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multimap::emplace_hint() Function in C++ STL

The std::multimap::emplace_hint() function in C++ is a member function of the std::multimap container from the C++ Standard Library. It allows you to efficiently insert an element into a multimap while providing a hint about the position where the element should be inserted. Using a hint can help improve the insertion performance, especially when you have some knowledge about the expected location of the element.

Here’s the syntax of the emplace_hint() function:

C++
iterator emplace_hint(const_iterator position, Args... args);
  • position: A hint indicating the position where the element should be inserted. The function will attempt to insert the element at or near this position.
  • Args... args: Arguments that are forwarded to the constructor of the element being inserted.

The emplace_hint() function returns an iterator pointing to the newly inserted element or to the element that prevented the insertion if it failed.

Here’s an example of how to use emplace_hint() with a multimap:

C++
#include <iostream>
#include <map>

int main() {
    std::multimap<int, std::string> studentMap;

    // Using emplace_hint to insert elements efficiently
    std::multimap<int, std::string>::iterator hint = studentMap.begin();
    hint = studentMap.emplace_hint(hint, 101, "Alice"); // Insert at or near the beginning
    hint = studentMap.emplace_hint(hint, 105, "Bob");   // Insert after Alice

    // Insert more elements
    studentMap.emplace_hint(hint, 103, "Charlie");
    studentMap.emplace_hint(hint, 104, "David");

    // Print the multimap
    for (const auto& pair : studentMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

In this example:

  • We create a std::multimap named studentMap that associates student IDs (integers) with their names (strings).
  • We use emplace_hint() to efficiently insert elements at or near the specified position in the multimap. The hint iterator is initially set to begin(), and we insert elements with student IDs and names.
  • The output will show the elements in the multimap in the order they were inserted. Using emplace_hint() can help optimize insertion when you have some knowledge about the desired position.

Note that the use of hints is optional, and the C++ Standard Library implementation will still ensure correct insertion even without hints. Hints can be beneficial in scenarios where you want to optimize the insertion performance.

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