Specifying language standards in CMake projects

This interactive example describes how to specify language standards in a CMake project when using IAR Compilers.

Helpful Resources

• C_STANDARD
• CXX_STANDARD
• target_compile_features()
• set_target_properties()

Setting Language Standards in CMake

In CMake, it is possible to select in which language standard each target will be produced. This can be performed globally or per-target. As for example:

• Specifying the project's C++ Standard to C++20, globally

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED OFF)
set(CMAKE_CXX_EXTENSIONS ON)

• Specifying the same standard, but for a single target

set_target_properties(app-cpp20 PROPERTIES
  CXX_STANDARD 20
  CXX_STANDARD_REQUIRED OFF
  CXX_EXTENSIONS ON)

The IAR Compilers and Language Standards

As a rule of thumb, each IAR Compiler conforms to IAR's latest language standard available at the time of their release. However, given the constrained nature of embedded systems development, there are exceptions:

• In C, there is typically C90 (a.k.a. C89) as a backward compatibility option.
• For C++, a dual runtime library stack might be offered as a user choice between IAR DLIB (up to C++14) or IAR Libc++ (for C++17 or later). This design choice provides flexibility between legacy code using DLIB and new applications being designed using the latest standard available.

Availability, naturally, depends on the product version. As an example, let's outline it for Arm:

Version	C Standards	C++ Standards	C++ Runtime Libs
10.10.1	C90/C17	C++20	DLIB/Libc++
9.30.1	C90/C17	C++17	DLIB/Libc++
8.40.1	C90/C17	C++17	DLIB only
8.32.1	C90/C11	C++14	DLIB only

The C++ library choice should be made based on the runtime library features in use. Say you want to #include <concepts> in your C++20 source file:

• It will work with --c++ --libc++ in iccarm-10.10.1 or later because it brings arm/inc/libcpp/concepts.
• It will not compile with --c++ --dlib_config ... since there is no concepts header implemented under arm/inc/cpp/.

CMake will automatically add the --c++ option for all C++ files in the project.

For strict conformance on C++20, CMake v4.4.0 or later will automatically add the --libc++ option.

For C++17, the option --libc++ needs to be manually added to target_compile_options() whenever available or required.

Interactive Example

A CMake project example is provided at examples/standards:

Project files
CMakeLists.txt
main.cpp
main.c

This interactive project example shows how you can select a language standard for its specific targets. It was created to demonstrate the behavior of enforcing strict language standard conformance in a CMake project when using the IAR Compilers.

Tasks

• Build the project with the IAR Compiler a first time.
• Perform the following tasks on CMakeLists.txt (click to show/hide answers):

TODO 1: Enable strict conformance for the app-cpp20 application

set_target_properties(app-cpp20 PROPERTIES
  CXX_STANDARD 20
  CXX_STANDARD_REQUIRED ON
  CXX_EXTENSIONS ON)

Now rebuild the project. Using a legacy compiler with no C++20 capabilities after enforcing the standard will result in a CMake fatal error when configuring the project:

CMake Error in CMakeLists.txt:
  Target "app-cpp20" requires the language dialect "CXX20" (with compiler
  extensions).  But the current compiler "IAR" does not support this, or
  CMake does not know the flags to enable it.

This means the compiler version used for building app-cpp20 did not offer C++20 support. Looking forward, the two options from here would be to:

• a) upgrade the compiler to a version supporting C++20 or
• b) use strict conformance only when making use of the language features provided by the compiler currently in use.

TODO 2: Enable strict conformance for the app-c90 application

set_target_properties(app-c90 PROPERTIES
  C_STANDARD 90
  C_STANDARD_REQUIRED ON
  C_EXTENSIONS ON)

Now rebuild the project with --verbose. Whenever required by the compiler, CMake will automatically add the --c89 option:

[3/16] [...]/arm/bin/iccarm  --silent [...]/main.c --c89 -e --dependencies=ns [...] -o [...]/main.o