Is the Order of Iteration in `std::map` Guaranteed by the Standard?

Iterating Order in std::map: Standard Guaranteed or Not?

In std::map, elements are sorted based on their keys. However, does the standard specify the order in which these elements are iterated? This question arises when iterating from begin() to end(), particularly for an integer-keyed map.

Standard Guarantee

Yes, the order of iteration from begin() to end() is guaranteed by the standard. This means that for an integer-keyed map, iterating through the elements will output the values associated with those keys in ascending order.

Internal Implementation

Internally, std::map uses a balanced binary search tree for efficient searching and insertion. Elements are stored in a way that maintains this sorted order. When iterating through the tree, the nodes are visited in such a way that the inorder traversal produces the elements in sorted order.

Determining Order

The default comparison function used in std::map is std::less, which determines the order as strictly increasing. This means that for integer keys, the elements will be sorted in ascending order.

Example

Consider the provided code snippet:

<code class="cpp">std::map<int, int> map_;
map_[1] = 2;
map_[2] = 3;
map_[3] = 4;
for (std::map<int, int>::iterator iter = map_.begin();
     iter != map_.end();
     ++iter)
{
    std::cout << iter->second;
}</code>

Output Guarantee:

The standard guarantees that the above code will output "234" because the elements will be iterated in ascending order of their keys. This ordering behavior is essential for efficient searching and maintaining the sorted nature of the map data structure.

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