NEST coding style guidelines for C++

In the code review process we want to enforce a consistent coding style to improve readability and maintainability. The article on why code readability matters describes the benefits of readable code. To simplify the process we use different tools that check compliance with our coding style and developers can reduce the workload of the review process by checking compliance of their code on their own.

See also

Before you make a pull request see which developer tools are required to ensure its compliant with our guidelines.

C++ language features

  1. Use only ISO C++ language features.

  2. Prefer ISO C++ library functions over their ISO C library equivalents.

  3. Prefer ISO C++ library containers (STL).

  4. Prefer C++ headers over their C equivalents.

  5. Don’t use printf and related functions.

  6. Use C++ style cast notation (see Books).

  7. Use the const qualifier where appropriate. Use it consistently (see Books)!

  8. Use namespaces and exceptions.

  9. Try to avoid static class members which need a constructor (non POD).

Language of comments and identifiers

  1. All comments should be written in English.

  2. All identifiers, class and function names should be in English.

Debugging and quality control

Use the assert macro intensively to check program invariants. Create expressive unit-tests using one of the supplied SLI and Python unit-testing infrastructure or the C++ testing framework based on Boost.

Compiler

NEST compiles with any recent version of the GNU C/C++ Compiler gcc. Support for and limitations of other compilers is described in the Installation Instructions

Resources

Online reference documents

  1. C++ Reference

  2. C++ Wikibooks

Books

We have found the following books to be useful.

  1. Stroustrup B (1997) The C++ Programming Language, 3rd Edition, Addison-Wesley

  2. Meyers S (1997) Effective C++, 2nd Edition, Addison Wesley

  3. Meyers S (1996) More Effective C++, Addison Wesley

  4. Coplien J O (1992) Advanced C++ programming styles and idioms, Addison-Wesley

  5. Eckle B (1995) Thinking in C++, Prentice Hall

  6. Plauger P J, Stepanov A, Lee M, and Musser D R (1998) The Standard Template Library, Comming June 1998, 1. Prentice Hall

  7. Plauger P J (1995) The (draft) Standard C++ Library, Prentice Hall

  8. Musser D R and Saini A (1996) STL Tutorial and Reference Guide, Addison-Wesley

  9. Kernighan B and Ritchie D (1988) The C Programming Language, 2nd Edition, Prentice Hall


Coding style

In the following the coding style guidelines are explained by example and some parts are adopted from Google C++ Style Guide.

The #define guard

All header files should have #define guards to prevent multiple inclusions. The format of the symbol name should be <FILE>_H. The file iaf_cond_alpha.h should have the following guard:

#ifndef IAF_COND_ALPHA_H
#define IAF_COND_ALPHA_H
...
#endif  // IAF_COND_ALPHA_H

Order of includes

Use standard order for readability and to avoid hidden dependencies: Related header, C library, C++ library, other libraries’ .h, your project’s .h.

NEST’s Makefiles add all project specific include paths to the compile commands, thus the file iaf_cond_alpha.h should be included as: #include "iaf_cond_alpha.h"

In iaf_cond_alpha.cpp, whose main purpose is to implement iaf_cond_alpha.h, order your includes as follows:

  1. iaf_cond_alpha.h.

  2. C system files.

  3. C++ system files.

  4. Other libraries’ .h files.

  5. Your project’s .h files.

With the preferred ordering, if iaf_cond_alpha.h omits any necessary includes, the build of iaf_cond_alpha.cpp will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.

Within each section the includes should be ordered alphabetically.

You should include all the headers that define the symbols you rely upon (except in cases of forward declaration). If you rely on symbols from bar.h, don’t count on the fact that you included foo.h which (currently) includes bar.h: include bar.h yourself, unless foo.h explicitly demonstrates its intent to provide you the symbols of bar.h. However, any includes present in the related header do not need to be included again in the related cpp (i.e., foo.cpp can rely on foo.h’s includes).

