Iterating over map and erasing element

Question

I have a global variable as a map and a function that iterates over the elements of this map such as:

void printMap(){
    for ( auto it = MyMap.begin(); it != MyMap.end(); ++it  ){
        std::cout << it->second;
    }
}

which works fine.

I want to add a functionality to the function which is after printing an element, it should be erased from the map like this:

void printMap(){
    for ( auto it = MyMap.begin(); it != MyMap.end(); ++it  ){
        std::cout << it->second;
        MyMap.erase(it);
    }
}

However, by adding the erase line I got an exception error of this type:

Thread 2: EXC_BAD_ACCESS (code=1, address=0x20000002)

I tried another way which is like this:

void myFunction(){
    printMap(); 
    MyMap.clear();
}

but I also got the same exception

Thread 2: EXC_BAD_ACCESS (code=1, address=0x0)

As I understand this kind of exception occurs when we refer to a memory location that does not exist. But I know it is there since the iterator got its value and it was printed. Even so I used the second method just in case that I don't refer to non-existing memory location but still I'm getting the exception.

So how can I iterate over the elements print the result then erase it?

UPDATE1
following the suggestions below and the linked topic I changed my function into this:

void printMap(){
    bool i = true;
    for (auto it = MyMap.cbegin(), next_it = MyMap.cbegin(); it != MyMap.cend(); it = next_it)
    {
        cout << it->second;
        next_it = it; ++next_it;
        if (i) {
            MyMap.erase(it);
        }

    }
}

I have also tried this https://stackoverflow.com/a/42820005/7631183 and this https://stackoverflow.com/a/42819986/7631183

The problem is still not solved and I'm getting the same error

UPDATE2
I run the same exact code on a different machine and it worked fine. I still don't know what is the reason so I would guess as suggested in the comments that std::map on the first had some problems.

P.S. The first machine was a mac and the second was a Linux

Answer 1

When you erase an element out of the map, you have invalidated the iterator you are using in the loop, thus causing error. The erase method returns an iterator for this reason.

for( auto it = MyMap.begin(); it != MyMap.end(); )
{
    std::cout << it->second;
    it = MyMap.erase(it);
}

This is indeed a duplicate question. You just don't understand it well enough to see that it is a duplicate. I hope the explanation of invalidating the iterator helps clear it up.

Answer 2

In C++11 and later, std:map::erase() returns an iterator to the element following the one being erased. Since you are using auto, you are using C++11 or later, so you can use the new iterator when erasing while looping through the std::map, eg:

void printMap(){
    auto it = MyMap.cbegin();
    while (it != MyMap.cend()) {
        std::cout << it->second;
        it = MyMap.erase(it);
    }
}

Also, since you are using C++11, your first print function can be simplified using a range-for loop:

void printMap(){
    for ( auto &it: MyMap ){
        std::cout << it.second;
    }
}