如何使用 C++ 中的非虚拟接口习语来实现接口类? [英] How to implement an interface class using the non-virtual interface idiom in C++?

问题描述

在 C++ 中，接口可以由其方法是纯虚拟的类来实现.

In C++ an interface can be implemented by a class whose methods are pure virtual.

这样的类可以是库的一部分，用于描述对象应该实现哪些方法才能与库中的其他类一起工作:

Such a class could be part of a library to describe what methods an object should implement to be able to work with other classes in the library:

class Lib::IFoo
{
    public:
        virtual void method() = 0;
};

:

class Lib::Bar
{
    public:
        void stuff( Lib::IFoo & );
};

现在我想使用类Lib::Bar，所以我必须实现IFoo接口.

Now I want to to use class Lib::Bar, so I have to implement the IFoo interface.

出于我的目的，我需要一整套相关的类，所以我想使用一个基类来保证使用 NVI 习惯用法的常见行为:

For my purposes I need a whole of related classes so I would like to work with a base class that guarantees common behavior using the NVI idiom:

class FooBase : public IFoo // implement interface IFoo
{
    public:
        void method(); // calls methodImpl;

    private:
        virtual void methodImpl();
};

非虚拟接口 (NVI) 习惯用法应该拒绝派生类覆盖在 FooBase::method() 中实现的常见行为的可能性，但是由于 IFoo使它成为虚拟似乎所有派生类都有机会覆盖 FooBase::method().

The non-virtual interface (NVI) idiom ought to deny derived classes the possibility of overriding the common behavior implemented in FooBase::method(), but since IFoo made it virtual it seems that all derived classes have the opportunity to override the FooBase::method().

如果我想使用 NVI 习语，除了已经建议的 pImpl 习语，我还有什么选择(感谢 space-c0wb0y).

If I want to use the NVI idiom, what are my options other than the pImpl idiom already suggested (thanks space-c0wb0y).

推荐答案

我认为您的 NVI 模式有误:http://en.wikibooks.org/wiki/More_C%2B%2B_惯用语/非虚拟_接口

I think you've got your NVI pattern around the wrong way: http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Non-Virtual_Interface

不确定这是否能解决您的问题.

Not sure if that solves your issue though.

class IFoo
{
    public:
       void method() { methodImpl(); }
    private:
       virtual void methodImpl()=0;
};

class FooBase : public IFoo // implement interface IFoo
{
    private:
        virtual void methodImpl();
};

这里有一个示例，说明为什么您可以使用从 XML 读取的读取器和从 DB 读取的另一个读取器执行此操作.请注意，公共结构移入 NVI readFromSource，而非公共行为移入私有虚拟 getRawDatum.这种方式只需要在一个函数中进行日志记录和错误检查.

Here's an example of why you might do this using a reader that reads from XML and another from DB. Note that common structure is moved into the NVI readFromSource, while non-common behaviour is moved into the private virtual getRawDatum. This way logging and error checking is only needed in the one function.

class IReader
{
  public:
    // NVI
    Datum readFromSource()
    {
       Datum datum = getRawDatum();
       if( ! datum.isValid() ) throw ReaderError("Unable to get valid datum");
       logger::log("Datum Read");
       return datum;
    }
  private:
    // Virtual Bits
    Datum getRawDatum()=0;
};

class DBReader : public IReader
{
  private:
    Datum getRawDatum() { ... }
};

class XmlReader : public IReader
{
   private:
     Datum getRawDatum() { ... }
};

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