在这种情况下，如何从向量中删除项目？

Question

I get a syntax error for everything I try. Can someone please tell me what to put in place of the comment? This is my first time working with vectors.

EntityList is a static vector of the Entity class.

for(int i = 0;i < (int)Entity::EntityList.size();i++) {
    if(!Entity::EntityList[i]) continue;

    if(Entity::EntityList[i]->isDead){
        //Erase from vector
        //Decrement i?
    }

    Entity::EntityList[i]->OnLoop();
}

What should I put in place of the comment? I've tried a few things but nothing works. For example, Entity::EntityList.erase(i); doesn't work. I get the following error, which I don't understand:

cannot convert parameter 1 from 'int' to 'std::_Vector_const_iterator<_Myvec>'

All the examples I see use ints for the parameter, so I don't know what to do.

Also, since elements in the vector are shifted down after removal, should I decrement i after removing an item so it performs the loop with the same value again? Or is there a more elegant way of doing it?

Answer 1

You could try:

Entity::EntityList.erase(Entity::EntityList.begin() + i);

And yes, you should decrement i.

Answer 2

I would recommend using the Erase/Remove idiom . You can supply the results of std::remove_if to vector::erase and you should be all set. It would look something like this:

entityList.erase(
    std::remove_if(entityList.begin(), entityList.end(), isEntityDead));

Here is an example:

#include <algorithm>
#include <vector>

class Entity {
public:
    bool isDead;
};

// Need this since isDead isn't a member function of Entity
bool isEntityDead(Entity& entity) {
    return entity.isDead;
}

int main(int argc, char** argv) {
    Entity ent1 = Entity();
    ent1.isDead = false;
    Entity ent2 = Entity();
    ent2.isDead = true;
    Entity ent3 = Entity();
    ent3.isDead = false;

    std::vector<Entity> entityList;
    entityList.push_back(ent1);
    entityList.push_back(ent2);
    entityList.push_back(ent3);

    std::cout << "Before removing anything: " << entityList.size() << std::endl;

    entityList.erase(std::remove_if(entityList.begin(), entityList.end(), isEntityDead));

    std::cout << "After remove/erase: " << entityList.size() << std::endl;

    return 0;
}

I would recommend not modifying the contents of the vector while you're looping through it if you can help it. If you need to remove the dead entities at the end of your game loop then you're better off figuring out which entities are "dead", putting them in a "dead list" and then removing anything in the "dead list" from your entity list all at once.