虚函数和向量迭代器

Question

I'm having trouble with this specific piece of code: It seems like virtual functions don't work as I expected.

#include <cstdio>
#include <string>
#include <vector>

class CPolygon
{
protected:
    std::string name;
public:
    CPolygon()
    {
        this->name = "Polygon";
    }
    virtual void Print()
    {
        printf("From CPolygon: %s\n", this->name.c_str());
    }
};

class CRectangle: public CPolygon
{
public:
    CRectangle()
    {
        this->name = "Rectangle";
    }
    virtual void Print()
    {
        printf("From CRectangle: %s\n", this->name.c_str());
    }
};

class CTriangle: public CPolygon
{
public:
    CTriangle()
    {
        this->name = "Triangle";
    }
    virtual void Print()
    {
        printf("From CTriangle: %s\n", this->name.c_str());
    }
};

int main()
{
    CRectangle rect;
    CTriangle trgl;
    std::vector< CPolygon > polygons;
    polygons.push_back( rect );
    polygons.push_back( trgl );

    for (std::vector<CPolygon>::iterator it = polygons.begin() ; it != polygons.end(); ++it)
    {
        it->Print();
    }

    return 0;
}

I expected to see:

From CRectangle: Rectangle
From CTriangle: Triangle

instead I get:

From CPolygon: Rectangle
From CPolygon: Triangle

Is this expected behavior? How should I call Print() function to get output expected by me?

Answer 1

Is this expected behavior? How should I call Print() function to get output expected by me?

Yes, it is expected behavior.

The problem is that standard containers, including vector , have value semantics : they store copies of the objects you pass to push_back() . Polymorphism, on the other hand, is based on reference semantics - it requires references or pointers in order to work correctly.

What happens in your case is that your CPolygon object gets sliced , which is not what you want. You should store pointers (possibly smart pointers) in your vector rather than objects of type CPolygon .

This is how you should rewrite your main() function:

#include <memory> // For std::shared_ptr

int main()
{
    std::vector< std::shared_ptr<CPolygon> > polygons;
    polygons.push_back( std::make_shared<CRectangle>() );
    polygons.push_back( std::make_shared<CTriangle>() );

    for (auto it = polygons.begin() ; it != polygons.end(); ++it)
    {
        (*it)->Print();
    }

    return 0;
}

Here is a live example .