cmakepp: Template Generation in CMake

Find it at http://github.com/toeb/cmakepp

Download cmakepp standalone file here

As with all my work I appreciate all feedback given and am particularly happy about virtual internet points 🙂

To the Post:

While working on the documentation of cmakepp I grew wary of always doubling the documentation – one in the source code and a mirror in the README.md. Also I was frustrated when I refactored and renamed functions or changed signatures which is not uncommon – especially when working on a new feature. So I needed a tool which could generate the documentation by copying the comment header and the function signature from my CMake files.

While the comment/signature/cmake script parsing is done by cmake_script_parse(...) and is another subject, I still stood before the problem of combining all the information into useful documentation. I especially did not want a too special solution which does not allow for special cases / different formatting. So I took inspiration from some of existing template engines like razor or asp and managed to implement the core logic in less than 60 lines of CMake code.

While my original thought was only ‘how is the best way to generate markdown’ I soon realized that This kind of templating would allow for quite some more domains to be served. For Example generating C++ source code (i.e. data structures) – any kind of text based format could easily be generated.

CMake itself allows you to do something not quite similar. You can use the string(CONFIGURE) and file(CONFIGURE) functions to replace @vars@ within text with values that you have specified. This however will never allow you to generate complex structures as simple as an inline foreach() loop.

When I was finished with implementing my solution I noticed I had a far more mighty tool at hand than I anticipated: I could let the functions which I was documenting run and add the result to my documentation. For Example see the comments for the log function and the resulting README.md

Template Process

The template process is as follows:

                                                       |cmake scope|
                                                            vvv             
(|file| =read=>) |string| =template_compile=> |cmake code| =eval=> |string| (=write=> |file|)

Template Syntax

The template syntax is quite easy:

  • <% %> encloses cmake code
  • <%% and %%> escape <% and %> resp.
  • <%= runs the function specified if possible (only single line function calls allowed) or formats the following string using the format() function (allows things like <%="${var} {format.expression[3].navigation.key}%>)
  • single line function calls are func(.....) but not func(... \n ....)
  • shorthands
    • @<cmake function call> is replaced with <%= <cmake function call> %>
    • @<navigation expression> is replaced with <%= {<navigation expression>}%>

Example

This example takes an template input file and creates output using some given data.  You can also check out my [Samples Repository](#samples) for templating.

*Input file - mytemplate.in*

Hello this is <%={user.first_name}%>,

I am writing you from within a template file!  Here are some things I like:
<% foreach(thing ${things}) %>* The Thing: <%={thing.name}%> - <%={thing.description}%>
<% endforeach() %>

Thanks for Reading! Write me at <%={user.email}%>
(path of this file: <%=${template_path}%>)

CMake

## create a user object
data("{
    first_name:'Tobias',
    last_name:'Becker',
    email:'toeb@thetoeb.de'
}")
ans(user)

## create a list of things
data("[
    {
        name:'Swimming',
        decsription:'humanoid aquatic propulsion'
    },
    {
        name:'Programming',
        decsription:'converting ideas to instructions'
    },
    {
        name:'Squash',
        decsription:'destroying your body while chasing a ball'
    }
]")
ans(things)

## now run the template file

template_run_file("mytemplate.in")
ans(generated_content)

message("${generated_content}")

stdout

Hello this is Tobias,

I am writing you from within a template file!  Here are some things I like:
* The Thing: Swimming - humanoid aquatic propulsion
* The Thing: Programming - converting ideas to instructions
* The Thing: Squash - destroying your body while chasing a ball


Thanks for Reading! Write me at toeb@thetoeb.de
(path of this file: C:/Users/Tobi/Documents/projects/cmakepp/cmake/templating/README.md.in)

Implementation

The implemenation is straightforward: Using CMake‘s regex functionality I split the input template into code fragment s and literal fragments source fragments are such which start with <% and end with %>. Out of these fragments I generate a cmake code listing which can be executed. This is done by appending code fragments directly and appending a template_out(<literal fragment>) for every literal fragment. The template itself works by creating an accumulator variable which can be appended to via template_out .

Some more transformations are performed but the gist ist captured in this description.

Example

template string

Hello <%={user.name}%>, 
<%foreach(i RANGE 1 3)%>* <%=${i}%>
<%endforeach()%>

result of template_compile Note *the strings seem may seem strange. That is because they are cmake_escape

template_begin()
template_out("Hello\ ")
template_out_format({user.name})
template_out(",\ \n")
foreach(i RANGE 1 3)
template_out("*\ ")
template_out_format(${i})
template_out("\n")
endforeach()
 template_end()

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