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Adding c++11

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Henry Fredrick Schreiner 2017-10-18 09:33:06 -04:00
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SUMMARY.md
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## Making a CMakeLists
* [Introduction to the basics](chapters/basics.md)
* [Adding features (like C++11)](chapters/features.md)
* [How to structure your project](chapters/structure.md)
* [Including small projects](chapters/smallinc.md)
* [Including large projects](chapters/largeinc.md)
* [Running other programs](chapters/programs.md)
* [Introduction to the Basics](chapters/basics.md)
* [C++11 and Beyond](chapters/cpp11.md)
* [Adding Features](chapters/features.md)
* [The Options Cache](chapters/options)
* [How to Structure Your Project](chapters/structure.md)
* [Including Small Projects](chapters/smallinc.md)
* [Including Large Projects](chapters/largeinc.md)
* [Running Other Programs](chapters/programs.md)
* [Testing](chapters/testing.md)
* [Tidy and Format](chapters/testing.md)
* [IDEs](chapters/IDEs.md)

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Now we see even more syntax. Strings are quoted, white space doesn't matter [^3], and the name of the project is the first argument (positional). All the keyword arguments here are optional. The version sets a bunch of variables, like `MyProject_VERSION` and `PROJECT_VERSION`. The languages are C, CXX, FORTRAN, and CUDA (CMake 3.7+). `C CXX` is the default. Docs [here](https://cmake.org/cmake/help/v3.9/command/project.html).
{% hint style='danger' %}
CMake doesn't care about white space, and you can add comments with the `#` character, but never put a comment inside the function call parenthesis.
{% endhint %}
There's really nothing special about the project name. No targets are added at this point.
## Making an executable
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[^2]: You will sometimes see `FATAL_ERROR` here, that was needed to support nice failures when running this in CMake <2.6, which should not be a problem anymore.
[^3]: The `::` syntax was originally intended for `INTERFACE IMPORTED` libraries, which were explicitly supposed to be libraries defined outside the current project. But, because of this, most of the `target_*` commands don't work on `IMPORTED` libraries, making them hard to set up yourself. So don't use the `IMPORTED` keyword for now, and use an `ALIAS` target instead. There's a project in the works to fix this limitation for CMake 3.11.
[^3]: The `::` syntax was originally intended for `INTERFACE IMPORTED` libraries, which were explicitly supposed to be libraries defined outside the current project. But, because of this, most of the `target_*` commands don't work on `IMPORTED` libraries, making them hard to set up yourself. So don't use the `IMPORTED` keyword for now, and use an `ALIAS` target instead. There's a project in the works to fix this limitation for CMake 3.11.

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# C++11 and beyond
C++11 is supported by CMake. Really. Just not in CMake 2.8, because, guess what, C++11 didn't exist in 2009 when 2.0 was released. As long as you are using CMake 3.1 or newer, you should be fine, there are two different ways to enable support. And as you'll soon see, there's even better support in CMake 3.8+. I'll start with that, since this is Modern CMake.
## CMake 3.8+: Meta compiler features
As long as you can require that a user install CMake, you'll have access to the newest way to enable C++ standards. This is the most powerful way, with the nicest syntax and the best support for new standards, and the best target behavior for mixing standards and options. Assuming you have a target named `myTarget`, it looks like this:
```cmake
target_compile_features(myTarget PUBLIC cxx_std_11)
set_target_properties(myTarget PROPERTIES CXX_EXTENSIONS OFF)
```
For the first line, we get to pick between `cxx_std_11`, `cxx_std_14`, and `cxx_std_17`. The second line is optional, but will avoid extensions being added; without it you'd get things like `-std=g++11` replacing `-std=c++11`. The first line even works on `INTERFACE` targets; only actual compiled targets can use the second line.
If a target further down the dependency chain specifies a higher C++ level, this interacts nicely. It's really just a more advanced version of the following method, so it interacts nicely with that, too.
## CMake 3.1+: Compiler features
You can ask for specific compiler features to be available. This was more granular than asking for a C++ version, though it's a bit tricky to pick out just the features a package is using unless you wrote the package and have a good memory. Since this ultimately checks against a list of options CMake knows your compiler supports and picks the highest flag indicated in that list, you don't have to specify all the options you need, just the rarest ones. The syntax is identical to the section above, you just have a list of options to pick instead of `cxx_std_*` options. See the [whole list here](https://cmake.org/cmake/help/v3.9/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html).
If you have optional features, you can use the list `CMAKE_CXX_COMPILE_FEATURES` and use `if(... IN_LIST ...)` from CMake 3.3+ to see if that feature is supported, and add it conditionally. See [the docs here](https://cmake.org/cmake/help/v3.9/manual/cmake-compile-features.7.html) for other use cases.
A related feature, (`WriteCompilerDetectionHeader`)[https://cmake.org/cmake/help/v3.9/module/WriteCompilerDetectionHeader.html], is worth checking out. It is a module that lets you make a file with macros allowing you to check and support optional features for specific compilers. Like any header generator, this will require that you build with CMake so that your header can be generated as part of the build process (only important if you care about supporting multiple build systems, or if you are making a no-build header-only library).
## CMake 3.1+: Global and property settings
There is another way that C++ standards are supported; a specific set of three properties (both global and target level). The global properties are:
```cmake
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
```
The first line sets a C++ standard level, and the second tells CMake to use it, and the final line is optional and ensures `-std=c++11` vs. something like `-std=g++11`. This method isn't bad for a final package, but shouldn't be used by a library. You can also set these values on a target:
```cmake
set_target_properties(myTarget PROPERTIES
CXX_STANDARD 11
CXX_STANDARD_REQUIRED YES
CXX_EXTENSIONS NO
)
```
Which is better, but still doesn't have the sort of explicit control that compiler features have for populating `PRIVATE` and `INTERFACE` properties.
You can find more information about the final two methods on [Craig Scott's useful blog post][crascit].
{% hint style='danger' %}
Don't set manual flags yourself. You'll then become responsible for mainting correct flags for every release of every compiler, error messages for unsupported compilers won't be useful, and some IDEs might not pick up the manual flags.
{% endhint %}
[crascit]: https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/