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C++20 STL Cookbook

Efficiently Insert Elements into a Map

Understand how to efficiently insert key-value pairs into C++ STL map containers. Explore differences between emplace and try_emplace methods to avoid unnecessary object construction, improving runtime performance especially with large payloads. Learn to implement and test these methods in practical C++20 code.

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The map class is an associative containerAn associative container is a type of data structure that enables efficient retrieval of values based on a unique key or identifier, without requiring traversal through other elements. that holds key/value pairs, where keys must be unique within the container.

There are a number of ways to populate a map container. Consider a map defined like this: 

map<string, string> m;

We can add an element with the [] operator: 

m["Miles"] = "Trumpet"

We can use the insert() member function: 

m.insert(pair<string,string>("Hendrix", "Guitar"));

Or, we can use the emplace() member function:

 m.emplace("Krupa", "Drums");

Many programmers tend to gravitate toward the emplace() function. Introduced with C++11, emplace() uses perfect forwarding to emplace (create in place) the new element for the container. The parameters are forwarded directly to the element constructors. This is quick, efficient, and easy to code.

Though it's certainly an improvement over the other options, the problem with emplace() is that it constructs an object even when it's not needed. This involves calling the constructors, allocating memory, and moving data around, and ...