Error with attempt at polymorphism

Question

Working on a little thing that randomly generates colored blocks. Anyway, for organization, I have each of the generators - with a method, generate() - in their own class, all of which descend from Generator. The World class holds a collection of Generator * to these, and so can be called as generators[randomIndex]->generate().

//in World.h
static std::vector<Generator *> generators;

//in World.cpp
generators.push_back(&Forest());

//Generator.h
class Generator
{
public:
    virtual void generate(sf::Color ** grid, sf::Vector2i size) = 0;
};


//Forest.h
class Forest : Generator
{
public:
        void generate(sf::Color ** grid, sf::Vector2i size);
};

The error:

'type cast' : conversion from 'Forest *' to 'Generator *' exists, but is inaccessible

Why does this occur, and how to fix it?

Kerrek SB · Accepted Answer · 2013-06-30 20:36:43Z

8

You have to inherit publicly:

class Forest : public Generator
//             ^^^^^^

answered Jun 30, 2013 at 20:36

Kerrek SB

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3 Comments

Aman Deep Gautam Over a year ago

I was just interested in knowing that is this error apt for this scennario? if yes, how?

Kerrek SB Over a year ago

@AmanDeepGautam: Yes. Only public inheritance creates an "is-a" relationship upwards.

kfsone Over a year ago

The inheritance policy sets a bar for everything being inherited and the default, as for a class definition itself, is private. So by leaving out the "public" you are saying "inherit from Generator but mark everything private". Which allows "Forest" to see everything belonging to Generator, but you cannot see it from a Forest-derived class or outside of Forest.

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