SFML奇怪的射击错误

Question

I'm making a game in C++ and SFML. I have a weird bug, whenever I have exactly 1 enemy and 3 shots on the screen and a bullet collides with the enemy, the game crashes. The error I get is:

Expression: vector subscript out of range

This is how I check for collision between shots and enemies:

for (int i = 0; i < enemies.size(); i++)
{
    for (int s = 0; s < shots.size(); s++) {
        if (Collision::PixelPerfectTest(enemies[i].getSprite(), shots[s].getSprite())) {
            enemies[i].setHealth(enemies[i].getHealth() - player.getDamage());
            if (enemies[i].getHealth() <= 0) {
                enemies.erase(enemies.begin() + i);
            }
            shots.erase(shots.begin() + s);
        }
    }
}

"enemies" and "shots" are vectors that I insert into every x seconds in the main loop. Here's the code for that:

    if (enemySpawner.getElapsedTime().asSeconds() >= 1.5f) {
        enemies.push_back(Enemy(spriteManager.enemySprite));
        std::cout << enemies.size() << " enemies" << std::endl;
        enemySpawner.restart();
    }

    if (shotSpawner.getElapsedTime().asSeconds() >= 0.3f &&      sf::Mouse::isButtonPressed(sf::Mouse::Left)) {
        shots.push_back(Shot(spriteManager.shotSprite, player.getPosition(), *window));
        std::cout << shots.size() << " shots" << std::endl;
        shotSpawner.restart();
    }

As I said, this only happens when I have 1 enemy and 3 shots, otherwise it seems to work fine.

EDIT: Now the game freezes when the first enemy spawns. Here's the updated code:

//Shot vs enemy
for (auto eit = enemies.begin(); eit != enemies.end();)
{
    for (auto sit = shots.begin(); sit != shots.end();) {
        if (Collision::PixelPerfectTest((*eit).getSprite(), (*sit).getSprite())) {
            (*eit).setHealth((*eit).getHealth() - player.getDamage());
            if ((*eit).getHealth() <= 0) {
                enemies.erase(eit);
                shots.erase(sit);
            }
            shots.erase(sit);
            eit = eit++;
            sit = sit++;
        }
    }
}

Answer 1

you can't just erase stuff from vectors while iterating over them the way you are doing because it will yield i values that won't be in the range of the vector anymore (after its size got decreased by the call to erase), which will basically result in trying to index an array value that does not exist anymore. If you need that behavior, it's a good idea to use iterators:

auto eit = enemies.begin();
for (; eit != enemies.end(); )
{
    if((*eit).isDead())
    {
         eit = enemies.erase(eit);
    }
    else
    {
         eit++;
    }
}

you would do the same for the shot array.

Answer 2

@moka's answer explains the core problem with your code, and his/her code using iterators will fix your problem. However, there is another way to deal with the problem: splitting your code into a "computation pass" and an "array modification pass." The code would look like:

for (int i = 0; i < enemies.size(); i++)
{
    for (int s = 0; s < shots.size(); s++) {
        if (!enemies[i].alive || !shots[s].active) {
            continue;
        }
        if (Collision::PixelPerfectTest(enemies[i].sprite(), shots[s].sprite())) {
            shots[s].active = false;
            enemies[i].setHealth(enemies[i].getHealth() - player.getDamage());
            if (enemies[i].getHealth() <= 0) {
                enemies[i].alive = false;
            }
        }
    }
}

enemies.erase(std::remove_if(enemies.begin(), enemies.end(),
    [](Enemy const &e) { return !e.alive; }), enemies.end());

shots.erase(std::remove_if(shots.begin(), shots.end(),
    [](Shot const &s) { return !s.active; }), shots.end());

This style is also good because it separates the game logic from the implementation a bit more. For example, if you want to allow a shot to damage more than one enemy, you would have to use an approach like this for the shots array anyway.

Chapter on Double Buffering in the excellent Game Programming Patterns book goes into more depth.