How to use an abstract class as the type of the index variable of a for each loop?

Question

I was translating a Java program of mine to C++. I got to a problem trying to use polymorphism the same way as in Java.

My code looks something like this:


class Base
{
public:
    virtual void print() = 0;
};

class Derived_1 : public Base
{
public:
    void print()
    {
        std::cout << "1" << std::endl;
    }
};

class Derived_2 : public Base
{
public:
    void print()
    {
        std::cout << "2" << std::endl;
    }
};

The next are two versions of my main method that I have tried, both give me compiler errors:

1:

int main(int argc, char const *argv[])
{
    std::vector<Base> v;
    
    v.push_back(Derived_1());
    v.push_back(Derived_2());

    for(Base i: v)
    {
        i.print();
    }

    return 0;
}

Error:

object of abstract class type "Base" is not allowed:C/C++(322)
main.cpp(35, 14): function "Base::print" is a pure virtual function

2:

int main(int argc, char const *argv[])
{
    std::vector<Base*> v;
    
    v.push_back(new Derived_1());
    v.push_back(new Derived_2());

    for(Base* i: v)
    {
        i.print();
    }

    return 0;
}

Error:

expression must have class type but it has type "Base *"C/C++(153)

How would you guys solve this?

In the first case, see [What is object slicing?](https://stackoverflow.com/questions/274626/what-is-object-slicing). In the second case, `i` is a pointer, so use `->` instead of `.` to access members. Java doesn't make that distinction. — Remy Lebeau, May 26 '22 at 08:58

score 6 · Accepted Answer · edited May 26 '22 at 09:04

Coming from java to c++, there are a lot to take-care; Especially memory management!

In your first shown case, you are slicing the objects. See What is object slicing?

In order to access the virtual functions, you should have used std::vector<Base*> (i.e. vector of pointer to Base), or std::vector<std::unique_ptr<Base>> or std::vector<std::shared_ptr<Base>>(i.e. vector of smart pointer to Base).

In your second shown code, your v is a vector of pointer to Base, meaning you need to access the members via the -> operator.

i.e. You should have accessed the print() as follows:

i->print();
^^^

Or alternatively:

(*i).print();
^^^^^

Also note that:

In your second case, whatever you newed must be deleted, so that the program doesn't leak memory. Alternately, we have smart memory management
The Base is missing a virtual destructor, which will lead to undefined behavior when deleteing the ovjects. You should add one for defined behavior. See: When to use virtual destructors?

Using std::unique_ptr, your code will look like (example code):

#include <memory> // std::unique_ptr, std::make_unique

class Base
{
public:
    virtual void print() /* const */ = 0;
    // add a virtual destructor to the Base
    virtual ~Base() = default;
};

class Derived_1 : public Base
{
public:
    void print() override /* const */ {
        std::cout << "1" << std::endl;
    }
};

class Derived_2 : public Base
{
public:
    void print() override /* const */ {
        std::cout << "2" << std::endl;
    }
};

int main()
{
    std::vector<std::unique_ptr<Base>> v;
    v.reserve(2); // for unwanted re-allocations of memory
    v.emplace_back(std::make_unique<Derived_1>());
    v.emplace_back(std::make_unique<Derived_2>());

    for (const auto& i : v) // auto == std::unique_ptr<Base>
    {
        i->print();
    }
    return 0;
}

How to use an abstract class as the type of the index variable of a for each loop?

1 Answers1