Modern way to set compiler flags in cross-platform cmake project

Your approach would – as @Tsyvarev has commented – be absolutely fine, just since you’ve asked for the “new” approach in CMake here is what your code would translate to:

cmake_minimum_required(VERSION 3.8)

project(HelloWorld)

string(
    APPEND _opts
    "$<IF:$<CXX_COMPILER_ID:MSVC>,"
        "/W4;$<$<CONFIG:RELEASE>:/O2>,"
        "-Wall;-Wextra;-Werror;"
            "$<$<CONFIG:RELEASE>:-O3>"
            "$<$<CXX_COMPILER_ID:Clang>:-stdlib=libc++>"
    ">"
)

add_compile_options("${_opts}")

add_executable(HelloWorld "main.cpp")

target_compile_features(HelloWorld PUBLIC cxx_lambda_init_captures)

---

You take add_compile_options() and – as @Al.G. has commented – “use the dirty generator expressions“.

There are some downsides of generator expressions:

1. The very helpful $<IF:...,...,...> expression is only available in CMake version >= 3.8
2. You have to write it in a single line. To avoid it I used the string(APPEND ...), which you can also use to “optimize” your set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ... calls.
3. It’s difficult to read and understand. E.g. the semicolons are needed to make it a list of compile options (otherwise CMake will quote it).

So better use a more readable and backward compatible approach with add_compile_options():

if(MSVC)
    add_compile_options("/W4" "$<$<CONFIG:RELEASE>:/O2>")
else()
    add_compile_options("-Wall" "-Wextra" "-Werror" "$<$<CONFIG:RELEASE>:-O3>")
    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
        add_compile_options("-stdlib=libc++")
    else()
        # nothing special for gcc at the moment
    endif()
endif()

---

And yes, you don’t explicitly specify the C++ standard anymore, you just name the C++ feature your code/target does depend on with target_compile_features() calls.

For this example I’ve chosen cxx_lambda_init_captures which would for e.g. an older GCC compiler give the following error (as an example what happens if a compiler does not support this feature):

The compiler feature "cxx_lambda_init_captures" is not known to CXX compiler

"GNU"

version 4.8.4.

---

And you need to write a wrapper script to build multiple configurations with a “single configuration” makefile generator or use a “multi configuration” IDE as Visual Studio.

Here are the references to examples:

• Does CMake always generate configurations for all possible project configurations?
• How do I tell CMake to use Clang on Windows?
• How to Add Linux Compilation to Cmake Project in Visual Studio

So I’ve tested the following with the Open Folder Visual Studio 2017 CMake support to combine in this example the cl, clang and mingw compilers:

CMakeSettings.json

{
    // See https://go.microsoft.com//fwlink//?linkid=834763 for more information about this file.
    "configurations": [
        {
            "name": "x86-Debug",
            "generator": "Visual Studio 15 2017",
            "configurationType": "Debug",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "buildCommandArgs": "-m -v:minimal",
        },
        {
            "name": "x86-Release",
            "generator": "Visual Studio 15 2017",
            "configurationType": "Release",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "buildCommandArgs": "-m -v:minimal",
        },
        {
            "name": "Clang-Debug",
            "generator": "Visual Studio 15 2017",
            "configurationType": "Debug",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "cmakeCommandArgs": "-T\"LLVM-vs2014\"",
            "buildCommandArgs": "-m -v:minimal",
        },
        {
            "name": "Clang-Release",
            "generator": "Visual Studio 15 2017",
            "configurationType": "Release",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "cmakeCommandArgs": "-T\"LLVM-vs2014\"",
            "buildCommandArgs": "-m -v:minimal",
        },
        {
            "name": "GNU-Debug",
            "generator": "MinGW Makefiles",
            "configurationType": "Debug",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "variables": [
                {
                    "name": "CMAKE_MAKE_PROGRAM",
                    "value": "${projectDir}\\mingw32-make.cmd"
                }
            ]
        },
        {
            "name": "GNU-Release",
            "generator": "Unix Makefiles",
            "configurationType": "Release",
            "buildRoot": "${env.LOCALAPPDATA}\\CMakeBuild\\${workspaceHash}\\build\\${name}",
            "variables": [
                {
                    "name": "CMAKE_MAKE_PROGRAM",
                    "value": "${projectDir}\\mingw32-make.cmd"
                }
            ]
        }
    ]
}

mingw32-make.cmd

@echo off
mingw32-make.exe %~1 %~2 %~3 %~4

So you can use any CMake generator from within Visual Studio 2017, there is some unhealthy quoting going on (as for September 2017, maybe fixed later) that requires that mingw32-make.cmd intermediator (removing the quotes).