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Wrong member function. Is this the slice problem?

This is a discussion on Wrong member function. Is this the slice problem? within the C++ Programming forums, part of the General Programming Boards category; Hello. I am having a problem with a program. I have a bunch of classes all derived from the same ...

  1. #1
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    Wrong member function. Is this the slice problem?

    Hello. I am having a problem with a program. I have a bunch of classes all derived from the same base class. I want to loop through a vector of objects, calling a function in each. The problem is that it doesn't matter which class the objects are, only the function defined in the base class is called.

    I simplified the code as far as possible to replicate the problem. As you see, I would like a mix of numbers 1,2,3 as the output, however using the vector the only number output is 1. Here is a copy of the output by the way:

    base->num() : 1
    a->num() : 2
    b->num() : 3
    (*it)->num() : 1
    (*it)->num() : 1
    (*it)->num() : 1

    I suspect this is the "slice" problem, because the vector is defined with pointers to the base class so it uses the base class functions? The question is how to get around it? How can I loop through a vector of objects sharing the same base class but calling each by their correct member functions?

    I am thankful for any help.

    Code:
    #include <iostream>
    #include <vector>
    
    class Base
    {
    public:
        int num() { return 1;}
    };
    
    class HeirA : public Base
    {
    public:
        int num() { return 2;}
    };
    
    class HeirB : public Base
    {
    public:
        int num() { return 3;}
    };
    
    int main(int argc, char** argv)
    {
        Base* base = new Base();
        HeirA* a = new HeirA();
        HeirB* b = new HeirB();
    
        // Explicitly specifying each object.
        std::cout << "base->num() : " << base->num() << std::endl;
        std::cout << "a->num() : " << a->num() << std::endl;
        std::cout << "b->num() : " << b->num() << std::endl;
    
        // Looping through a vector.
        std::vector<Base*> list;
        list.push_back(base);
        list.push_back(a);
        list.push_back(b);
        std::vector<Base*>::iterator it = list.begin();
        while (it != list.end())
        {
            std::cout << "(*it)->num() : " << (*it)->num() << std::endl;
            ++it;
        }
    }

  2. #2
    Kiss the monkey. CodeMonkey's Avatar
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    902
    It looks like you want to declare Base::num() virtual. This way, regardless of whether you call num() via a base pointer/reference or a derived base/reference, it will use the most derived implementation at runtime.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  3. #3
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    6,157
    This is not slicing.

    The code that "explicitly specifies each object" is demonstrating effects of the hiding rule: in the context of both derived classes, Base::num() is hidden. The code that is iterating over the vector is using static dispatch (always calling the Base::num()) since that function is non-virtual.
    Right 98% of the time, and don't care about the other 3%.

  4. #4
    ZuK
    ZuK is offline
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    This would cause slicing
    Code:
    #include <iostream>
    #include <vector>
     
    class Base
    {
    public:
        virtual int num() { return 1;}
    };
     
    class HeirA : public Base
    {
    public:
        virtual int num() { return 2;}
    };
     
    class HeirB : public Base
    {
    public:
        virtual int num() { return 3;}
    };
     
    int main(int argc, char** argv)
    {
        Base base;
        HeirA a;
        HeirB b;
     
        // Looping through a vector.
        std::vector<Base> list;
        list.push_back(base);
        list.push_back(a);
        list.push_back(b);
        std::vector<Base>::iterator it = list.begin();
        while (it != list.end())
        {
            std::cout << "(*it).num() : " << (*it).num() << std::endl;
            ++it;
        }
    }
    Kurt

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
    Join Date
    Oct 2007
    Posts
    22,411
    You also want to look at smart pointers, std::unique_ptr and std::shared_ptr especially.
    Elkvis likes this.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    Registered User
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    Mar 2012
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    Ah, virtual! I had completely forgotten about that. Thank you all.

    Quote Originally Posted by Elysia View Post
    You also want to look at smart pointers, std::unique_ptr and std::shared_ptr especially.
    Yes, I suppose I should do that.

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