“向量迭代器不可递增”错误

Question

I added coin system to my cocos2d-x c++ game. I have a problem. I want to auto release coin groups when they go out of sight. I mean, if a coin group slides through the scene and exits the scene, i want to destroy it. How to do that?

Like this:

for (auto gr : coinGroups) {
    gr->setPositionX(gr->getPositionX() - scrollSpeed);

    if(gr->getBoundingBox().getMaxX() < origin.x) {
        this->removeChild(gr);
        coinGroups.erase(coinGroups.begin());
    }
} 

coinGroups is a

std::vector<CoinGroup*>

But this gives me error (“vector iterator not incrementable” bla bla bla).

How can I solve this error? Thanks.

Answer 1

You can not delete elements from a container where you traverse through, because this would cause an undefined behavior. Note the loop uses an internal iterator.

The following code is only to better understand the issue, but you would have to do something like that:

std::vector<CoinGroup*> tempCoins;
tempCoins.swap( coinGroups );
for (auto gr : tempCoins)
{
    .....

    if(gr->getBoundingBox().getMaxX() < origin.x)
        this->removeChild(gr);
    else
        coinGroups.push_back(gr)
}

The common solution in c++ is to use std::remove_if :

#include <algorithm> // std::remove_if

auto endIt = std::remove_if( coinGroups.begin(), coinGroups.end(), [&](CoinGroup *gr) -> bool
{
    gr->setPositionX(gr->getPositionX() - scrollSpeed);
    if (gr->getBoundingBox().getMaxX() < origin.x)
    {
        this->removeChild(gr);
        return true;
    }
    return false; 
} );
coinGroups.erase(endIt, coinGroups.end());