How do I reserve capacity and shrink-to-fit with std::multimap?

The C++ std::multimap is a standard template library container that stores elements in a sorted order and allows multiple equivalent keys. However, it does not provide direct methods to reserve capacity or shrink its size, similar to other containers like std::vector or std::string. Below is an explanation and examples of effectively managing capacity with std::multimap.

Reserving Capacity

std::multimap does not have a reserve() method because its internal structure (usually a tree) is not based on contiguous memory allocation. However, one can optimize performance by pre-inserting the expected number of elements using insert operations which can help minimize reallocations and copies.

Shrinking Capacity

While you cannot directly shrink the capacity of a std::multimap, you can remove elements to reduce size. Although the storage will not be released, it will reflect the updated size. If extremely low memory is required, consider moving your data to a different container.

Example

#include <iostream> #include <map> int main() { std::multimap mmap; // Inserting elements mmap.insert({1, "One"}); mmap.insert({2, "Two"}); mmap.insert({1, "Uno"}); // Displaying elements for (const auto& pair : mmap) { std::cout << pair.first << ": " << pair.second << std::endl; } // Removing an element mmap.erase(1); // Removes all elements with key 1 // Displaying elements again std::cout << "After erase:" << std::endl; for (const auto& pair : mmap) { std::cout << pair.first << ": " << pair.second << std::endl; } return 0; }

C++ std::multimap reserve capacity shrink-to-fit container management

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