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Coding conventions

General coding rules

This section provides lists of general coding rules for the GammaLib development.

C++ rules

All GammaLib C++ code must be compatible with ISO standard C++98. Do not use C++11 features (see here if you don’t know what C++98 and C++11 is and note that C++03 is just a minor correction of C++98).

The reason for this restriction is that C++ compilers / standard libraries such as gcc / libstdc++ or clang / libc++ only finished implementing C++11 in 2013 and using C++11 features in GammaLib would make it impossible to use GammaLib on user and server machines with an old compiler.

Code format

  • Blocks are indented by 4 characters.

  • Do not use tabs (code formatting should be independent of editor configurations).

  • Do not exceed a line length of 80 characters (a few more characters are acceptable in exceptional cases).

  • Put a blank line at the end of each file (this is required by some compilers).

  • Each function starts with a curly bracket in the line following the function name. The return type is in the same line as the function name. Example:

    int function(void)
        int i = 0;
        return i;
  • Code blocks should be encompassed in curly brackets, even if the block consists only of a single line.

  • The opening curly bracket of a block starts in the same line as the related statement. Example:

    for (int i = 0; i < 10; ++i) {
        sum += i;
  • Separate code elements by spaces; for example, do not write int i=0; but write int i = 0;.

  • Align successive similar lines on common elements. Here a common example illustrating the alignment on the = symbol:

    m_max    = par.m_max;
    m_prompt = par.m_prompt;
    sum     += par.m_sum;

    And here an example illustrating the alignment in a class definition on the member function name:

    void        log10GeV(const double& eng);
    void        log10TeV(const double& eng);
    std::string print(void) const;

Code semantics

  • Each function and/or method terminates with a return statement.

  • Each function and/or method has only a single exit point (i.e. a single return statement).

  • Use explicit for constructors with single arguments to prevent unintended type conversions. The only exception to this rule is the copy constructor or type conversion constructors.

  • Specify void for function definitions without arguments, i.e. use void function(void) instead of void function() in the function declaration.

  • Use pre-incrementation in loops (pre-incrementation is faster than post-incrementation). Example:

    for (int i = 0; i < 10; ++i) {
        sum += i;
  • Where possible (and appropriate), use std::vector containers instead of allocating memory. In other words: avoid direct memory allocation with new.

  • Use the std:: namespace prefix where possible; write for example


    You may not believe it, but droping the std:: may on some systems lead to considerably slower code for trigonometric functions!

  • Provide comments, comments, comments!!!

Language features

  • Do not use macros.
  • Do not use #define directives for the declaration of constants. Use const instead.
  • Do not use std::strncpy, std::memcpy or similar as these functions are corrupted on some systems.
  • If possible, pass arguments by reference.
  • Output arguments should be passed as pointers.
  • Use C++ (std::string) instead of C-style (char*) strings.
  • Use C++ casts instead of C-style casts.
  • Avoid using templates.
  • Do not use an integer for a floating point argument (i.e. write 10.0 instead of 10). Some older compilers give an error when using integers in some floating point functions, such as log10().

Python rules

All code must be compatible with Python 2.6, 2.7 as well as 3.2 or later. As of 2015 most users still have Python 2, even though Python 3 has been out for seven years, so using Python 3 features is out of question. Also GammaLib is mostly a C++ library with only some high-level analysis scripts (e.g. running a complete simulation or analysis or plotting some results) that are simple Python code and wouldn’t profit much from using new Python 3 features.

Code format

  • Python code should follow the official PEP8 Python style: It says (among many other things) that you should indent with four spaces, not tabs. Following PEP8 is simple, because there’s a pep8 tool that you should run on your Python code before committing, e.g. like so:

    $ pep8 test/
    test/ W191 indentation contains tabs
    test/ W391 blank line at end of file
    test/ W293 blank line contains whitespace

    If you want you can even use the autopep8 tool which can automatically fix the formatting for almost all cases. Run pep8 -h and autopep8 -h to see the options you can use. PEP8 compliance is automatically checked by the continuous integration system.

Code semantics

  • When writing scripts, the following import conventions should be used:

    >>> import gammalib
    >>> energy = gammalib.GEnergy(10, 'TeV')

    In interactive sessions (IPython is recommended) to save typing you can use import * to import everything into your local namespace:

    >>> from gammalib import *
    >>> energy = GEnergy(10, 'TeV')

    Using import * in Python scripts is not recommended. Some reason are given in this section in the Python docs.

  • Most GammaLib code can be written equivalently in C++ and / or Python, however some things have been made “Pythonic” in the SWIG Python wrapper. GammaLib objects can be printed by passing them to the Python print function. GammaLib container classes feature a Pythonic list-like interface, i.e. they can be iterated over and elements can be accessed with []. Examples:

    # Create a container with two elements
    >>> from gammalib import GEnergy, GEnergies
    >>> energies = GEnergies()
    >>> energies.append(GEnergy(1, 'TeV'))
    >>> energies.append(GEnergy(3, 'TeV'))
    # Print GammaLib object Python-style.
    # C++ style would be calling a member function: energies[1].print()
    >>> print(energies[1])
    2 TeV
    # Use the container like a Python list
    >>> len(energies)
    >>> for energy in energies:
    1000 GeV
    2 TeV

Language features

You can find lots of information on how to write Python code that works with Python 2 and 3 on the web, e.g. here and here. If you don’t have a Python 2 or Python 3 interpreter on your development machine, don’t worry. This will be checked during code review or during continuous integration testing.

The most common Python 2 / 3 issues are print and integer division.

  • You should use the print() function, because the print statement was removed in Python 3:

    >>> print('hello')  # print function. Good. Works in Python 2 and 3.
    >>> print 'hello'   # print statement. Bad. Only works in Python 2.
    SyntaxError: invalid syntax
  • Integer division behaves differently in Python 2 and 3.

    >>> 5 / 2  # Python 3: integer division results in float
    >>> 5 / 2  # Python 2: integer division results in int

    If you do need integer division in your Python code, you should use one of these explicit forms that work in Python 2 and 3:

    >>> int(5 / 2)  # Python 2 and 3: truncate int part
    >>> 5 // 2  # Python 2 and 3: integer division

A good way to enforce Python 3 compatible behaviour in your script concerning print and integer division even when running it in Python 2 is to put the following line at the top of your Python script:

from __future__ import print_function, division