3

我想请教您有关此代码的一些建议。它有效，但我认为它可以用更优雅的方式编写。这是一段 C++11 代码，所以在编译时请记住它；）！

#include <iostream>
#include <type_traits>
#include <typeinfo>

using namespace std;

class A {};
class B: public A {};

class C {};
class D: public C {};

class E: public A, public C {};

template<class T, typename = void>
class Example;

template<class T>
class Example<T, typename enable_if<is_base_of<A, T>::value and not is_base_of<C, T>::value>::type>
{
  public:
    string a() const
    {
      return string(typeid(T).name()) + " have A as base class";
    }
};

template<class T>
class Example<T, typename enable_if<not is_base_of<A, T>::value and is_base_of<C, T>::value>::type>
{
  public:
    string b() const
    {
      return string(typeid(T).name()) + " have C as base class";
    }
};

template<class T>
class Example<T, typename enable_if<is_base_of<A, T>::value and is_base_of<C, T>::value>::type> :
    public Example<A>,
    public Example<C>
{
};

int
main()
{
  Example<B> example1;
  Example<D> example2;
  Example<E> example3;

  cout << example1.a() << endl;
  //cout << example1.b() << endl;   It must raise a compile error
  //cout << example2.a() << endl;   It must raise a compile error
  cout << example2.b() << endl;
  cout << example3.a() << endl;
  cout << example3.b() << endl;
}

如您所见，我正在尝试编写一个可以处理从A和C派生的类的类模板。问题是当A和C像类E一样被继承时。事实上，我们也可以有这样的东西......

template<class T>
class Example<T, typename enable_if<is_base_of<A, T>::value> { /* ... */ };

template<class T>
class Example<T, typename enable_if<is_base_of<C, T>::value> { /* ... */ };

...但是当一个类（如E）继承A和C时，它将失败。

有更好的代码的想法吗？谢谢

4

1 回答 1

4

更简单的方法是使用static_assert.

template <typename T>
class Example
{
public:
    std::string a() const
    {
        static_assert(std::is_base_of<A, T>::value, "T must derive from A to use a()");
        return std::string(typeid(T).name()) + " have A as base class";
    }

    std::string b() const
    {
        static_assert(std::is_base_of<C, T>::value, "T must derive from C to use b()");
        return std::string(typeid(T).name()) + " have C as base class";
    }
};
于 2012-03-21T08:03:17.990 回答