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Const Correctness for C++ Builds

· 6 min read
Christopher McArthur
C++ DevOps Expert

In the ever-evolving world of software development, ensuring code quality and maintainability is paramount. Two seemingly unrelated concepts, const correctness in C++ and ephemeral build environments from DevOps, share a surprising connection, both aiming to build a strong foundation for reliable software.

Const Correctness: Enforcing Immutability in Code

Const correctness is a programming paradigm in C++ that emphasizes the use of the const keyword to explicitly declare variables and objects that shouldn't be modified. This enforces a form of immutability within your code. Just like an immutable object in other languages, a const variable cannot have its value changed after initialization.

Here's the magic:

  1. Prevents Accidental Modifications: const acts as a safeguard against accidental changes, preventing bugs caused by unintended assignments.
  2. Improved Code Clarity: It clearly communicates intent to both the compiler and future developers, making code easier to understand and maintain.
  3. Compiler Optimizations: The compiler can leverage const to improve performance by making assumptions about the data's immutability.

Let's implement a hypothetical C++ CI service.

#include <string_view>

class build_image {
build_image(std::string_view version, std::string_view name) : version_(version), name_(name) {
download_build_image(*this); // Takes a non-const to populate in `this->id`

std::string version() const { return std::string(version_); }
std::string name() const { return std::string(name_); }
std::string id() const { return id_; }
void id(std::string_view id) { id_ = std::string(id); }

const std::string_view version_;
const std::string_view name_;
std::string id_;

void download_build_image(build_image& image);
void open_shell_handle(const build_image& image);

// Contents of a build_image can not be modified while commands are executed
bool run_command(const build_image& image, const std::string& cmd) {
// Simulate opening a shell and executing commands
return true; // Code never fails to compile

int main() {
const std::string_view version = "24.04"sv;
const std::string_view name = "ubuntu"sv;
const build_image ubuntu24(version, name);

const auto build_result =
run_command(ubuntu24, "./ debug all")

std::cout << "Build complete! " << build_result << std::endl;

return 0;

Ephemeral Build Images: Fresh Starts for Every Build

Ephemeral, meaning "short-lived," refers to a build environment that is created, used for a specific task (like building software), and then discarded. This approach is gaining traction due to its numerous benefits:

  1. Reproducible Builds: Every build starts with a clean slate, ensuring consistency regardless of the developer's environment or previous build artifacts. Version controlling these ephemeral build images guarantees reproducibility across builds.
  2. Reduced Complexity: Ephemeral builds eliminate the need to manage complex build environments on developer machines, simplifying the development process.
  3. Security Enhancements: Since the build environment is discarded after use, it minimizes the attack surface and potential vulnerabilities.
  • Docker: A containerization platform that allows packaging applications and their dependencies into lightweight, portable units called containers. Docker excels at creating isolated and consistent build environments.
  • Kubernetes: An open-source system for automating deployment, scaling, and management of containerized applications. Kubernetes can orchestrate the creation and management of multiple Docker containers that form a complex build environment.
  • Azure Pipelines: A cloud-based CI/CD platform from Microsoft that provides pre-built agents for Windows environments. These agents offer isolated and pre-configured build environments for running build tasks.

Infrastructure as Code (IaC) and Ephemeral Builds

Infrastructure as Code (IaC) refers to the practice of managing infrastructure (like servers, networks, and storage) in a programmatic way using code. Tools like Terraform allow developers to define the configuration of their build environment in code files. These code files can then be version controlled alongside the application code, ensuring consistency and reproducibility.

Version Control: Tying it All Together

Version control systems (VCS) like Git play a crucial role in both const correctness and ephemeral builds. It allows you to manage changes to your codebase (enforcing const usage) and the definitions of your ephemeral build environments, fostering a unified Software Development Lifecycle (SDLC) for both.

Artifact Management with Version Control

  • Codebase Versioning: Git tracks changes to your C++ source code. This enables reverting to previous versions if necessary and identifying who made specific changes that might have introduced issues. Enforcing const correctness through code reviews and linting tools further strengthens the codebase's integrity.
  • Build Environment Definitions: You can store the configuration files or scripts that define your ephemeral build environments (e.g., Dockerfiles, Terraform configurations) alongside your code. Version controlling these definitions ensures everyone uses the same build environment and allows for rollbacks to previous configurations if needed.

Benefits of a Unified SDLC

A Single Source of Truth: By leveraging version control, you establish a single source of truth for both your code and build environment definitions. This streamlines your SDLC by ensuring everyone works with the latest and approved versions of both components.

Combining const correctness with ephemeral builds and managing them through version control creates a robust and cohesive foundation for reliable software development. You enforce code immutability, guarantee clean build environments for each build, and establish a clear audit trail for changes, leading to fewer bugs, easier maintenance, and ultimately, more dependable software.

The Connection: Immutable Code, Fresh Builds

Const correctness and ephemeral builds, seemingly separate concepts, share a surprising connection: building a strong foundation for reliable software. Const correctness enforces immutability in your C++ code, while ephemeral builds provide a clean slate for each build. Together, they create a powerful synergy:

  • Immutable Code, Consistent Builds: Const correctness safeguards code, and ephemeral builds provide a fresh starting point.
  • Simplified Maintenance: Version control streamlines development by managing both code and build environments.
  • Ephemeral builds provide a clean, consistent starting point for every build, eliminating the risk of inconsistencies from previous builds.

Imagine a scenario where a non-const variable in your C++ code accidentally gets modified during the build process. This could lead to unexpected behavior in the final product. Ephemeral builds mitigate this risk by ensuring a fresh, unmodified codebase for each build.

Similarly, consider an ephemeral build environment containing cached data or temporary files from previous builds. This can lead to inconsistencies if these remnants influence the build outcome. Const correctness in your code helps prevent such issues by ensuring the code itself remains immutable, regardless of the build environment.


In conclusion, combining const correctness with ephemeral builds and managing them through version control creates a robust and cohesive foundation for reliable software development. You enforce code immutability, guarantee clean build environments for each build, and establish a clear audit trail for changes, leading to fewer bugs, easier maintenance, and ultimately, more dependable software.