Question

我有一个B类，它来自A：

template<class T>
class A
{
    class iterator; // Defined fully

    iterator begin ()
    {
        // Returns a pointer to the first element
    }
    iterator end ()
    {   
        // Returns a pointer to the last element
    }
}
template <class T>
class B : public A
{
    // It automatically inherits the iterator
}

template <typename InputIterator>
void foo (InputIterator first,InputIterator last)
{
    // Some code to infer whether it is of type A or B
}

现在有些函数说foo()一次使用B::begin()，有时使用A::begin()调用。

我需要在运行时确定类型以推断类型并设置一些标志变量。我该怎么做呢？我尝试使用typeinfo()，但它为两个迭代器返回相同的值。

Answer 1

From library type_traits you can use some type magic:
is_base_of - returns true if Base is base of Derived.
is_same - returns true if A is the same type as B.
Everything with type_traits can be found here http://www.cplusplus.com/reference/type_traits/?kw=type_traits

They are not so runtime, it's only some magic with structs and templates, C++ does not support type as data by default. If you want so you can use Boost library, it does support types as I know.

UPD:
As comments under the question mentioned A::iterator is absolutely the same with B::iterator, so without looking at classes they are the same memory chunk.
So solution (maybe) is to create a little different function, what depends on classes actually:

 template <typename LeftClass, typename RightClass>
 void foo (LeftClass left, RightClass right)
 { 
     if(is_same<LeftClass, RightClass>::value)
     {

     }
 //Or that
     if(is_same<LeftClass, A>::value && is_same<RightClass, A>::value)
 }

Just don't forget to make this "friend" with classes.

Answer 2

typeinfo()返回相同的值，因为A::begin()和B::begin()都会为您提供相同类型的值。

你应该让B::iterator继承自A::iterator，或者在你的迭代器中有一个特殊的函数，它返回一个引用它的容器的引用/指针（它的类型为{{1}或者输入A）。

如何在C ++运行时确定数据类型？

2 个答案: