Specifying compiler versions in travis for cmake builds - c++

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, i.e.:
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, e.g.:
$ 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, i.e.: 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