I read the documentation.
It says:
A CMake Generator is responsible for writing the input files for a native build system.
What exactly does that mean?
If I have a set of C++ files in my project, are these the input files?
If I'm using Linux, what is my native build system by default? Make?
Why do the input files have to be written by the generator if they already exist?
What's a generator?
To understand what a generator is, we need to first look at what is a build system. CMake doesn't compile or link any source files. It used a generator to create configuration files for a build system. The build system uses those files to compile and link source code files.
So what's a build system?
A build system is a broad term that groups together a set of tools used to generally compile and link source code, but it can also include auxiliary tools used during a build process.
For example, in a multi-stage build system, one executable might be built to be used in the build process of another build.
Depending on the tool chain used on a system, CMake will generate multiple files and folders to allow the building of the source files referenced in the CMakeLists.txt and supporting .cmake files.
Sometimes multiple build systems may be installed on a computer, like for Windows you could have a Visual Studio and MinGW build system. CMake allows you to specify which if these build systems to generate configuration files for.
CMake includes a number of Command-Line, IDE, and Extra generators.
Command-Line Build Tool Generators
These generators are for command-line build tools, like Make and Ninja. The chosen tool chain must be configured prior to generating the build system with CMake.
The following are supported(**):
Makefile Generators
Ninja Generators
IDE Build Tool Generators
These generators are for Integrated Development Environments that include their own compiler. Examples are Visual Studio and Xcode which include a compiler natively.
The following are supported(**):
Extra Generators
These are generators that create a configuration to work with an alternative IDE tool and must be included with either an IDE or Command-Line generator.
The following are supported(**):
If I have a set of C++ files in my project, are these the input files?
Yes, they are some of the input files. For a make build system you also have a MakeFile. For Visual Studio you have a solution file (.sln). With both systems there are additional files needed that CMake knows how to create given a proper CMakeLists.txt file.
If I'm using Linux, what is my native build system by default? Make?
Generally, yes, but other build systems could be setup like Ninja.
Why do the input files have to be written by the generator if they already exist?
Some source files may already exist, but CMake has the ability to generate header and source files. Also as mentioned above, there are configuration files that must be generated that depend on the source files supplied in the CMakeLists.txt file.
** According to the documentation for CMake Version 3.9 & 3.15 & 3.25
Cmake -h, the help output will list which generators are available.
Maybe a picture is worth a thousand words.
If there is any mistake in the picture, please correct me to avoiding misleading. Nice picture. I have considered making one myself. Your representation is closer to my mental image but the key concept in my mental image is that it is a tree with branches and as you descend down the tree each branch at that level is independent of a sibling branch. A branch can be dependent upon its parent but is independent of its sibling, so the child rectangles spanning more than one parent should be independent, e.g. compiler. Continued
Commented
Aug 9, 2022 at 13:59
A CMake Generator is responsible for writing the input files for a native build system.
means that CMake prepares build scripts for a native build system when no generator is specified. In Linux the default build system is Make and its input file are makefiles, which are then interpreted and a build is executed accordingly. Before the first execution of CMake build scripts do not exist.
C++ source files (or any other source files) are not input files to a build system. Build system scripts specify how to handle source file in order to produce binary executables.
