C++ Basics: Keep a Deque Reference Across End Insertions

Use the reference guarantee for deque end insertion without retaining invalid iterators.

Prepare a scratch directory

Tested on Linux x86_64 with GCC 14.2.0. Install a C++20-capable compiler and its standard-library headers, with g++ on PATH. You also need Bash, a text editor, and the mktemp, rm, and rmdir utilities. No third-party library is required.

Keep the same terminal open. Stop if setup fails, before saving the source:

start_directory="$PWD"
work_directory="$(mktemp -d)" || exit 1
cd -- "$work_directory" || exit 1

Run the example

Save this as deque-end-insertion-references-demo.cpp:

#include <deque>
#include <iostream>
#include <string>

int main() {
    std::deque<std::string> jobs{"compile", "test"};
    std::string& original_first = jobs.front();
    jobs.push_front("configure");
    jobs.push_back("package");
    original_first += " complete";
    std::cout << "Retained element: " << original_first << '\n';
    std::cout << "Current front: " << jobs.front() << '\n';
    std::cout << "Jobs:";
    for (const auto& job : jobs) {
        std::cout << " [" << job << ']';
    }
    std::cout << '\n';
}

Compile and run it. The second command runs only after compilation succeeds:

g++ -std=c++20 -Wall -Wextra -Werror -pedantic deque-end-insertion-references-demo.cpp -o deque-end-insertion-references-demo &&
./deque-end-insertion-references-demo

Observed output:

Retained element: compile complete
Current front: configure
Jobs: [configure] [compile complete] [test] [package]

Read the result

The retained reference still names compile, although that element is no longer first. The range loop obtains new iterators after both insertions.

Apply it carefully

Insertion at either deque end preserves element references but invalidates iterators. Do not keep an old begin or end iterator for later traversal. Middle insertion has different invalidation rules; this guarantee is not a general promise that every deque mutation preserves references.

The C++ working draft documents the relevant container rules. This example uses only features available in C++20, even though the linked draft also includes newer additions.

Clean up

Remove only the source and executable created here, then remove the empty practice directory:

rm -- deque-end-insertion-references-demo.cpp deque-end-insertion-references-demo
cd -- "$start_directory" || exit 1
rmdir -- "$work_directory"
unset start_directory work_directory

If the directory is not empty, inspect its remaining contents before removing anything else.

Acknowledgements

This post was created with assistance from AI.