How Can I Safely Remove Elements from a C Map During Iteration?

Removing Elements from a Map During Iteration

In C , iterating over a standard library map while removing elements can be challenging, as using erase() invalidates iterators. However, there is a widely accepted idiom to address this issue:

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

Here's how this idiom works:

• No hoisting: We do not hoist the increment operator outside the loop condition. This is critical, as hoisting would cause the iterator to advance even if the element is removed, potentially leading to undefined behavior.
• Incrementing safely: Within the loop, the increment operator is only applied after checking if the element should not be removed.
• Caching the next iterator (C 11 or later): When an element is removed, we cache the iterator to the next element before erasing. This allows us to continue iterating seamlessly without disrupting the iterator sequence.

Pre-C 11 Syntax:

Before C 11, erasing const iterators required a slightly different syntax:

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

Constness and Removal:

Erasing an element from a map does not violate the constness of the elements, similar to deleting a pointer-to-constant. Constness primarily constrains the ability to modify the value, not its lifetime.

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