Question

For simplicity,

class Parent {}
class Child1 : Parent {}
class Child2 : Parent {}

Elsewhere, I created instances of Child1 and Child2 and store it in same vector under Parent:

// . . . in .h file, for example
vector<Parent> vector_of_parent;

// . . . in one particular method
Child1 c1; 
Child2 c2; 
vector_of_parent.push_back(c1);
vector_of_parent.push_back(c2);
// . . .

Then in another method which has access to vector_of_parent, I tried

 void doSomething(Parent& some_child) { 
 // wrapped in a try block somehow...
 Child1& c = dynamic_cast<Child1&> some_child;
 // do something if the cast is successful
 }

 void otherMethod() {
      doSomething(vector_of_parent.at(0)); // vector_of_parent.at(0) is a Child1
 }

Why is there a std:bad_cast when I call otherMethod()?

Answer 1

Your std::vector is declared as std::vector<Parent>. It holds only instances of Parent – when you insert the Child1 and Child2 instances, they get sliced.

If you want to use a vector of polymorphic objects with a common base class of Parent, you need to use a container of pointers (or, for ease of lifetime and memory management, smart pointers).

Appropriate container types to consider include std::vector<Parent*>, std::vector<std::tr1::shared_ptr<Parent> > and boost::ptr_vector<Parent>.

I’d recommend against the std::vector<Parent*> unless you’re very comfortable with manual memory management.

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Also, you need to use public inheritance instead of private, and the base class must have a virtual destructor. I assume you left these out for brevity though.