For example, the includes in <nestdir>/models/iaf_cond_alpha.cpp might look like this:

#include "iaf_cond_alpha.h"

#include <sys/types.h>
#include <unistd.h>
#include <hash_map>
#include <vector>

#include "config.h"
#include "foo.h"
#include "node.h"

Exception

Sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized. Example:

#include "iaf_cond_alpha.h"

#include "port.h"  // For LANG_CXX11.

#ifdef LANG_CXX11
#include <initializer_list>
#endif  // LANG_CXX11

Indentation, formatting and naming

Files

Files are named in lower_case_under_lined notation. C/C++ header files have the extension .h. C implementation files have the extension .c. C++ implementation files have the extension .cpp. The use of .cc is deprecated and is only left for compatibility.

All files in NEST start with a preamble, which contains the filename and the NEST copyright text (see example below).

  • Lines should not exceed 120 character

  • Files should not be too long (max. 2000 lines)

  • No trailing whitespace

Folders

Use lower_case_under_lined notation for folder names.

Variables and class members

In general, use meaningful, non-abbreviated names or follow naming conventions from the neuroscience field, for example, the membrane potential is V_m. Use the lower_case_under_lined notation. Private member variables should end with an underscore (name_). If applicable, the general rule is use is to use the same notation for biophysical quantities as is used in Dayan&Abbot, 2001.

Constants should be defined with enums and not with #define, and use the UPPER_CASE_UNDER_LINED notation:

enum StateVecElems
{
  V_M = 0,
  DG_EXC,
  G_EXC,
  DG_INH,
  G_INH,
  STATE_VEC_SIZE
};

Built-in types

All code for the NEST kernel should use the type aliases, defined in nest.h. Thus, use nest::float_t instead of float.

Functions and class methods

In general, use meaningful, non-abbreviated names or follow naming conventions from the neuroscience field, e.g. the membrane potential is V_m. Use the lower_case_under_lined notation.

There should be a line-break after the method’s return type (implementation only). Parameters of methods should either fit into one line or each parameter is on a separate line.

inline void
nest::Stopwatch::print( const char* msg,
                        timeunit_t timeunit,
                        std::ostream& os ) const
{
  // code
}

Namespaces

Use lower_case_under_lined notation for namespaces. Do not use using namespace statements in header files. The closing brace of a namespace should be followed by a comment containing the namespace statement. Do not indent the body of namespaces.

namespace example
{
// code
} // namespace example

All symbols for the NEST kernel are declared in the namespace nest.

Structs and classes

Use a struct only for passive objects that carry data; everything else is a class. Use CamelCase notation for naming classes, structs and enums, e.g. GenericConnBuilderFactory. Private, nested classes and structs end with an underscore (State_).

The access modifier (public, protected, private) in class definitions are not indented.

Do not implement methods inside the class definition, but implement small inline methods after the class definition and other methods in the corresponding implementation file.

Template class declarations follow the same style as normal class declarations. This applies in particular to inline declarations. The keyword template followed by the list of template parameters appear on a separate line. The < and > in template expressions have one space after and before the sign, respectively, e.g., std::vector< int >.

template< typename T >
class MyClass: public T
{
public:
  // code
private:
  // more code
};

Further indentation and formatting

Avoid committing indentation and formatting changes together with changes in logic. Always commit these changes separately._

As a general rule of thumb, always indent with two spaces. Do not use TAB character in any source file. Always use braces around blocks of code. The braces of code blocks have their own line.

Control structures (if, while, for, …) have a single space after the keyword. The parenthesis around the tests have a space after the opening and before the closing parenthesis. The case labels in switch statements are not indented.

if ( x > 0 )
{
  // code
}
else
{
  // code
}

switch ( i )
{
case 0:
  // code
default:
  // code
}

Binary operators (+, -, *, ||, &, …) are surrounded by one space, e.g. a + b.

Unary operators have no space between operator and operand, e.g. -a. Do not use the negation operator ! since it can easily be overseen. Instead use not, e.g. not vec.empty().

