无法将一个类推入for循环内的对象向量中

Question

I cannot call a function that does a push_back into a vector

void GameState::InitialiseBullet(float x, float y, float vx, float vy)
{
    Bullet* bullets = new Bullet();
    bullets->SetSize(5.f, 20.f);
    bullets->AddFrame("./images/bullet.png");
    bullets->Play();
    bullets->SetX(x);
    bullets->SetY(y);
    bullets->velocityX = vx;
    bullets->velocityY = vy;

    bullets->isActive = true;
    gameObjects.push_back(bullets);
}

when it is inside the following for loop

for (auto& object : gameObjects)
{
    //Determine the type at runtime
    if (dynamic_cast<Player*>(object) != 0)
    {
        //Process player-specific logic
        PlayerLogic(dynamic_cast<Player*>(object), a_fTimeStep);
    }

//Determine the type at runtime
if (dynamic_cast<Bullet*>(object) != 0)
{
    //Process bullet-specific logic
    BulletLogic(dynamic_cast<Bullet*>(object), a_fTimeStep);
}
if (dynamic_cast<Enemy*>(object) != 0)
{
    //Process enemy-specific logic
    Enemy* enemy = dynamic_cast<Enemy*>(object);
    EnemyLogic(enemy, lowerAliens);
    if (enemy->GetIsActive() == true)
    {
        allDead = false;
    }
}

//Update and draw our objects
object->Update(a_fTimeStep);
object->Draw();
}

The piece of code that calls the function:

if (createBullet == true)
{
    InitialiseBullet(bulletX, bulletY, 0, 500);
    createBullet = false;
}

That code works when outside the for loop. However, I need the for loop to provide access to each of my player, enemy and bullet objects. Is there a way to push_back to a vector inside a for loop that is based on the same vector? I get a "Expression: Vector iterators incompatible" error when it's inside the loop. Any ideas? New to C++ programming.

Answer 1

It looks like you are pushing into the same vector you are iterating, that means, you are forcing items realocation and iterator invalidation; in other words - your data moves to different location and used iterator becomes invalid.

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I rarely see situation where you really need to iterate and append same vector, so take a look into your code again.

If you really need to do that, iterate this way:

for (size_t i = 0; i < gameObjects.size(); ++i)
{/*Some code*/}

Also using this method you should use gameObjects[i]. instead of it->

Answer 2

It's just a vector of pointers, so it's not very big.

The objects being added is probably even smaller.

You could make a copy of the vector and iterate over the copy while inserting into the real one.

You could put new items into a new, empty vector while you iterate, and then splice them onto the real one at the end.

To delete objects, you could do either of those things, or you could simply set a flag "isZombie" and then remove all the zombies at the end.

These aren't the only answers, but they all work.

Answer 3

When using iterators to loop through your vector you can't in this 'for-loop' modify the vector.

A quick google gave me this; which seemd to fit your case pretty well.

Probably because the push_back ... caused an internal reallocation in the vector thus all its iterators were invalidated.

Source: http://www.cplusplus.com/forum/beginner/64854/

Do I understand you right when I'm assuming your using iterators due to your error message.

One question you should ask yourself is why you would ever want to add instances to this vector, maybe you should rethink your design slightly to avoid this.