C++的纯虚函数实现和头文件
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C++'s pure virtual function implementation and header files
提问by Neo
I'm having some trouble implementing pure virtual functions inherited from some abstract class, when the classes in question are divided into *.hand *.cppfiles. The compiler (g++) tells me that the derived class cannot be instantiated because of the existence of pure functions.
当有问题的类被分为*.h和*.cpp文件时，我在实现从某个抽象类继承的纯虚函数时遇到了一些麻烦。编译器( g++) 告诉我，由于纯函数的存在，无法实例化派生类。
/** interface.h**/
namespace ns
{
class Interface {
public:
virtual void method()=0;
}
}
/** interface.cpp**/
namespace ns
{
//Interface::method()() //not implemented here
}
/** derived.h **/
namespace ns
{
class Derived : public Interface {
//note - see below
}
}
/** derived.cpp **/
namespace ns
{
void Derived::Interface::method() { /*doSomething*/ }
}
/** main.cpp **/
using namespace ns;
int main()
{
Interface* instance = new Derived; //compiler error
}
Does this mean that I have to declarethe method() twice - in the Interface's *.hand in the derived.htoo? Is there no other way around?
这是否意味着我必须两次声明method() - 在接口*.h和derived.h太？没有其他办法了吗？
回答by Lightness Races in Orbit
You forgot to declare Derived::method().
你忘记申报了Derived::method()。
You tried to define it at least, but wrote Derived::Interface::method()rather than Derived::method(), but you did not even attempt to declare it. Therefore it doesn't exist.
您至少尝试定义它，但写入Derived::Interface::method()而不是Derived::method()，但您甚至没有尝试声明它。因此它不存在。
Therefore, Derivedhas no method(), therefore the pure virtual function method()from Interfacewas not overridden... and therefore, Derivedis also pure virtual and cannot be instantiated.
因此，Derivedhas no method()，因此纯虚函数method()fromInterface没有被覆盖......因此，Derived也是纯虚函数，不能被实例化。
Also, public void method()=0;is not valid C++; it looks more like Java. Pure virtual member functions have to actually be virtual, but you did not write virtual. And access specifiers are followed by a colon:
此外，public void method()=0;不是有效的 C++；它看起来更像Java。纯虚成员函数实际上必须是虚的，但您没有编写virtual. 访问说明符后跟一个冒号：
public:
virtual void method() = 0;
回答by robert
You have to declare your method in the subclass.
您必须在子类中声明您的方法。
// interface.hpp
class Interface {
public:
virtual void method()=0;
}
// derived.hpp
class Derived : public Interface {
public:
void method();
}
// derived.cpp
void
Derived::method()
{
// do something
}
