JeVois  1.17
JeVois Smart Embedded Machine Vision Toolkit
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General programming guidelines for contributors to core JeVois framework
These programming guidelines are for programmers who wish to contribute to the core JeVois framework ( and are quite strict. For programmers who just wish to program a new machine vision module that uses the JeVois core framework, quidelines are much looser and are described in the documentation of jevoisbase.

General rules and indentation

  • JeVois uses doxygen for documentation, with the exclamation mark (!) comment style. You should learn about doxygen and look at how the rest of JeVois is documented before you start writing code. Undocumented or improperly documented code is subject to deletion. Also see here for more explanations of tricky documentation cases (once you know the basics of doxygen).
  • JeVois treats all warnings as errors. So your code will not compile if you have warnings. This is a feature: if your code has warnings, you are doing something wrong! If you are including files from a library not written by you and that has warnings, you can use the following:
  • Source code line length is 120. Add this to your ~/.emacs to set it:
  ;; set default line wrap len:
  (setq default-fill-column 120)
  • JeVois uses astyle to ensure uniform indentation styles across our various text editors. A style file is included with JeVois, and can be found in jevois/scripts/astylerc. To use this style file, invoke astyle using the –options flag, e.g.
    astyle myfile --options=/path/to/jevois/scripts/astylerc
  • Indentation matching this style is as defined by the following emacs rules, on top of the default C mode of emacs indentation (ad this to your ~/.emacs to activate):
      ;; JeVois indentation style for C++ and such
      (defun my-c-mode-common-hook ()
        (local-set-key "\C-h" 'backward-delete-char)
        ;; this will make sure spaces are used instead of tabs
        (setq tab-width 4 indent-tabs-mode nil)
        (setq indent-tabs-mode 'nil)
        (setq c-basic-offset 2)
        (c-set-offset 'substatement-open 0)
        (c-set-offset 'statement-case-open 0)
        (c-set-offset 'case-label 0)
        (c-set-offset 'brace-list-open 0)
        (c-set-offset 'access-label -2)
        (c-set-offset 'inclass 4)
        (c-set-offset 'member-init-intro 4)
        ;; include possible ! as comment start string so that indentation starts after it
        (setq comment-start-skip "/\\*+!* *\\|//+ *")
        ;; type C-c C-s or C-c C-o while editing to see what other rules to add here...
      (add-hook 'c-mode-hook 'my-c-mode-common-hook)
      (add-hook 'c++-mode-hook 'my-c-mode-common-hook)
      (add-hook 'perl-mode-hook 'my-c-mode-common-hook)
      (add-hook 'cperl-mode-hook 'my-c-mode-common-hook)
      (add-hook 'emacs-lisp-mode-hook 'my-c-mode-common-hook)
      (add-hook 'nroff-mode-hook 'my-c-mode-common-hook)
      (add-hook 'tcl-mode-hook 'my-c-mode-common-hook)
      (add-hook 'makefile-mode-hook 'my-c-mode-common-hook)
  • If you use vi, use this configuration:
      "-------------Essential JeVois Style Compliance Settings-------------
      " Disable old-school vi compatability
      set nocompatible
      " Allow plugins to control our indentation
      filetype plugin indent on
      " Set each auto-indent level to equal two spaces
      set shiftwidth=2
      " Let each tab equal two spaces
      set tabstop=2
      " Make sure vim turns all tabs into spaces
      set expandtab
      " Make vim indent our code properly
      set smartindent
      " Make the maximum line length equal 120
      set textwidth=120
      "-------------Other cool vim tricks-------------
      " Use a cool menu when autocompleting filenames, commands, etc...
      set wildmenu
      set wildmode=list:longest
      " Make vim automatically change directories to the directory of any file you open. 
      " This means that when you open a file, then want to open another using :tabe, :o, etc,
      " you can just type in the relative path from the file you're currently editing.
      set autochdir
      " When editing the JeVois library, it is a total pain when you are editing a .H file in jevois/include/whatever/whatever, 
      " and then decide you need to edit the source .C file in the jevois/src/whatever/whatever. This little function will 
      " automatically back track in the directory tree for you, find the corresponding .C or .H file, and open it in a new
      " tab. 
      " To use it, just type ,o (that's a comma, and then a lower-case o). 
      function! OpenOther()
        if expand("%:e") == "C"
          exe "tabe" fnameescape(expand("%:p:r:s?src?include?").".H")
        elseif expand("%:e") == "H"
          exe "tabe" fnameescape(expand("%:p:r:s?include?src?").".C")
      nmap ,o :call OpenOther()<CR>

