9.1 KiB
Contributing
Community
Mapnik is a creative community focused on making beautiful maps with beautiful software.
We host our code on github.com/mapnik and encourage anyone interested to fork the repository and provide pull requests or patches for things they want to see added or fixed.
If you just have a question about the code, or a feature you want to discuss then feel free to create a new issue at github.com/mapnik-support.
Code Philosophy
Look through the code to get an idea, and do not hesitate to ask questions.
Also read the design philosophy page for the motivations that lead to code decisions.
Templates are good, within reason. We seek to use templates where possible for flexible code, but not in cases where functional patterns would be just as concise and clear.
Since version 3.0 we use C++11 which brings many advantages and makes the code easier to write and to read.
In general we use Boost, it makes more possible in C++. It is a big build time dependency (as in time to compile against and # of headers) but ultimately compiles to small object code and is very fast (particularly spirit). It also has no dependencies itself (it's really an extension to the C++ language) so requiring it is much easier than requiring a hard dependency that itself has other dependencies. This is a big reason that we prefer AGG to Cairo as our primary renderer. Also AGG produces the best visual output and strikes an excellent balance between speed and thread safety by using very lightweight objects. Cairo not so much.
You will also notice that we don't use many of the standard geo libraries when we could. For instance we don't use GDAL, OGR, or GEOS anywhere in core, and only leverage them in optional plugins. We feel we can often write code that is faster and more thread safe than these libraries but that still does the job. If this ever changes we can adapt and start using these libraries or others as dependencies - nothing is nicer than standing on the shoulders of giants when it makes sense.
Code commits best practices.
Big changes - awesome as pull requests
We love big, aggressive refactoring - but ideally in branches. Even if the changes should go directly into the mainline code and are stable, very big changes are useful to see as a group and branches are cheap. So, branch and commit then create a pull request against master so that other developers can take a quick look. This is a great way for informal code review when a full issue is not warrented.
Commits that fix issues should note the issue
git commit plugins/input/ogr/ -m "implemented sql query in OGR plugin (closes #472)"
Commits that relate to issues should reference them:
git commit tests/python_tests/sqlite_test.py -m "more robust sqlite tests - refs #928"
Commits that add a new feature or fix should be added to the CHANGELOG
Ideally the CHANGELOG can be a very concise place to look for the most important recent development and should not read like a full commit log. So, some developers may prefer to weekly or monthly look back over their commits and summarize all at once with additions to the CHANGELOG. Other developers may prefer to add as they go.
License
Mapnik is licensed LGPL, which means that you are a free to use the code in any of your applications whether they be open source or not. It also means that if you contribute code to Mapnik that others are free to continue using Mapnik in the same way, even with your new additions. If you choose to redistribute an application using Mapnik just make sure to provide any source code modifications you make back to the community. For the actual details see the full LGPL license in the COPYING doc.
Copyright
Mapnik is an open source project and will always be. Your contributions to Mapnik should be motivated by your desire to contribute to a community effort and by the knowledge that your open code will stay that way.
Artem, as the founder and leader of the Mapnik project, is the primary copyright holder and therefore also the primary contact for any current or future license questions around Mapnik. It is important that the copyright holder is respected, trusted, and known to the community so maintaining copyright with Artem is key to maintaining the project as open source.
Therefore all files created by any core developers or patch authors should have a copyright declaration like:
/*****************************************************************************
*
* This file is part of Mapnik (c++ mapping toolkit)
*
* Copyright (C) {YEAR} Artem Pavlenko
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
*****************************************************************************/
Coding Conventions
Mapnik is written in C++, and we try to follow general coding guidelines.
If you see bits of code around that do not follow these please don't hesitate to flag the issue or correct it yourself.
Prefix cmath functions with std::
The avoids ambiguity and potential bugs of using old C library math directly.
So always do std::abs() instead of abs(). Here is a script to fix your code in one fell swoop:
DIR=./bindings
for i in {abs,fabs,tan,sin,cos,floor,ceil,atan2,acos,asin}; do
find $DIR -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/ $i\(/ std::$i\(/g;"
find $DIR -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/\($i\(/\(std::$i\(/g;"
done
Avoid boost::lexical_cast
It's slow both to compile and at runtime.
Avoid sstream objects if possible
They should never be used in performance critical code because they trigger std::locale usage which triggers locks
Spaces not tabs, and avoid trailing whitespace
Indentation is four spaces
Use C++ style casts
static_cast<int>(value); // yes
(int)value; // no
Use const keyword after the type
std::string const& variable_name // preferred, for consistency
const std::string & variable_name // no
Pass built-in types by value, all others by const&
void my_function(int double val); // if int, char, double, etc pass by value
void my_function(std::string const& val); // if std::string or user type, pass by const&
When to use shared_ptr and unique_ptr
Sparingly, always prefer passing objects as const& except where using share_ptr or unique_ptr express more clearly your intent. See http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/ for more details.
Shared pointers should be created with boost::make_shared where possible
Since Mapnik 3.0 use std::make_shared.
Use assignment operator for zero initialized numbers
double num = 0; // please
double num(0); // no
Function definitions should not be separated from their arguments:
void foo(int a) // please
void foo (int a) // no
Separate arguments by a single space:
void foo(int a, float b) // please
void foo(int a,float b) // no
Space between operators:
if (a == b) // please
if(a==b) // no
Braces should always be used:
if (!file)
{
throw mapnik::datasource_exception("not found"); // please
}
if (!file)
throw mapnik::datasource_exception("not found"); // no
Braces should be on a separate line:
if (a == b)
{
int z = 5;
// more...
}
Prefer empty() over size() == 0 if container supports it
This avoids implicit conversions to bool and reduces compiler warnings.
if (container.empty()) // please
if (container.size() == 0) // no
Other C++ style resources
Many also follow the useful Google style guide which mostly fits our style. However, Google obviously has to maintain a lot of aging codebases. Mapnik can move faster, so we don't follow all of those style recommendations.
Emacs helper
To auto-convert to the above syntax you can put this in an .emacs file:
;; mapnik c++
(setq c-default-style "bsd")
;; no tabs please
(setq indent-tabs-mode nil)
;; ident by four spaces
(setq c-basic-offset 4)
;; don't ident inside namespace decl
(c-set-offset 'innamespace 0)
;;
(c-set-offset 'template-args-cont 'c-lineup-template-args)