# Building C++ Libraries Sample

version 7.0.2

- Groovy DSL

- Kotlin DSL

| You can open this sample inside an IDE using the IntelliJ native importer or Eclipse Buildship.

This guide demonstrates how to create a C++ library with Gradle using `gradle init`. You can follow the guide step-by-step to create a new project from scratch or download the complete sample project using the links above.

## What you’ll build

You’ll generate a C++ library that follows Gradle’s conventions.

## What you’ll need

- A text editor or IDE - for example IntelliJ IDEA

- A Java Development Kit (JDK), version 8 or higher - for example AdoptOpenJDK

- The latest Gradle distribution

- An installed C++ compiler. See which C++ tool chains are supported by Gradle.

## Create a project folder

Gradle comes with a built-in task, called `init`, that initializes a new Gradle project in an empty folder. The `init` task uses the (also built-in) `wrapper` task to create a Gradle wrapper script, `gradlew`.

The first step is to create a folder for the new project and change directory into it.

    $ mkdir demo
    $ cd demo

## Run the init task

From inside the new project directory, run the `init` task using the following command in a terminal: `gradle init`. When prompted, select the `3: library` project type and `1: C++` as implementation language. Next you can choose the DSL for writing buildscripts - `1 : Groovy` or `2: Kotlin`. For the other questions, press enter to use the default values.

The output will look like this:

    $ gradle init

    Select type of project to generate:
      1: basic
      2: application
      3: library
      4: Gradle plugin
    Enter selection (default: basic) [1..4] 3

    Select implementation language:
      1: C++
      2: Groovy
      3: Java
      4: Kotlin
      5: Scala
      6: Swift
    Enter selection (default: Java) [1..6] 1

    Select build script DSL:
      1: Groovy
      2: Kotlin
    Enter selection (default: Groovy) [1..2] 1

    Project name (default: demo):

    BUILD SUCCESSFUL
    2 actionable tasks: 2 executed

The `init` task generates the new project with the following structure:

    ├── gradle (1)
    │   └── wrapper
    │       ├── gradle-wrapper.jar
    │       └── gradle-wrapper.properties
    ├── gradlew (2)
    ├── gradlew.bat (2)
    ├── settings.gradle (3)
    └── lib
        ├── build.gradle (4)
        └── src
            ├── main
            │   └── cpp (5)
            │   │   └── hello.cpp
            │   └── headers
            │       └── app.h
            └── test
                └── cpp (6)
                    └── hello_test.cpp

    ├── gradle (1)
    │   └── wrapper
    │       ├── gradle-wrapper.jar
    │       └── gradle-wrapper.properties
    ├── gradlew (2)
    ├── gradlew.bat (2)
    ├── settings.gradle.kts (3)
    └── lib
        ├── build.gradle.kts (4)
        └── src
            ├── main
            │   └── cpp (5)
            │   │   └── hello.cpp
            │   └── headers
            │       └── app.h
            └── test
                └── cpp (6)
                    └── hello_test.cpp

1 | Generated folder for wrapper files
2 | Gradle wrapper start scripts
3 | Settings file to define build name and subprojects
4 | Build script of `lib` project
5 | Default C++ source folder
6 | Default C++ test source folder

You now have the project setup to build a C++ library.

## Review the project files

The `settings.gradle(.kts)` file has two interesting lines:

settings.gradle

    rootProject.name = 'demo'
    include('lib')

settings.gradle.kts

    rootProject.name = "demo"
    include("lib")

- `rootProject.name` assigns a name to the build, which overrides the default behavior of naming the build after the directory it’s in. It’s recommended to set a fixed name as the folder might change if the project is shared - e.g. as root of a Git repository.

- `include("lib")` defines that the build consists of one subproject called `lib` that contains the actual code and build logic. More subprojects can be added by additional `include(…​)` statements.

Our build contains one subproject called `lib` that represents the C++ library we are building. It is configured in the `lib/build.gradle(.kts)` file:

lib/build.gradle

    plugins {
        id 'cpp-library' (1)

        id 'cpp-unit-test' (2)
    }

    library {
        targetMachines.add(machines.linux.x86_64) (3)
    }

lib/build.gradle.kts

    plugins {
        `cpp-library` (1)

        `cpp-unit-test` (2)
    }

    library {
        targetMachines.add(machines.linux.x86_64) (3)
    }

1 | Apply the cpp-library plugin to add support for building C++ libraries
2 | Apply the cpp-unit-test plugin to add support for building and running C++ test executables
3 | Set the target operating system and architecture for this library

The file `src/main/cpp/hello.cpp` is shown here:

Generated src/main/cpp/hello.cpp

    /*
     * This C++ source file was generated by the Gradle 'init' task.
     */

    #include <iostream>
    #include <stdlib.h>
    #include "demo.h"

    std::string demo::Greeter::greeting() {
        return std::string("Hello, World!");
    }

The generated test, `src/test/cpp/hello.cpp` is shown next:

Generated src/test/cpp/hello_test.cpp

    /*
     * This C++ source file was generated by the Gradle 'init' task.
     */

    #include "demo.h"
    #include <cassert>

    int main() {
        demo::Greeter greeter;
        assert(greeter.greeting().compare("Hello, World!") == 0);
        return 0;
    }

The generated test class has a single test. The test instantiates the `Hello` class, invokes a method on it, and checks that it returns the expected value.

## Build the library

    $ ./gradlew build

    BUILD SUCCESSFUL in 0s
    4 actionable tasks: 4 executed

| The first time you run the wrapper script, `gradlew`, there may be a delay while that version of `gradle` is downloaded and stored locally in your `~/.gradle/wrapper/dists` folder.

The `build` task compiles the C++ sources, links the object files into a shared library, and runs the tests. To build a static library, please refer to the static library sample.

| Dependencies on other projects isn’t covered in this guide. To learn more about this subject, have a look at the transitive dependency sample for a demonstration.

| Publishing libraries to Maven repositories is outside the scope of this guide. To learn more about this subject, have a look at the simple library sample for a demonstration.

| Gradle integrates with several IDEs: Visual Studio, Xcode and Clion. To learn more, have a look at their respective linked documentation to configure those IDE integration in your project.

## Publish a Build Scan

The best way to learn more about what your build is doing behind the scenes, is to publish a build scan. To do so, just run Gradle with the `--scan` flag.

    $ ./gradlew build --scan

    BUILD SUCCESSFUL in 0s
    4 actionable tasks: 4 executed

    Publishing a build scan to scans.gradle.com requires accepting the Gradle Terms of Service defined at https://gradle.com/terms-of-service.
    Do you accept these terms? [yes, no] yes

    Gradle Terms of Service accepted.

    Publishing build scan...
    https://gradle.com/s/5u4w3gxeurtd2

Click the link and explore which tasks where executed, which dependencies where downloaded and many more details!

## Summary

That’s it! You’ve now successfully configured and built a C++ library project with Gradle. You’ve learned how to:

- Initialize a project that produces a C++ library

- Build the library, which includes running its tests

## Next Steps

- Make your way to the native samples repository to see the C++ plugins in action for common scenarios such as transitive dependencies, custom source layout, and static library.

- Check out the user manual chapter on Building C++ projects