Creating a Multi-Stage Docker Build for a C++ Application

--- title: Create a multi-stage build for your C++ application linkTitle: Containerize your app using a multi-stage build weight: 5 keywords: C++, containerize, multi-stage description: Learn how to create a multi-stage build for a C++ application. aliases: - /language/cpp/multistage/ - /guides/language/cpp/multistage/ --- ## Prerequisites - You have a [Git client](https://git-scm.com/downloads). The examples in this section use a command-line based Git client, but you can use any client. ## Overview This section walks you through creating a multi-stage Docker build for a C++ application. A multi-stage build is a Docker feature that allows you to use different base images for different stages of the build process, so you can optimize the size of your final image and separate build dependencies from runtime dependencies. The standard practice for compiled languages like C++ is to have a build stage that compiles the code and a runtime stage that runs the compiled binary, because the build dependencies are not needed at runtime. ## Get the sample application Let's use a simple C++ application that prints `Hello, World!` to the terminal. To do so, clone the sample repository to use with this guide: ```bash $ git clone https://github.com/dockersamples/c-plus-plus-docker.git ``` The example for this section is under the `hello` directory in the repository. Get inside it and take a look at the files: ```bash $ cd c-plus-plus-docker/hello $ ls ``` You should see the following files: ```text Dockerfile hello.cpp ``` ## Check the Dockerfile Open the `Dockerfile` in an IDE or text editor. The `Dockerfile` contains the instructions for building the Docker image. ```Dockerfile # Stage 1: Build stage FROM ubuntu:latest AS build # Install build-essential for compiling C++ code RUN apt-get update && apt-get install -y build-essential # Set the working directory WORKDIR /app # Copy the source code into the container COPY hello.cpp . # Compile the C++ code statically to ensure it doesn't depend on runtime libraries RUN g++ -o hello hello.cpp -static # Stage 2: Runtime stage FROM scratch # Copy the static binary from the build stage COPY --from=build /app/hello /hello # Command to run the binary CMD ["/hello"] ``` The `Dockerfile` has two stages: 1. **Build stage**: This stage uses the `ubuntu:latest` image to compile the C++ code and create a static binary. 2. **Runtime stage**: This stage uses the `scratch` image, which is an empty image, to copy the static binary from the build stage and run it. ## Build the Docker image To build the Docker image, run the following command in the `hello` directory: ```bash $ docker build -t hello . ``` The `-t` flag tags the image with the name `hello`. ## Run the Docker container To run the Docker container, use the following command: ```bash $ docker run hello ``` You should see the output `Hello, World!` in the terminal. ## Summary In this section, you learned how to create a multi-stage build for a C++ application. Multi-stage builds help you optimize the size of your final image and separate build dependencies from runtime dependencies. In this example, the final image only contains the static binary and doesn't include any build dependencies. As the image has an empty base, the usual OS tools are also absent. So, for example, you can't run a simple `ls` command in the container: ```bash $ docker run hello ls ``` This makes the image very lightweight and secure.

This section guides you through creating a multi-stage Docker build for a simple C++ "Hello, World!" application. It covers cloning the sample repository, examining the Dockerfile with build and runtime stages, building the image, and running the container. The multi-stage approach optimizes image size by separating build dependencies from runtime dependencies, resulting in a lightweight and secure final image containing only the executable.