Specifying compiler versions in travis for cmake builds

Question

The travis Building a C++ Project documentation shows how to specify gcc and clang compiler versions in build matrices. However, it does not show how to build projects with those compilers using cmake.

I amended the .travis.yml file here to specify gcc 9 and clang 8 as per the travis documentation, ie:

matrix:
  include:
    - compiler: gcc
      addons:
        apt:
          sources:
            - ubuntu-toolchain-r-test
          packages:
            - g++-9
      env:
        - MATRIX_EVAL="CC=gcc-9 && CXX=g++-9"

    - compiler: clang
      addons:
        apt:
          sources:
            - ubuntu-toolchain-r-test
            - llvm-toolchain-bionic-8
          packages:
            - clang-8
            - libstdc++-8-dev
      env:
        - MATRIX_EVAL="CC=clang-8 && CXX=clang++-8"

before_install:
  - eval "${MATRIX_EVAL}"
  - pip install --user cpp-coveralls
...

script:
  - mkdir _builds
  - cd _builds
  - cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} .. 
  - make
  - ./via-httplib_test

But is caused build errors when running cmake , eg:

$ cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} ..
CMake Error at /usr/local/cmake-3.12.4/share/cmake-3.12/Modules/CMakeDetermineCCompiler.cmake:48 (message):
  Could not find compiler set in environment variable CC:

  gcc-9.
Call Stack (most recent call first):
  CMakeLists.txt:9 (project)


CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!
See also "/home/travis/build/kenba/via-httplib/_builds/CMakeFiles/CMakeOutput.log".
The command "cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} .." exited with 1.

I tried fixing the errors by specifying CMAKE_C_COMPILER and CMAKE_CXX_COMPILER for cmake, but I could not get it to work. However, it builds correctly with:

  env:
    - MATRIX_EVAL="CC=gcc && CXX=g++"
  ...
  env:
    - MATRIX_EVAL="CC=clang && CXX=clang++"

but builds with the default bionic gcc and clang compilers, ie: GCC 7.4.0 and Clang 7, not GCC 9 and Clang 8.

How to write a .travis.yml file so that cmake can find and use the compiler versions specified in a matrix?

Answer 1

Perhaps it was a Travis issue? Simply adding the g++-9 package and changing the variable to MATRIX_EVAL="CC=gcc-9 && CXX=g++-9" results in a successful compilation of your project with GCC 9.3.0

This is a minimal example that I use for building my project:

language: cpp

matrix:
  include:
    - os: linux
      addons:
        apt:
          sources:
            - sourceline: 'ppa:ubuntu-toolchain-r/test'
          packages:
            - clang-8
      env:
        - MATRIX_EVAL="CC=clang-8 CXX=clang++-8"
    - os: linux
      addons:
        apt:
          sources:
            - sourceline: 'ppa:ubuntu-toolchain-r/test'
          packages:
            - g++-9
      env:
        - MATRIX_EVAL="CC=gcc-9 CXX=g++-9"

before_install:
  - eval "${MATRIX_EVAL}"

script:
  - cmake .
  - make