Files organization

  • JeVois uses exclusively filename extensions .H and .C for C++ files.
  • JeVois uses CamelCase convention for file names and for class names. An exception is helper classes used for template meta-programming, which usually use lowercase_with_underscores names, to resemble the STL (see more details below).
  • The JeVois Library is mainly split into the jevois/include/jevois/ and jevois/src/jevois/ directories. These directories are then further split into Core, Debug, Image, etc. The build system creates a shared library, from these files. Vision modules programmers use the JeVois library by including the appropriate header (.H) files under jevois/include and by linking against (details below). When adding to the core JeVois framework (as opposed to just writing new vision modules), please keep in mind:
    • Each directory under jevois/ should contain files which can be logically grouped together - e.g. the Debug directory is for debugging functions and classes, Image is for image processing, etc. Each of these directories contains a group of classes which implement a specific feature.
    • All header files must be named with a .H extension, and should go into the include directory. Header files should be split such that all backend functionality which users don't need do know about should be hidden away in a details subdirectory. In general, such backend functionality can be split into a Helpers file which contains any helper classes and can be included before the body of a class definition, and a Impl implementation file which includes any inline code. For example, the Parameter framework involves the main definitions and programming interface in jevois/include/jevois/Component/Parameter.H, which relies on some helper classes (which programmers using Parameter do not need to knwo about) defined in jevois/include/jevois/Component/details/ParameterHelpers.H, and has some inline implementation code (which programmers using Parameter also do not need to know about) defined in jevois/include/jevois/Component/details/ParameterImpl.H. This organization makes it easy to generate user documentation that omits all of the nasty implementation details.
    • All non-inline, non-template implementation source files should be named with a .C extension, and should go into the jevois/src/ directory following the same relative path as its .H file.
    • As much code as possible should be moved into .C files so that users don't have to recompile our .H files over and over again. The big exception to this is template code, which in most cases must go into .H files.
    • All header and source files should include the boilerplate preamble/license. Just copy it from another file.
  • Class member variables have names starting with "its" to indicate to humans reading the code that they are member variables.
  • JeVois member functions use camelCase starting with a lowercase letter.
  • JeVois free functions usually use name_with_underscores, or camelCase starting with a lowercase letter.
  • Generally speaking, one file per class. File name and class name must match exactly. If several classes conceptually belong together, then it is ok to put them in the same file.
  • All .H files use include guards, which will prevent the file from being included several times, thereby generating some errors about things being re-defined. No .C file uses include guards. We use a pragma supported by g++ for include guards:
#pragma once
// ....
  • A new class will hence typically involve the following set of files:

    • include/jevois/XXX/MyClass.H: only contains declarations and documentation. Absolutely no actual implementation code. Only declare in this file things of interest to programmers who will use your class and who do not need to care about exactly how it works internally. Everything in this file should be documented using doxygen markup.
    • include/jevois/XXX/details/MyClassHelpers.H: contains supporting declarations that must be known before the main declarations in MyClass.H can take effect. For example, if the end user will only use a derived class and the base class contains no information that they should care about, declare the base class in details/MyClassHelpers.H and towards the top of MyClass.H include details/MyClassHelpers.H. There is no doxygen markup in this file, and documentation is optional, mainly geared towards advanced programmers.
    • include/jevois/XXX/details/MyClassImpl.H: contains inlined and template implementation ONLY. There is no doxygen markup in this file, and documentation is optional, mainly geared towards advanced programmers.
    • src/jevois/XXX/MyClass.C: contains all non-template, non-inline implementation. Documentation is optional.

    See for example the following files:

    • jevois/include/jevois/Component/details/ParameterHelpers.H preliminaries of no interest to programmers just using the JeVois framework
    • jevois/include/jevois/Component/Parameter.H documented interface for programmers using the JeVois framework
    • jevois/include/jevois/Component/details/ParameterImpl.H inline functions and templates implementation of no interest to programmers just using the JeVois framework
    • jevois/src/jevois/Component/Parameter.C does not exist because Parameter is a fully-templated, fully inline class, but could contain any non-template, non-inline implementation of no interest to programmers just using the JeVois framework
  • To find particular words in JeVois source code, we recommend adding the following macro to your ~/.bash_aliases or ~/.bashrc:
    # do a grep on c sources (e.g., for kernel, u-boot, etc) or c++ sources (for JeVois, NRT, etc)
    xg () {
       grep $* `find . -name "*.[hcHC]"`

You can use it as follows, for example to find all files that refer to the function warnAndRethrowException() provided by JeVois to help programmers with handling of exceptions:

itti@iLab1:~/jevois/software/jevois$ xg warnAndRethrowException
./include/jevois/Debug/Log.H:      try { do_something_risky(); } catch (...) { jevois::warnAndRethrowException(); }
./include/jevois/Debug/Log.H:  void warnAndRethrowException[[noreturn]]();
./src/jevois/Debug/Log.C:void jevois::warnAndRethrowException()

Reveals that it is declared in Log.H and implemented in Log.C, so you can now open those files for more information.

