2
{ };
class Derived: public A1
{ };
class Derived: public A2
{ };
Sameways, why isn't the following, a compile time error?
Wouldn't there be 2 versions of Derived class, one inheriting from A1 while the other from A2?
class A1
{ };
class A2
{ };
template<class T>
class Derived: public T
{ };
int main()
{
Derived<A1> *ptr1 = new Derived<A1>;
Derived<A2> *ptr2 = new Derived<A2>;
}
答案1
得分: 4
以下是您要翻译的内容：
不会有两个继承自A1和A2的Derived类，而只会有一个名为Derived<A1>和Derived<A2>的类。您传递给模板参数的数据会成为类的一部分。声明Derived<A1>并将A1传递给模板参数T，您会实例化一个类模板。
我认为这里解释得更清楚：
> 模板实例化 涉及生成一个特定模板参数组合的具体类或函数（实例）。
并且不会有Derived类存在。正如max66在评论中指出的那样，Derived不是一个类，而是一个类模板，如果没有实例化，它将无法单独编译。
英文:
There would be not two Derived classed, inherited from A1 and A2, but literally one class Derived<A1> and Derived<A2>. The data you pass to template argument, becomes part of the class. Delcaring Derived<A1> and passing A1 to the template argument T you instantiate a class template.
I think here is explained better:
> Template instantiation involves generating a concrete class or
> function (instance) for a particular combination of template
> arguments.
And there would be no Derived class at all. As max66 pointed out in comment, Derived is not a class, but a class template, which will not be compiled alone, without instantiation.
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