Coding Guidelines

The main development language of Arbor is C++. For Arbor we start with the community guidelines set out in the C++ Core Guidelines. These guidelines are quite generic, and only give loose guidelines for some important topics like variable naming and indentation.

This wiki will describe the specific extensions and differences to the C++ Core Guidelines - variable naming. - code formatting (indentation, placement of curly braces, int const& vs const int& etc.) - rules for topics not covered in the Core Guidelines (e.g. CUDA). - exceptions to the rules given in the Core Guidelines. - rules for the CMake build system and directory structure. - rules for external dependencies (e.g. Boost).


This page needs revision.

Code organisation

Source files naming conventions


describe the public/private header and source code organization.

  • .cpp extension for source files

  • .hpp extension for header files

  • Names must be lowercase with words separated with underscores.


All code is in namespace arb.

As an example, the std::make_unique<> function template was not provided as part of C++11 (it wasn’t introduced until C++14), and we would like to use it in Arbor. The code sample below shows how namespaces are declared and formatted in Arbor:

namespace arb {
namespace util {

// just because we aren't using C++14, doesn't mean we shouldn't have make_unique
template <typename T, typename... Args>
std::unique_ptr<T> make_unique(Args&&... args) {
    return std::unique_ptr<T>(new T(std::forward<Args>(args) ...));

} // namespace util
} // namespace arb

In the example above the namespaces are not indented. However, namespaces should be indented if they are declared in the middle of code, to make their existance obvious to the person reading the code.

Use an impl namespace to hide implementation details that should not be exposed to user space.

You can use using statements to import individual types or functions from a namespace - only if it really improves the readability of your code - only in a function or class scope: don’t pollute namespaces

Formatting statements

A lot of the rules here are purely a matter of personal taste, imposed by the guy who got to set the rules. That said, it follows accepted practice in the C++ community (if we accept that not everybody has the same accepted practice), and if followed consistently will make code easier to understand.

template <typename T>
class array {
    using value_type = T;

    value_type& operator[] (std::size_t i) {
        return data_[i];

    const value_type& operator[] (std::size_t i) const {
        return data_[i];

    std::size_t size() const {
        return size_;

    value_type* data_;
    std::size_t size_;

// use new lines and indentation to make complex template expressions
// human readable
template <
    typename T,
    typename = typename  // assert that T is a built-in arithmetic type
T sum(const array<T>& in) {
    return std::accumulate(in.begin(), in.end(), 0);

TODO: When declaring an operator, should we leave a space between the operator and the following opening parenthesis or should we follow the convention we use for functions, where we don’t leave a space?

Indentation and whitespace cleanup

  • No tabs, 4 spaces

  • Take the extra effort to remove trailing whitespace (at the end of the lines and the file).

  • Respect 80-column limit, but go for longer lines when they make sense (and make the code clearer)

Variable naming conventions

All lowercase, words separated by _, but template parameters follow camel case:

template <typename ValueType>
class my_class {
    // ...
    ValueType val_;

Single letter template parameters should be preferred.

TODO: Or should we force single letter parameters aliased by more meaningful type names inside the class (either public or private depending on our intent)?

Avoid obfuscated names of old C heritage.

Recurring variables naming conventions

TODO: Some variable names are recurring a lot inside every code. It would be nice if we could decide on the most common ones.

  • count or cnt

  • index or idx

  • iter or it

Ben says “depends… I would use count or index unless the scope of the variable is very small. Using it is standard C++ short hand, but again for fairly limited scope.”

Member variables

Private member variables must be suffixed by _, while public member variables must not.

TODO: Any conventions about static variables, consts or global consts?

Member initialisation lists

Constructors member initialisation lists should be as follows:

// everything goes on one line if clear
class my_class {
    my_class(int a):

    my_class(int a, int b, int c):
        a_(a), b_(b) , c_(c)

    int a_ = 0;
    int b_ = 0;
    int c_ = 0;

// use one entry per line if multiple lines needed
class my_class {
    my_class(int a, int o, int p):

    int apple_;
    int orange_;
    int pear_;

Member functions

Make sure to declare const if it is not changing the object’s state.

