Style guide for contributors

When contributing to DIPlib, please follow the style and layout of other files. Specifically:

Programming style

  • Everything should be declared within the dip namespace, or a sub-namespace. The exception is functionality that interfaces DIPlib with other libraries or software, which should be defined in their own namespaces (e.g. the dml namespace for the DIPlib–MATLAB interface).

  • Don’t put dip:: in front of every identifier within the library code, but do always do so for dip::sint and dip::uint, as they might be confused with types commonly present in the base namespace or as preprocessor macros.

  • Do explicitly state the namespace for identifiers from every other library, including std::. This makes it easier to find references to specific types or functions.

  • All functions local to a translation unit must either be declared static or within an unnamed (anonymous) namespace, to prevent name space pollution and to prevent them from being exported out of the library.

  • Prefer using over typedef.

  • Use struct for classes without any private members.

  • Do not declare more than one variable on the same line of code. That is, avoid things like float *a, b = 2.5, *c = NULL, which is confusing.

  • Declare variables where they are first initialized, or as close as possible to that point. Uninitialized variables need some description.

Naming conventions

  • Use camel case for variable, function and class names. Variable names start with a lowercase letter, function and class names start with an uppercase letter. Don’t use underscores except for in a few special cases. Private class member variables end in an underscore. Constants are in all uppercase letters, with underscores to separate words if necessary. Internal names (the ones that are not meant to be used directly by library users), if public, do not have documentation and clearly warn the user by their name (such as dip::Image::ShiftOriginUnsafe), or are declared in the dip::detail namespace; private internal names (declared static or inside an unnamed namespace) do not have any specific requirements.

  • Setter member functions start with Set. But getter member functions do not start with Get. Query functions that return a boolean start with Is. Member functions that do something have a name that resembles a verb: Forge, Convert, PermuteDimensions.

  • The exception is in classes such as dip::DimensionArray, which is meant to emulate the std::vector class, and therefore follows the naming convention of the C++ Standard Library. Also, most classes define a swap operator that needs to be named as in the C++ Standard Library to be useful.

  • Use all uppercase letters for preprocessor macros. Separate words with an underscore. Macros always start with DIP_. Include guards also start with DIP_, and end with _H; the part in the middle is an all-uppercase version of the file name.

  • File names are in all lowercase and use underscores between words. There’s no need to shorten names to 8 characters, so don’t make the names cryptic.


  • All loops and conditional statements should be surrounded by braces, even if they are only one statement long.

  • Indents are three spaces, don’t use tab characters. Continuation indents are double regular indents, as are the indents for class definitions. Namespace scope is not indented.

  • The keyword const comes after the type name it modifies: dip::Image const& img.

  • Braces and brackets have spaces on the inside, not the outside.

  • Keep each statement on its own line.

Function signatures

  • Option parameters to high-level functions (those that should be available in interfaces to other languages such as MATLAB) should be strings or string arrays, which are easier to translate to scripted languages.

  • Option parameters to low-level functions (those that are meant to be called only from C++ code) should be defined as enum class, these are simpler and more efficient than strings. Use the DIP_DECLARE_OPTIONS macro to turn the enumerator into a flag set. Declare these option types within the dip::Option:: namespace or another sub-namespace if more appropriate.

  • Don’t use bool as a function parameter, prefer meaningful strings in high-level functions (e.g. “black”/”white”), and enum class with two options defined in dip::Option:: namespace for low-level functions. Only private functions can deviate from this.

  • Multiple return values are preferably combined in a struct, rather than a std::tuple or similar, as a struct has named members and is easier to use. Output should rarely be put into the function’s argument list, with the exception of images (see Function signatures).

  • For every function that produces an output image, there should be two signatures, the main one with the out image as an input argument, and a second one, defined as inline in the header file, as follows:

    inline Image Function( Image const& in, ... ) {
       Image out;
       Function( in, out, ... );
       return out;
  • Add default values to as many input parameters as possible in the high-level functions. Sort the parameters such that the more important ones (the ones that the user is most likely to want to set) appear first. Image input parameters always appear first, with input image as first parameter, and output image as last image parameter:

    void Function(
       Image const& in,
       Image const& kernel,
       Image& out,
       dfloat size = 1,
       BooleanArray process = {}

Header files

  • In header files that define “modules” or parts of them (e.g. diplib/linear.h), always first include diplib.h. In header files that define helper classes and don’t reference the dip::Image object, you can instead include only the minimal subset of header files (e.g. include only diplib/library/types.h).

  • Try to pull in as few header files as possible in public header files. Include those header files that define any function input types (e.g. in diplib/linear.h we include diplib/kernel.h). Types that are output arguments can be forward-declared if that makes sense. For example, dip::Kernel has a method that creates a dip::PixelTable, which is used mostly internally. Most users of dip::Kernel will not use this method. Therefore, there is no point in pulling in diplib/pixel_table.h for all users of diplib/kernel.h, and we forward-declare dip::PixelTable instead.

  • Some STL containers such as std::vector and std::set are defined when including diplib.h. There is no need to explicitly include the corresponding standard headers for these. The current list of headers guaranteed to be included by diplib.h is: <algorithm>, <cctype>, <cmath>, <complex>, <cstddef>, <cstdint>, <cstdlib>, <cstring>, <exception>, <functional>, <initializer_list>, <iostream>, <iterator>, <limits>, <memory>, <numeric>, <set>, <string>, <type_traits>, <utility>, <vector>.


  • The first line of a documentation block is the brief string. You can add a \brief command to extend the brief string to a whole paragraph, but please keep it brief. Add additional paragraphs of documentation if needed.

  • Add \see to link to functions in other modules, or in the same module but “far away”. Linking to the next or previous function is not very important, but do so if it makes sense and there is a \see section anyway. Note that the documentation lists the functions in the same order as that they appear in the header files.

  • Use !!! par "name" for a named info box. It will be rendered with a gray background.

  • Use !!! attention and !!! warning for two levels of attention-grabbing info boxes. These are rendered with lighter and darker 2nd color background (highly contrasting!).

  • Don’t use !!! note or !!! remark.