How to Safely Remove Elements from a C Map During Iteration?

Removing Elements from a Map During Iteration

When iterating through a map, the common approach is to use a range-based for loop. However, modifying the map while iterating can lead to unexpected behavior, as the iterators will become invalid.

The recommended technique to remove elements from a map while iterating is known as the "associative-container erase idiom":

for (auto it = m.cbegin(); it != m.cend() /* not hoisted */; /* no increment */) {
  if (must_delete) {
    m.erase(it++);    // or "it = m.erase(it)" since C++11
  } else {
    ++it;
  }
}

This idiom ensures that the iterator remains valid after the element is removed by using the prefix increment operator.

Note that it is important to use a traditional for loop instead of a range-based for loop in this case, as modifying the container will invalidate the iterators of the range-based loop.

Prior to C 11, it was necessary to use a separate iterator type when iterating through a map to remove elements:

for (std::map<K, V>::iterator it = m.begin(); it != m.end(); ) { /* ... */ }

In both cases, erasing an element does not violate the constness of the element, as constness in C does not affect the lifetime of an object.

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