Const Correctness

JeVois uses exclusively the right-to-left convention for const qualifications. This is because it is the best way to unambiguously read statements that have const in them, just read them aloud from right to left. For example:

int const * prt1; // ptr1 is pointer to const int (read from right to left; can change pointer address, but not the int value pointed to)
int * const ptr2; // ptr2 is a const pointer to int (cannot change the address, but can change the int value)
int const * const ptr3; // ptr3 is a const pointer to a const int (const address, const value)

See for more details and examples. Also see this one: and that one:

  • JeVois is const-correct code (except for bugs and omissions!). When writing jevois code, make sure it is const-correct. For example:
    • input arguments to a function are typically received by const reference. For example:
      void myfunc(std::string const & arg);
    • a member function that does not modify any member variables of an object should be declared const. For example:
      class MyClass
      int getX() const
      { return x; } // since we do not modify x or anything else in MyClass, getX() is declared const
      int x;
    • a temporary variable that will not be modified should be declared const. For example:
      double const perimeter = 2.0 * M_PI * radius; // assuming perimeter will not be modified later
    • a mutex in a class should typically be declared mutable, which will allow it to be locked/unlocked on const instances of the class or in const member functions of the class.
    • For return values of functions: return a const ref if possible (i.e., you are returning a const ref to something which will not disappear soon, typically use this for accessor functions of your classes, when returning one of the data members of the class), otherwise return by value (non-const). While returning by non-const value may pose dangers, it also allows move semantics. This is particularly important if you return a vector, a string, etc as by returning it by non-const value it will actually be moved (in most cases) as opposed to copied.
    • on rare occasions, it is more desirable to keep a member function const even though it might modify some member non-mutable variables of a class, if logically speaking this makes more sense. You have to use your judgments for these cases, and use a const_cast inside the function. For example, consider the implementation of the className() accessor function in Component, which returns the name of the component's class. itsClassName is declared in Component.H as a const string data member of Component, which makes sense since the class name cannot be modified for any Component. As for all const data members, the only way to assign a value to itsClassName is hence during construction. But we would like to report the class name of any class derived from Component, which is not accessible during construction of the base Component class. For this reason, we will set the class name the first time it is requested (at which point we know the object derived from Component will be fully constructed):
      std::string const & jevois::Component::className() const
      boost::shared_lock<boost::shared_mutex> lck(itsMetaMtx);
      // We need the (derived!) component to be fully constructed for demangle to work, hence the const_cast here:
      if (itsClassName.empty()) *(const_cast<std::string *>(&itsClassName)) = jevois::demangle(typeid(*this).name());
      return itsClassName;
      so we use const_cast to set the const member variable itsClassName the first time it is requested.
    • see for more details and examples.

Local variables

  • Local variable names should favor longer more descriptive names over shorter ones. E.g.
// Bad...
for (int i = 0; i < c; ++i);
// Better...
for (int itemIdx = 0; itemIdx < itemCount; ++itemIdx);

Proper use of inline, virtual, override, final, etc

  • In declarations, do not write inline (e.g., in class declarations). Inline is an implementation detail and people just looking at your interface (declarations) should not be bothered with it.
  • In definitions (implementation of functions), that's the right place to add inline. For template functions, add inline to the same line as the template.


template <class T>
class Stuff
template <class T> inline
{ ... }
  • The rule for virtual, override, final is the opposite as for inline: specify it in your declarations (users of your classes need to know what is virtual and can safely be reimplemented by derived classes), omit it from your definitions (once a function has been declared virtual, it will stay that way).

Capitalization rules

  • Class names should be written in CamelCase starting with an uppercase character, for example:
class MyCoolClass;
class ImageSegmenter;
  • Variables should be camelCase starting with a lowercase character, for example:
    bool isRunning;
    size_t arraySize;
  • Member variables should be camelCase starting with the prefix “its”, for example:
class MyClass
size_t itsCounter;
std::vector<int> itsStorage;
  • Function names should be camelCase starting with a lowercase character, for example:
void doSomething(int paramOne);
class MyClass
void doSomethingElse(int paramTwo);
  • Typedef's which are just simple aliases for types should be CamelCase starting with an uppercase character, for example:
typedef jevois::Dims<float> FloatingDims;
  • Anything that is the result of any fancy metaprogramming (e.g. static const variables or typedefs set by compile-time checks) should be written in all lowercase with underscores separating the words, for example:
template <class T1, class T2>
struct simple_promotion
// Even though this is a typedef, we use lowercase and an underscore to emphasize
// that it is part of some fancy metaprogramming
typedef decltype(*(T1*)nullptr + *(T2*)nullptr)) promoted_type;
const std::string & className() const
The class name of this component.
Definition: Component.C:39
Definition: Concepts.dox:1
std::string demangle(std::string const &mangledName)
Demangle a mangled name.
Definition: CompilerUtil.H:99
Definition: CompilerUtil.H:84