Coding Guidelines


Coding style is important. A clean, consistent style leads to code that is more readable, debuggable, and maintainable. To this end, we prescribe (and proscribe) a variety of practices. Our goal is to encourage agile but reasoned development of code that can be easily understood by others.

C/C++ foundational guidelines: this document uses as its foundation the coding guidelines developed by Stroustrup and Sutter.

Python foundational guidelines: the PEP 8 guidelines are to be followed for python code development.

The both of these foundational sets of guidelines are extensive and where the Basilisk document is silent these guidelines will prevail.

These are guidelines, not rules. With very few exceptions, this document does not completely ban any particular C/C++ or Python pattern or feature, but rather describes best practice, to be used in the large majority of cases. When deviating from the guidelines given here, just be sure to consider your options carefully, and to document your reasoning, in the code.

Above all else, be consistent. Follow this guide whenever possible, but if you are editing a package written by someone else, follow the existing stylistic conventions in that package (unless you are retrofitting the whole package to follow this guide, for which you deserve an award).

What about non-conforming code?

Some Basilisk code was written prior to the release (and updates) of this style guide. Thus, the codebase may contain code that doesn’t conform to this guide. The following advice is intended for the developer working with non-conforming code:

  1. All new code should conform to this guide.

  2. Unless you have copious free time, don’t undertake converting the existing codebase to conform to this guide.

  3. If you are the author of a non-conforming package, try to find time to update the code to conform.

  4. If you are doing minor edits to non-conforming code, follow the existing stylistic conventions in that code (if any). Don’t mix styles.

  5. If you are doing major work on non-conforming code, take the opportunity to re-style it to conform to this guide.


The following shortcuts are used in this document to denote naming schemes:

  • CamelCased: The name starts with a capital letter, and has a capital letter for each new word, with no underscores.

  • camelCased: Like CamelCase, but with a lower-case first letter

  • under_scored: The name uses only lower-case letters, with words separated by underscores.

  • ALL_CAPITALS: All capital letters, with words separated by underscores.

General Guidelines


No single letter variables. The only exceptions are ‘i’ as an iteration index and a select list of mathematical symbols.

Naming Variable for Mathematics

The following section specifies general guidelines for the naming of variable to be used in code which implements mathematical operations. The naming convention is heavily influenced by the textbook Analytical Mechanics of Space Systems by Schaub and Junkins.

Indicating Reference Frames

A vector variable expressed with components in a reference frame \(\cal B\), is represented with the variable name followed by an underscore and a capital letter denoting the frame \({}^{\cal B}\bf v\) as vector_B.

An angular rate variable expressed in one frame \(\cal B\) with respect to a second \(\cal R\), where components are expressed in the frame \(\cal B\), \({}^{\cal B}\pmb\omega_{\mathcal{B}/\mathcal{R}}\), is given as omega_BR_B.

MRP’s and DCM’s

A direction cosine matrix is expressed as \([BN]\), a mapping of an \(\cal N\) frame vector into a \(\cal B\) frame vector, is written dcm_BN. Similarly for the Modified Rodrigues Parameters (MRP) attitude parameterization the \(\pmb\sigma_{\mathcal{B}/\mathcal{N}}\) is written sigma_BN.


If you are using the Intel Eigen library library to do linear algebra, the mapping from an attitude description such as quaternions or MRPs to a direction cosine matrix (DCM) using .toRotationMatrix() will return \([NB]\), not \([BN]\).

Inertia Tensor

The Inertia tensor \([I_C]\) of the hub about the point \(C\) is defined in the body frame \(\cal B\) components using the variable IHubPntC_B.


The first and second time derivatives of scalar (\(\dot{x}\), \(\ddot{x}\)) or vector (\(\dot{\bf{x}}\), \(\ddot{\bf{x}}\)) quantities, respectively are written as xDot and xDDot.

The first and second time derivatives with respect to a variable other than time should use the same pattern as time derivatives but with a different modifier. For example, \(f '(x)\) and \(f ''(x)\) are written as xPrime and xDPrime respectively.

Common Usage Examples

  • Position vector from \(\cal N\) to \(\cal B\) in inertial frame components \({}^{\cal N} \bf r_{\mathcal{B/N}}\): r_BN_N

  • Inertial time derivative of position vector from \(\cal N\) to \(\cal B\) in inertial frame components \({}^{\cal N} \dot{\bf r}_{\cal B/N}\): rDot_BN_N

  • Time derivative with respect to the body of position vector from \(B\) to \(H\) in body frame components \({}^{\cal B} \bf r'_{H/B}\): rPrime_HB_B

  • Unit direction vector from \(B\) to \(S\) in body frame components \({}^{\cal B} \hat{\bf s}_{S/B}\): sHat_SB_B

  • Inertial time derivative of body angular rate with respect to the inertial frame in body frame components \({}^{\cal B} \dot{\pmb\omega}_{\mathcal{B}/\mathcal{N}}\): omegaDot_BN_B

  • DCM of the body frame with respect to the inertial frame \([BN]\): dcm_BN



Variables holding message names are to be composed in the following manner.

SomeMsg_C descriptionInMsg;                      // C interface to input msg
SomeMsg_C descriptionOutMsg;                     // C interface to output msg
ReadFunctor<SomeMsgPayload> descriptionInMsg;    // C++ interface to input message
Message<SomeMsgPayload> descriptionOutMsg;       // C++ interface to output message
  • SomeMsgPayload: message structure definition

  • In (Out): indicates the direction of the message with respect to the module.

  • Msg: explicitly identifies the variable as a message.

Variables holding data from a read message are to be composed in the following manner.

SomeMsgPayload descriptionInBuffer;
  • description: description of the data.

  • In (Out): indicates the direction of the data being written to the buffer with respect to the module.

  • Buffer: explicitly identifies the variable as having a data buffer functionality.

Message Definitions

The C based messages are stored in src/architecture/msgPayloadDefC as a *.h file. The C++ messages are stored in src/architecture/msgPayloadDefCpp as a *.h file. The file name uses Upper Camel Case and should be identical to the message name within the file. The last letters should be MsgPayload. For example, a particular spacecraft sensor message could be named SpecialSensorMsgPayload.h. The contents could be


/*! @brief Describe the purpose of the message */
typedef struct {
    double sensorOutput_B[3];   //!<        sensor vector in B frame components */
    double sensorSignal;        //!<        raw sensor signal
    int status;                 //!<        sensor status flag


When running the Basilisk setup command python3 the related message interface files are then automatially created and included in the project.

C/C++ Exceptions

  • Currently no language specific exceptions

Python Exceptions

  • Variables are to be lower camelCase. This is done to maintain consistency across the C/C++ and Python code bases which are interfaced via SWIG.

  • Inline comments are accepted so long as they are kept brief.

  • Binary operator spaces will be adhered to as specified in PEP 8, however, not for math symbols operations. E.g. no spaces are included around *, /, +, -, etc

# Yes
x = (4*9/2)-1
# No
x = (4 * 9 / 2) - 1