There is no space between a statement and its corresponding semicolon:

return a + 3 ; // bad
return a + 3;  // good

Stopwatch example

For example, the stopwatch.h file could look like:

/*
 *  stopwatch.h
 *
 *  This file is part of NEST.
 *
 *  Copyright (C) 2004 The NEST Initiative
 *
 *  NEST is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  NEST 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with NEST.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#ifndef STOPWATCH_H
#define STOPWATCH_H

// C includes:
#include <sys/time.h>

// C++ includes:
#include <cassert>
#include <iostream>

namespace nest
{

/***********************************************************************
 * Stopwatch                                                           *
 *   Accumulates time between start and stop, and provides             *
 *   the elapsed time with different time units.                       *
 *                                                                     *
 *   Partly inspired by com.google.common.base.Stopwatch.java          *
 *   Not thread-safe: - Do not share stopwatches among threads.        *
 *                    - Let each thread have its own stopwatch.        *
 *                                                                     *
 *   Usage example:                                                    *
 *     Stopwatch x;                                                    *
 *     x.start();                                                      *
 *     // ... do computations for 15.34 sec                            *
 *     x.stop(); // only pauses stopwatch                              *
 *     x.print("Time needed "); // > Time needed 15.34 sec.            *
 *     x.start(); // resumes stopwatch                                 *
 *     // ... next computations for 11.22 sec                          *
 *     x.stop();                                                       *
 *     x.print("Time needed "); // > Time needed 26,56 sec.            *
 *     x.reset(); // reset to default values                           *
 *     x.start(); // starts the stopwatch from 0                       *
 *     // ... computation 5.7 sec                                      *
 *     x.print("Time "); // > Time 5.7 sec.                            *
 *     // ^ intermediate timing without stopping the stopwatch         *
 *     // ... more computations 1.7643 min                             *
 *     x.stop();                                                       *
 *     x.print("Time needed ", Stopwatch::MINUTES, std::cerr);         *
 *     // > Time needed 1,8593 min. (on cerr)                          *
 *     // other units and output streams possible                      *
 ***********************************************************************/
class Stopwatch
{
public:
  typedef size_t timestamp_t;
  typedef size_t timeunit_t;

  enum
  {
    MICROSEC = ( timeunit_t ) 1,
    MILLISEC = MICROSEC * 1000,
    SECONDS = MILLISEC * 1000,
    MINUTES = SECONDS * 60,
    HOURS = MINUTES * 60,
    DAYS = HOURS * 24
  };

  static bool correct_timeunit( timeunit_t t );

  /**
   * Creates a stopwatch that is not running.
   */
  Stopwatch()
  {
    reset();
  }

  /**
   * Starts or resumes the stopwatch, if it is not running already.
   */
  void start();

  /**
   * Stops the stopwatch, if it is not stopped already.
   */
  void stop();

  /**
   * Returns, whether the stopwatch is running.
   */
  bool isRunning() const;

  /**
   * Returns the time elapsed between the start and stop of the
   * stopwatch. If it is running, it returns the time from start
   * until now. If the stopwatch is run previously, the previous
   * runtime is added. If you want only the last measurment, you
   * have to reset the timer, before stating the measurment.
   * Does not change the running state.
   */
  double elapsed( timeunit_t timeunit = SECONDS ) const;

  /**
   * Returns the time elapsed between the start and stop of the
   * stopwatch. If it is running, it returns the time from start
   * until now. If the stopwatch is run previously, the previous
   * runtime is added. If you want only the last measurment, you
   * have to reset the timer, before stating the measurment.
   * Does not change the running state.
   * In contrast to Stopwatch::elapsed(), only the timestamp is returned,
   * that is the number if microseconds as an integer.
   */
  timestamp_t elapsed_timestamp() const;

  /**
   * Resets the stopwatch.
   */
  void reset();

  /**
   * This method prints out the currently elapsed time.
   */
  void print( const char* msg = "", timeunit_t timeunit = SECONDS, std::ostream& os = std::cout ) const;

  /**
   * Convenient method for writing time in seconds
   * to some ostream.
   */
  friend std::ostream& operator<<( std::ostream& os, const Stopwatch& stopwatch );

private:
#ifndef DISABLE_TIMING
  timestamp_t _beg, _end;
  size_t _prev_elapsed;
  bool _running;
#endif

