C++ .emplace()

The .emplace() method in C++ for std::map efficiently constructs and inserts an element directly into the map. Unlike .insert(), .emplace() constructs the element in-place, avoiding unnecessary copies or moves.

Syntax

mapName.emplace(key, value)

• mapName: Refers to the specific map being modified.
• key: The key for the new element.
• value: The value associated with the key.

Example

This example demonstrates using the .emplace() method to add elements to a map and the behaviour on inserting elements with the same key:

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

int main() {
  // Initialize a map of products and their prices
  std::map<std::string, double> product_prices;

  // Using std::pair instead of structured binding to support C++11 and later
  std::pair<std::map<std::string, double>::iterator, bool> prod1 = product_prices.emplace("Laptop", 999.99);
  std::pair<std::map<std::string, double>::iterator, bool> prod2 = product_prices.emplace("Phone", 499.99);
  std::pair<std::map<std::string, double>::iterator, bool> prod3 = product_prices.emplace("Laptop", 799.99);

  // Print results
  std::cout << "Laptop insertion (first attempt): " << (prod1.second ? "Successful" : "Failed") << std::endl;
  std::cout << "Phone insertion: " << (prod2.second ? "Successful" : "Failed") << std::endl;
  std::cout << "Laptop insertion (second attempt): " << (prod3.second ? "Successful" : "Failed") << std::endl;

  // Print the content of the map
  for (const auto& pair : product_prices) {
    std::cout << pair.first << ": $" << pair.second << std::endl;
  }

  return 0;
}

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The code above results in the following output:

Laptop insertion (first attempt): Successful
Phone insertion: Successful
Laptop insertion (second attempt): Failed
Laptop: $999.99
Phone: $499.99

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Codebyte Example

Run the following codebyte example to understand how the .emplace() method adds new elements to a map:

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