Error when printing a vector of another subclass

Question

I am learning c++ and I want to create two classes B and C that inherit from A. And create a constructor for both classes: B should receive a string in the constructor. C should receive a vector from B.

I also want to implement the print function for both classes. B should output the name and C should output all B names.

This is the base class :

class A {
    public:
        virtual void print(std::ostream& os) = 0;
        virtual ~A() = default;
};

I implemented the two subclasses and when compiling the code doesn't work and with a long error, this is just a small part of the error:

test.cpp: In member function ‘virtual void C::print(std::ostream&)’:
test.cpp:34:18: error: no match for ‘operator<<’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘__gnu_cxx::__alloc_traits<std::allocator<B>, B>::value_type’ {aka ‘B’})
   34 |               os << this->b[i];
In file included from /usr/include/c++/9/iostream:39,
                 from test.cpp:1:

Here are the two subclasses :

class B : public A{

        std::string name;
    public:
        B(std::string name) : A(), name(name){}
        void print(std::ostream& os) override {
            os << this->name;
        }

};

class C : public A{
    std::vector<B> b;
    public:
        C(std::vector<B> b) : A(), b(b){}

        void print(std::ostream& os) override {
            for(size_t i = 0; i < this->b.size(); ++i){
              os << this->b[i];
            }
        }
};


int main(){

    B b("b 1");
    B b2{"b 2"};
    B b3{"b 3"};
    B b4{"b 4"};

    std::vector<B> bs {b, b2, b3, b4};
    
    C c {bs};
    c.print(std::cout);
    return 0;
}

Answer 1

As said in the comments you have to overload the operator<<.

I would use this->b[i].print(os).

But if you want to define a operator<< for class B, you should read about the stream extraction operator. https://www.learncpp.com/cpp-tutorial/overloading-the-io-operators/

Answer 2

There is no << stream insertion operator for B .

You could write this->b[i].print(os); instead, but since you have a virtual print function, you only need to define one operator<< for the base type:

std::ostream& operator<<(std::ostream& os, const A& a)
{
    a.print(os);
    return os;
}

and then you can << anything derived from A .

Also, the print function should be marked const since you never want anything to be modified just from printing it.