“模板多态性”当调用基类型的模板参数的函数时，使用派生类型？ [英] "template polymorphism" when calling function for templated parameter of base type, with derived type?

问题描述

我有一个模板类：

  template< class T& 
 class TemplateClass 
 {
 //不相关这个类做什么
} 
  

b $ b

有两个类，一个基类和一个派生自它的类：

  class Base 
 {
 //不管什么
} 
 
类派生：public Base 
 {
} 
  pre> 
 
 
现在我想组装一个函数，它可以处理模板类型为Base *的TemplateClass以及模板类型为Derived *的TemplateClass，而无需单独实现该函数两种类型。 
 
 
 
在代码中，这是我想要做的：
  void DoSomething（const TemplateClass< Base *>& tc）
 {
 // do something 
} 
  pre> 
 
 
和代码中的其他位置：
  TemplateClass< Base *>一个; 
 TemplateClass< Derived *> b; 
 
 DoSomething（a）; // Works 
 DoSomething（b）; //不工作，但我想它工作
  
这不必手动重载DoSomething for Derived *？
解决方案
很遗憾，C ++不支持模板协方差，所以你不能这样做自动。作为解决方法，您可以这样做：
  template< typename T& 
 void DoSomething（const TemplateClass< T *>& tc）{
 //在C ++ 03中，使用BOOST_STATIC_ASSERT和boost :: is_convertible 
 static_assert（
 std： ：is_convertible< T *，Base *> :: value，
模板参数必须可转换为Base *
）; 
 
 // ... 
} 
  
 在ideone上演示。
 
I've got a template class:
template <class T>
class TemplateClass
{
   //irrelevant what this class does
}
And two classes, one base and one class which derives from it:
class Base
{
    //does whatever
}

class Derived : public Base
{
}
Now I want to assemble a function which can deal with TemplateClass of templated type Base* as well as TemplateClass of templated type Derived* without having to implement the function separately for both types. 

In code, this is what I'd like to be able to do:
void DoSomething(const TemplateClass<Base *> &tc)
{
   //do something
}
And somewhere else in the code:
TemplateClass<Base *> a;
TemplateClass<Derived *> b;

DoSomething(a); //Works
DoSomething(b); //Doesn't work, but I'd like it to work
Is there a way to implement this without having to manualy overload DoSomething for Derived* ?
解决方案
Sadly, C++ does not support template covariance, so you cannot do that automatically. As a workaround, you could do something like this:
template <typename T>
void DoSomething(const TemplateClass<T*>& tc) {
    // in C++03, use BOOST_STATIC_ASSERT and boost::is_convertible
    static_assert(
        std::is_convertible<T*, Base*>::value,
        "Template argument must be convertible to Base*"
    );

    // ...
}
Demo on ideone.

                        
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