C++ 默认类继承访问
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Default class inheritance access
提问by paul simmons
Suppose I have a base and derived class:
假设我有一个基类和派生类：
class Base
{
public:
virtual void Do();
}
class Derived:Base
{
public:
virtual void Do();
}
int main()
{
Derived sth;
sth.Do(); // calls Derived::Do OK
sth.Base::Do(); // ERROR; not calls Based::Do
}
as seen I wish to access Base::Do through Derived. I get a compile error as "class Base in inaccessible" however when I declare Derive as
正如所见，我希望通过派生访问 Base::Do。但是，当我将 Derive 声明为
class Derived: public Base
it works ok.
它工作正常。
I have read default inheritance access is public, then why I need to explicitly declare public inheritance here?
我已经读过默认继承访问是公共的，那为什么我需要在这里显式声明公共继承？
采纳答案by Peter G.
You might have read something incomplete or misleading. To quote Bjarne Stroustrup from "The C++ programming Language", fourth Ed., p. 602:
您可能阅读了不完整或误导性的内容。引用 Bjarne Stroustrup 中的“C++ 编程语言”，第四版，p。602：
In a
class, members are by defaultprivate; in astruct, members are by defaultpublic(§16.2.4).
在 a 中
class，成员是默认的private；在 a 中struct，成员默认为public（第 16.2.4 节）。
This also holds for members inherited without access level specifier.
这也适用于没有访问级别说明符继承的成员。
A widespread convention is, to use struct only for organization of pure data members. You correctly used a classto model and implement object behaviour.
一个普遍的约定是，仅将 struct 用于组织纯数据成员。您正确使用 aclass来建模和实现对象行为。
回答by liaK
From standard docs, 11.2.2
来自标准文档，11.2.2
In the absence of an access-specifier for a base class, publicis assumed when the derived class is defined with the class-keystructand privateis assumed when the class is defined with the class-keyclass.
在没有访问说明符为基类的，公共当派生的类与定义的假定 的类键结构和私人当类与定义的假定的类键类。
So, for structs the default is publicand for classes, the default is private...
因此，对于structs，默认值为public，对于classes，默认值为private...
Examples from the standard docs itself,
标准文档本身的示例，
class D3 : B { / ... / }; // B private by default
class D3 : B { / ... / }; // B private by default
struct D6 : B { / ... / }; // B public by default
struct D6 : B { / ... / }; // B public by default
回答by Prasoon Saurav
The default inheritance level (in absence of an access-specifier for a base class )for classin C++ is private. [For structit is public]
classC++ 中的默认继承级别（在没有基类的访问说明符的情况下）是私有的。[因为struct它是公开的]
class Derived:Base
class Derived:Base
Baseis privately inherited so you cannot do sth.Base::Do();inside main()because Base::Do()is private inside Derived
Base是私人继承的，所以你不能在sth.Base::Do();里面做，main()因为里面Base::Do()是私有的Derived
回答by Dev
The default type of the inheritance is private. In your code,
继承的默认类型是私有的。在您的代码中，
class B:A
B级：A
is nothing but
无非是
class B: private A
B类：私人A