Getters and Setters

Before filling up a class with getters and setters, consider seriously if those members are meant actually to be public. If nonetheless getters and/or setters are needed, don’t use the get_ and set_ prefixes.

template <typename T>
class my_class {
    // ...
    T value() const {
        return value_;

    void value(const T& val) {
        // perhaps do something before assigning, otherwise it could be just public
        value_ = val;
    T value_;

Declaring references and pointers

// ok
std::string& s = ...;
const std::string& s = ...;
std::string* s = ...;
const std::string* s = ...;
std::string* const s = ...;

// not ok
std::string &s = ...;
const std::string &s = ...;
std::string *s = ...;
const std::string *s = ...;
std::string *const s = ...;

Generally, we follow C++’s convention for references and pointers, as it is the style used in the C++ standard, and also the recommendation of the C++ Core Guidelines NL.18. Precedence and the C++ language grammar may offer some support the other convention, but not enough support!


Macros are C-ish, so they must be avoided. If not possible, they must be written in capitals, with words separated by underscores.

Always use {}, even for single statement if, for, etc

It makes code clearer, and avoids nasty bugs that occur when refactoring. It also avoids some errors when merging with git.

// ok
for (auto& v: vector) {
    // increment the value!

// bad
for (auto& v: vector)
    // increment the value!

don’t put { on a new line

Except when indentation of arguments or when doing member initialization in constructors.

// it makes sense to have the { on a new line here for clarity
std::vector<std::string> foo(
    std::vector<std::vector<int>>& values,
    std::map<int, std::string>& name_table)
    // do some work

leave a space between if, for etc and following parenthesis

Accords with K&R style, and makes a visual distinction with function evaluation

// ok
for (auto& v: vector) {

// not ok
for(auto& v: vector) {

use using instead of typedef

It is easier to read, consistent with auto:

// good
using int_container = std::vector<int>;

// bad
typedef std::vector<int> int_container;

and can be used for template aliases

template <typename T>
using aligned_container = std::vector<T, my_fancy_aligned_allocator<T>>;

Use scoped enum instead of enum

// good
enum class ionKind {sodium, calcium};
// bad
enum ionKind {ion_sodium, ion_calcium};

And stick to the naming scheme for all enums of xxxKind to make it clear throughout the code whenever an enum is being used, for example:

auto i = current(voltage, ionKind::calcium);

Use struct for POD wrappers

But class if it has any sort of fancy logic associated with it

Memory management

use unique_ptr

Actually, feel free to use naked pointers in your code, but make sure that you use smart pointers to handle allocation and freeing of memory. When a developer sees a naked pointer in Arbor they can think “good, I don’t have to worry about responsibility for freeing that memory”. Furthermore, if unique_ptr handles allocation and freeing of memory, the user doesn’t have to concern themselves with freeing memory ever.

This practice implies that care must be taken to ensure that the resource managed by a unique_ptr has to outlive any raw pointers that are obtained from its get() member.

while avoiding shared_ptr whenever possible

If you think long and hard, you will probably realise that you actually want a unique_ptr. Shared pointers have performance overheads, and are quite easy to misuse. For example by creating circular references that ironically lead to memory never being freed.

Header files

use pragma once

Use #pragma once to guard against including the same header twice. This might not be completely standard compliant, but it is supported by every compiler under the sun, and is much cleaner than #ifdef guards.

don’t rely on headers being included elsewhere

For example, if you use std::vector<int> in a file, make sure to have #include <vector> at the top of the source file.

Relying on headers being include elsewhere can lead to portability problems, for example on OS X you have to #include <cmath> for some math functions that are imported via other header files with gcc on Linux.

Sort headers alphabetically

To make it easy to search for a header in a long list of includes.

For example:

#include <algorithm>
#include <fstream>
#include <map>
#include <queue>
#include <set>

use C++ wrappers for C standard headers

// good
#include <cmath>
#include <cstdio>

// bad
#include <math.h>
#include <stdio.h>

when calling C stdlib functions, use the std::-prefix versions, e.g., std::printf(...) instead of printf. Most of the times C++ wrappers just bring into std the C declarations, but sometimes the wrappers have more syntactic sugar and call the same internal builtins that their C counterparts call (for example GCC).

group headers together

In the following order

  1. C++ standard libary

  2. system C headers (POSIX, kernel interfaces etc.)

  3. external libraries

  4. public arbor headers

  5. private arbor headers

For example:

// first C++ standard headers
#include <algorithm>
#include <fstream>
#include <map>

// then system C headers
#include <signal.h>
#include <sys/select.h>

// externals
#include <vector/Vector.hpp>

// public arbor headers
#include <arbor/common_types.hpp>
#include <arbor/simulation.hpp>

// private arbor headers (note we use quotes for private project headers).
#include "cell_group.hpp"
#include "util/optional.hpp"