通过模板专门化继承

Question

Recently I found a case where is was easier to do a template specialization instead of real inheritance. The derived classes just had to implement a pure virtual function and had no own members. It was something like:

#include <iostream>

class Interface {
public:
    virtual void calculate() = 0;
    virtual float getResult() = 0;
};

class Base : public Interface {
    float result;
public:
    Base() : result(1) {};
    virtual ~Base() {};

    virtual void calculate();
    virtual float getValue() = 0; // do some very complex calculation here

    float getResult() { return result; }
};

class DerivedA : public Base {
public:
    DerivedA() : Base() {};
    ~DerivedA() {};

    float getValue();
};

class DerivedB : public Base {
public:
    DerivedB() : Base() {};
    ~DerivedB() {};

    float getValue();
};

void Base::calculate() {
    for (int i = 0; i < 10; i++)
        result += getValue();
}

float DerivedA::getValue() {
    return 1;
}

float DerivedB::getValue() {
    return 1.1;
}

int main() {
    Interface * a = new DerivedA();
    a->calculate();

    Interface * b = new DerivedB();
    b->calculate();

    std::cout << "Result A: " << a->getResult() << std::endl;
    std::cout << "Result B: " << b->getResult() << std::endl;

    delete a;
    delete b;
}

This can be written as specialized templates as:

#include <iostream>

class Interface {
public:
    virtual void calculate() = 0;
    virtual float getResult() = 0;
};

template<typename T>
class Base : public Interface {
    float result;
public:
    Base() : result(1) {};

    void calculate();
    float getValue(); // do some very complex calculation here

    float getResult() { return result; };
};

typedef Base<int>   DerivedA; // actually int and float are only examples
typedef Base<float> DerivedB; // and may be some much more complex types!

template<typename T>
void Base<T>::calculate() {
    for (int i = 0; i < 10; i++)
        result += getValue();
}

template<typename T>
float Base<T>::getValue() {
    return 0;
}

template<>
float Base<int>::getValue() {
    return 1;
}

template<>
float Base<float>::getValue() {
    return 1.1;
}

int main() {
    Interface * a = new DerivedA();
    a->calculate();

    Interface * b = new DerivedB();
    b->calculate();

    std::cout << "Result A: " << a->getResult() << std::endl;
    std::cout << "Result B: " << b->getResult() << std::endl;

    delete a;
    delete b;
}

Both examples give the same results, I guess the second one is faster as no virtual tables need to be evaluated (the methods getValue() may even be inlined in the second case).

So my question is: What are the limits of using template specialization instead of inheritance? Are there any side effects I haven't seen? Any benefits of inheritance over template specialization? I know that I can't create new members and methods for specialized classes as I can do for derived. But can this be a general, more performant approach for such use cases where I only need to implement some type-specific code?

Btw: Is there a name for this pattern?

Answer 1

Templates and Inheritance are not interchangeable.

• Templates express static polymorphism (ie polymorphism at compile-time)

• Inheritance allows runtime polymorphism : you can manipulate Base class pointers and expect the runtime to call the correct virtual function for you.

Whit your templates approach, what if you want to manipulate a container of Base<> objects (eg std::vector<Base<??>> ) and call calculate() on them ? You can't.

So although both inheritance and templates express interfaces and polymorphism, they really are different beasts: choosing one over the other depend on your context, and how your types are going to be used.

Note:

Performance considerations should not alter this choice

Answer 2

When using superclasses and subclasses, either DerivedA or DerivedB can be passed to a non-templated function or a method that takes a Base class instance.

void method(Base &base)
{
    // ...
}

The major limitation of the template-based approach is that this is no longer possible. method() will also have to be a template:

template<typename T>
void method(Base<T> &base)
{
    // ...
}

If method() is large, there's going to be quite a code bloat, here.