I want a vector of derived class pointers as base class pointers

Question

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}; } 

The error:

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)   ^ 

Answer 1

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()); 

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.

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()}   }; }