我想要一个派生类指针的向量作为基类指针 [英] I want a vector of derived class pointers as base class pointers

问题描述

在C ++中，vector类存储一个对象数组。在这种情况下，我存储指向派生类对象（Dogs）的指针。在某些时候，我想将此向量视为指向基类（动物）对象的指针。这是正确/无争议的方式吗？为什么我不能这样做？

In C++, the vector class stores an array of objects. In this case, I am storing pointers to derived class objects (Dogs). At some point, I want to treat this vector as pointers to objects of the base class (Animals). This is the "right"/non controversial way right? Why can't I do this?

#include <vector>
using namespace std;

class Animal { }; 
class Dog : public Animal { };

int main(int argc, char *argv[]) {
    vector<Dog*> dogs;
    dogs.push_back(new Dog());
    dogs.push_back(new Dog());
    vector<Animal*> animals = dogs; // This doesn't seem to work.

    // This is really what I want to do...
    vector<Animal*> all_animals[] = {dogs, cats, birds};
}

错误：

Untitled.cpp:11:18: error: no viable conversion from 'vector<class Dog *>' to 'vector<class Animal *>'
    vector<Animal*> animals = dogs;
                    ^         ~~~~
/usr/include/c++/4.2.1/bits/stl_vector.h:231:7: note: candidate constructor not viable: no known conversion from 'vector<Dog *>' to 'const std::vector<Animal *, std::allocator<Animal *> > &' for 1st argument
  vector(const vector& __x)
  ^

推荐答案

std :: vector 有一个复制构造函数，但它要求您复制完全相同类型的向量。幸运的是，还有另一个构造函数，它接受一对迭代器并添加范围内的所有元素，因此您可以这样做：

There is a copy constructor for a std::vector but it requires you to copy the exact same type of vector. Fortunately, there is another constructor which takes a pair of iterators and adds all the elements in the range, so you can do this:

vector<Animal*> animals(dogs.begin(),dogs.end());

这会创建一个 Animal 指针的新向量通过迭代每个 Dog 指针。每个 Dog 指针都会转换为 Animal 指针。

This creates a new vector of Animal pointers by iterating through each Dog pointer. Each Dog pointer is converted to an Animal pointer as it goes.

这是一个更完整的例子（使用C ++ 11）：

Here is a more complete example (using C++11):

#include <vector>

struct Animal { };

struct Dog : Animal { };

struct Cat : Animal { };

struct Bird : Animal { };

int main(int,char**)
{
  Dog dog1, dog2;
  Cat cat1, cat2;
  Bird bird1, bird2;
  std::vector<Dog *> dogs = {&dog1,&dog2};
  std::vector<Cat *> cats = {&cat1,&cat2};
  std::vector<Bird *> birds = {&bird1,&bird2};
  std::vector<std::vector<Animal *>> all_animals = {
    {dogs.begin(),dogs.end()},
    {cats.begin(),cats.end()},
    {birds.begin(),birds.end()}
  };
}

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