在 C++ 中创建动态类型

Question

I'm writing a piece of generic software that will be loaded on to many different variants of the same basic hardware. They all have the same processor, but with different peripherals and their own functions that need to be carried out. The software will know which variant it should run by reading a hardware switch value.

Here's my current implementation in a nutshell:

class MyBase
{
public:
    MyBase() { }
    virtual run() = 0;
}


class VariantA : public MyBase
{
public:
    VariantA () { }
    virtual run()
    {
        // Run code specific to hardware Variant-A
    }
}


class VariantB : public MyBase
{
public:
    VariantB () { }
    virtual run()
    {
        // Run code specific to hardware Variant-B
    }
}


void main()
{
    MyBase* variant;
    uint_8 switchValue = readSwitchValue();

    switch(switchValue)
    {
    case 0:
        variant = new VariantA();
        break;

    case 1:
        variant = new VariantB();
        break;
    }

    variant->run();
}

Now this works just fine. I read the hardware value and use a switch statement to create the new corresponding class.

The problem is that there are a lot of variants I have to deal with. Currently about 15, with the potential to add another 20-30 in the near future. I have really come to despise switch statements that run for hundreds of lines, so I'm really looking for a better way to do this, probably through templates.

I want to be able to use my hardware value to look up a type and use that type to create my new object. Ideally when I add a new variant, I create the new class, add that class type to my lookup table with it's matching hardware value, and it's good to go.

Is this possible at all? What's a good solution here?

Answer 1

As stated, you make a factory, but not necessarily with naive switch statements. What you can do is make a template class to create the relevant object and dynamically add these to your factory.

class VariantinatorBase {
  public:
    VariantinatorBase() {}
    virtual ~VariantinatorBase() {}
    virtual std::unique_ptr<Variant> Create() = 0;
};

template< class T >
class Variantinator : public VariantinatorBase {
  public:
    Variantinator() {}
    virtual ~Variantinator() {}
    virtual std::unique_ptr<Variant> Create() { return std::make_unique<T>(); }
};

Now you have a class factory that allows you to register these.

class VariantFactory
{
  public:
    VariantFactory()
    {
         // If you want, you can do all your Register() calls in here, and even
         // make the Register() function private.
    }

    template< uint8_t type, typename T >
    void Register()
    {
        Register( type, std::make_unique<Variantinator<T>>() );
    }

    std::unique_ptr<Variant> Create( uint8_t type )
    {
        TSwitchToVariant::iterator it = m_switchToVariant.find( type );
        if( it == m_switchToVariant.end() ) return nullptr;
        return it->second->Create();
    }

  private:
    void Register( uint8_t type, std::unique_ptr<VariantinatorBase>&& creator )
    {
        m_switchToVariant[type] = std::move(creator);
    }

    typedef std::map<uint8_t, std::unique_ptr<VariantinatorBase> > TSwitchToVariant;
    TSwitchToVariant m_switchToVariant;
};

At the beginning of your program, create the factory and register your types:

VariantFactory factory;
factory.Register<0, VariantA>();
factory.Register<1, VariantB>();
factory.Register<2, VariantC>();

Then later, you want to call on it:

std::unique_ptr<Variant> thing = factory.Create( switchValue );

Answer 2

You are looking for a factory

http://www.oodesign.com/factory-pattern.html

A factory is a software module (a method, a class) whose sole purpose is to create the right object for the job. An example using a factory class:

class VariantFactory
{
    MyBase* CreateObject(uint_8 value);
}

And the CreateObject method can be filled out to give you the type of object that you need.

In the case of a very small selection of objects with simple construction, a simple switch statement might suffice. As soon as you get a lot of objects or ones that require more detailed construction, a factory is quite useful.

Answer 3

I made this a comment; let's turn it into an answer:

Personally, I think a "switch/case" block to create the appropriate class is probably an optimal solution. Just put your case statement in a static "factory" method that returns a reference to the specific class. IMHO...

Here's a good example: factory method design pattern

Class Book : public Product
{
};

class Computer : public Product
{
};

class ProductFactory
{
public:
  virtual Product* Make(int type)
  {
    switch (type)
    {
      case 0:
        return new Book();
      case 1:
        return new Computer();
        [...]
    }
  }
}

Call it like this:

ProductFactory factory = ....;
Product* p1 = factory.Make(0); // p1 is a Book*
Product* p2 = factory.Make(1); // p2 is a Computer*
// remember to delete p1 and p2

Note that in his most excellent response, smink also suggests some other design alternatives, too.

BOTTOM LINE: There's nothing inherently "wrong" with a switch/case block. Even for a switch with many case options.

IMHO...

PS: This really isn't creating a "dynamic type". Rather, it's "creating a static type dynamically". That would be equally true if you used a template or an enum solution as well. But again - I vastly prefer the "switch/case".

Answer 4

Update: I am leaving my original solution here for posterity, but consider the solution provided by paddy to be superior and less error prone. With only a couple of slight improvements I think it's actually about as good as you can possibly get.

---

Consider this design:

class VariantA : public MyBase
{
    static MyBase *CreateMachineInstance() { return new VariantA; }
};

class VariantB : public MyBase
{
    static MyBase *CreateMachineInstance() { return new VariantB; }
};

Now, all you need is an std::map that uses a uint_8 as the key and maps it to a function pointer (returning MyBase ). Insert the identifiers in the map (pointing each to the appropriate machine creation function) and then read the code and just use the map to find what machine you're using.

This is loosely based on a concept/pattern called a "factory" but may break slightly if your machine constructors require different arguments or you need to perform additional per-machine initialization/operations - and from what you mention it sounds like you might.

If that's the case, you can still use this pattern but you will have to make some tweaks and rearchitect things a bit but you will end up with something much cleaner and easier to augment and maintain.

Answer 5

#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;

template<class T,class T1>
class HeroHonda
{
private:
    T millage;
    T1 *options;

public:
    HeroHonda() {
        puts("constructed");
        options=new T1[20];

        strcpy(options,"Good millage,Powerstart");
        millage=110;
    }

    virtual T features() {
        cout<<options<<"millage is"<<millage<<endl;
        return 1;
    }

    // virtual T Extrafeatures() = 0;

    ~HeroHonda() {
      cout<<"destructor"<<endl;
      delete [] options;    
    }
};

int main()
{
    HeroHonda <int,char> *Ptr=new HeroHonda <int,char>;
    Ptr->features();
    delete Ptr;
}