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C++ STL Iterators

bookIterators in Associative Containers

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Note
Definition

Associative containers store elements identified by keys, with some also storing mapped values. The main types are std::set, std::map, std::unordered_set, and std::unordered_map. Their iterators are bidirectional for ordered containers and forward for unordered ones, providing access to keys or key–value pairs. Keys cannot be modified through iterators.

When working with associative containers in C++, you interact with elements using iterators that reflect the unique structure of these containers. The most common associative containers are std::set, std::map, std::unordered_set, and std::unordered_map.

The ordering of iteration is guaranteed for ordered containers but not for unordered ones.

main.cpp

main.cpp

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#include <iostream>
#include <map>
#include <unordered_set>
#include <string>

int main()
{
    std::map<std::string, int> ordered_map = {
        {"apple", 3},
        {"banana", 1},
        {"cherry", 2}
    };

    std::unordered_set<std::string> unordered_set = {
        "zebra", "lion", "elephant"
    };

    std::cout << "Iterating over std::map (ordered):\n";
    for (const auto& pair : ordered_map)
        std::cout << pair.first << " => " << pair.second << '\n';

    std::cout << "\nIterating over std::unordered_set (unordered):\n";
    for (const auto& animal : unordered_set)
        std::cout << animal << '\n';
}

In the above code, you see two associative containers in action. When iterating over the std::map, the elements are traversed in sorted order by key: "apple", "banana", then "cherry". The iterator for std::map gives you access to both the key (pair.first) and the value (pair.second). In contrast, when iterating over the std::unordered_set, the order of elements is unspecified and may differ between runs, reflecting the lack of ordering guarantees. The iterator for std::unordered_set gives access to the key only, since sets do not store mapped values.

main.cpp

main.cpp

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#include <iostream>
#include <map>

int main()
{
    std::map<int, std::string> numbers = {
        {1, "one"},
        {2, "two"}
    };

    auto it = numbers.begin();
    // Uncommenting the next line would cause a compile-time error:
    // it->first = 99;

    std::cout << "Key: " << it->first << ", Value: " << it->second << '\n';
}

This code highlights an important rule of associative containers: keys cannot be modified through iterators. Attempting to change a key results in a compilation error, which protects the container’s ordering or hashing guarantees.

When iterating over std::map or std::unordered_map, you can access both the key (pair.first) and the value (pair.second), but only the value is mutable. For std::set and std::unordered_set, iterators expose only the key, which is also immutable. This design ensures correctness and consistent performance for all associative containers.

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Seção 3. Capítulo 2
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