  /**
   * Returns current time in microseconds since EPOCH.
   */
  static timestamp_t get_timestamp();
};

inline bool
Stopwatch::correct_timeunit( timeunit_t t )
{
  return t == MICROSEC || t == MILLISEC || t == SECONDS || t == MINUTES || t == HOURS || t == DAYS;
}

inline void
nest::Stopwatch::start()
{
#ifndef DISABLE_TIMING
  if ( not isRunning() )
  {
    _prev_elapsed += _end - _beg;  // store prev. time, if we resume
    _end = _beg = get_timestamp(); // invariant: _end >= _beg
    _running = true;               // we start running
  }
#endif
}

inline void
nest::Stopwatch::stop()
{
#ifndef DISABLE_TIMING
  if ( isRunning() )
  {
    _end = get_timestamp(); // invariant: _end >= _beg
    _running = false;       // we stopped running
  }
#endif
}

inline bool
nest::Stopwatch::isRunning() const
{
#ifndef DISABLE_TIMING
  return _running;
#else
  return false;
#endif
}

inline double
nest::Stopwatch::elapsed( timeunit_t timeunit ) const
{
#ifndef DISABLE_TIMING
  assert( correct_timeunit( timeunit ) );
  return 1.0 * elapsed_timestamp() / timeunit;
#else
  return 0.0;
#endif
}

inline nest::Stopwatch::timestamp_t
nest::Stopwatch::elapsed_timestamp() const
{
#ifndef DISABLE_TIMING
  if ( isRunning() )
  {
    // get intermediate elapsed time; do not change _end, to be const
    return get_timestamp() - _beg + _prev_elapsed;
  }
  else
  {
    // stopped before, get time of current measurement + last measurements
    return _end - _beg + _prev_elapsed;
  }
#else
  return ( timestamp_t ) 0;
#endif
}

inline void
nest::Stopwatch::reset()
{
#ifndef DISABLE_TIMING
  _beg = 0; // invariant: _end >= _beg
  _end = 0;
  _prev_elapsed = 0; // erase all prev. measurements
  _running = false;  // of course not running.
#endif
}

inline void
nest::Stopwatch::print( const char* msg, timeunit_t timeunit, std::ostream& os ) const
{
#ifndef DISABLE_TIMING
  assert( correct_timeunit( timeunit ) );
  double e = elapsed( timeunit );
  os << msg << e;
  switch ( timeunit )
  {
  case MICROSEC:
    os << " microsec.";
    break;
  case MILLISEC:
    os << " millisec.";
    break;
  case SECONDS:
    os << " sec.";
    break;
  case MINUTES:
    os << " min.";
    break;
  case HOURS:
    os << " h.";
    break;
  case DAYS:
    os << " days.";
    break;
  }
#ifdef DEBUG
  os << " (running: " << ( _running ? "true" : "false" ) << ", begin: " << _beg << ", end: " << _end
     << ", diff: " << ( _end - _beg ) << ", prev: " << _prev_elapsed << ")";
#endif
  os << std::endl;
#endif
}

inline nest::Stopwatch::timestamp_t
nest::Stopwatch::get_timestamp()
{
  // works with:
  // * hambach (Linux 2.6.32 x86_64)
  // * JuQueen (BG/Q)
  // * MacOS 10.9
  struct timeval now;
  gettimeofday( &now, ( struct timezone* ) 0 );
  return ( nest::Stopwatch::timestamp_t ) now.tv_usec
    + ( nest::Stopwatch::timestamp_t ) now.tv_sec * nest::Stopwatch::SECONDS;
}

} /* namespace timer */
#endif /* STOPWATCH_H */

And the corresponding stopwatch.cpp:

/*
 *  stopwatch.cpp
 *
 *  This file is part of NEST.
 *
 *  Copyright (C) 2004 The NEST Initiative
 *
 *  NEST is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  NEST 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with NEST.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include "stopwatch.h"

namespace nest
{
std::ostream& operator<<( std::ostream& os, const Stopwatch& stopwatch )
{
  stopwatch.print( "", Stopwatch::SECONDS, os );
  return os;
}
}