从基类指针到派生类指针的static_cast无效

Question

I am creating a simple test entity-component system. I have a base Component class with several derived classes. I then have several systems that apply some logic to these components.

// Component.h
// ------------
class Component
{
public:
    Component();
    ~Component();
}


// ControlComponent.h
// -------------------
#include <string>
#include "Component.h"

class ControlComponent : public Component
{
public:
    std::string input = ""; // store simple input instruction
    ControlComponent();
    ~ControlComponent();
};


// ControlSystem.cpp
void ControlSystem::update(Entity* entity)
{
    vector<Component*>* components = entity->getComponents();

    for (Component* component : *components)
    {
        PositionComponent* pc = static_cast<PositionComponent*>(component);
        ControlComponent* cc = static_cast<ControlComponent*>(component);

        if (pc != nullptr && cc != nullptr)
        {
            std::cout << "Which direction would you like to go?" << std::endl;
            std::string input;
            std::cin >> input;
            cc->input = input; // application breaks here

            // Apply some logic...
        }
    }
}

When I static_cast from base Component* to either of the derived components ( PositionComponent* or ControlComponent* ) and when both results are not nullptr (ie the cast was successful), I get invalid values, like cc->input not being able to read characters from string etc.

I wire up the components in my entity factory, like this:

void EntityFactory::wireUpPlayer(Entity* player)
{
    player->addComponent(new HealthComponent());
    player->addComponent(new ControlComponent());
    player->addComponent(new PositionComponent());
}

And the implementation for addComponent is as follows:

void Entity::addComponent(Component* component)
{
    m_components.push_back(component);
}

These components are shown to have valid memory addresses, so I'm not sure where the issue is coming from.

Answer 1

static_cast does not check validity at runtime; if the cast compiled, it assumes at runtime that the conversion is okay. If you aren't casting a null pointer, the result of a static_cast will not be a null pointer. To get a checked cast you need dynamic_cast and that, in turn, requires the pointer being converted to point to a polymorphic type, ie, one that has at least one virtual function. That means changing Component to have at least one virtual function.

Answer 2

When I static_cast from base Component* to either of the derived components ( PositionComponent* or ControlComponent* ) and when both results are not nullptr (ie the cast was successful)...

When casting from a base class to a derived class, static_cast is telling the compiler, "Trust me, I know what I'm doing." In other words, if it's even potentially legal, it will "succeed" and return non- nullptr . If, at runtime, it's not legal, you'll get undefined behavior, from trying to use an instance of one class as if it were of another class.

Use dynamic_cast instead.

Answer 3

As Pete Becker and Josh Kelley said, use dynamic_cast and I believe you also need to set at least one function as virtual . If you do not, the compiler will not record the inheritance and dynamic_cast will likely still return nullptr . When performing inheritance, I suggest making the class destructors virtual. This is also good practice when unmanaged resources need to be disposed of in the destructor of a derived class and you only have a pointer to the base class, the derived class destructor will only be called so long as the destructors are virtual. There was a post that explained it here: When to use virtual destructors?