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Open CASCADE Technology
7.2.0
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Table of Contents
Introduction
The purpose of this document is to define a common programming style for Open CASCADE Technology.
The common style facilitates understanding and maintaining a code developed cooperatively by several programmers. In addition, it enables construction of tools that incorporate knowledge of these standards to help in the programming.
OCCT programming style follows common and appropriate best practices, so some guidelines have been excerpted from the public domain.
The guide can be improved in the future as new ideas and enhancements are added.
Scope of the document
Rules in this document refer to C++ code. However, with minor exceptions due to language restrictions, they are applicable to any sources in Open CASCADE Technology framework, including:
- C/C++
- GLSL programs
- OpenCL kernels
- TCL scripts and test cases
Naming Conventions
General naming rules
The names considered in this section mainly refer to the interface of Open CASCADE Technology libraries or source code itself.
International language [MANDATORY]
Open CASCADE Technology is an open source platform available for an international community, thus all names need to be composed of English words or their abbreviations.
Meaningful names
Names should be meaningful or, at least, contain a meaningful part. To better understand this requirement, let us examine the existing names of toolkits, packages, classes and methods:
- Packages containing words Geom or Geom2d in their names are related to geometrical data and operations.
- Packages containing words TopoDS or BRep in their names are related to topological data and operations.
- Packages ending with ...Test define Draw Harness plugins.
- Methods starting with Get... and Set... are usually responsible for correspondingly retrieving and storing data.
Related names
Names related to a logically connected functionality should have the same prefix (start with the same letters) or, at least, have any other common part. For example, method GetCoord returns a triple of real values and is defined for directions, vectors and points. The logical connection is obvious.
Camel Case style
Camel Case style is preferred for names. For example:
Names of development units
Usually a unit (e.g. a package) is a set of classes, methods, enumerations or any other sources implementing a common functionality, which is self-contained and independent from other parts of the library.
No underscores in unit names [MANDATORY]
Names of units should not contain underscores, unless the use of underscores is allowed explicitly.
File name extensions [MANDATORY]
The following extensions should be used for source files, depending on their type:
- .cxx – C++ source files
- .hxx – C++ header files
- .lxx – additional headers containing definitions of inline methods and auxiliary code
Note that .lxx files should be avoided in most cases - inline method should be placed in header file instead.
Prefix for toolkit names [MANDATORY]
Toolkit names are prefixed by TK, followed by a meaningful part of the name explaining the domain of functionality covered by the toolkit (e.g. TKOpenGl).
Names of classes
Usually the names of source files located in a unit start from the unit name separated from the other part of the file name by underscore "_".
Thus, the names of files containing sources of C++ classes that belong to a package are constructed according to the following template:
For example, file Adaptor2d_Curve2d.cxx belongs to the package Adaptor2d
Files that contain sources related to the whole unit are called by the unit name with appropriate extension.
Names of functions
The term function here is defined as:
- Any class method
- Any package method
- Any non-member procedure or function
It is preferred to start names of public methods from an upper case character and to start names of protected and private methods from a lower case character.
Names of variables
There are several rules that describe currently accepted practices for naming variables.
Naming of variables
Name of a variable should not conflict with the existing or possible global names (for packages, macros, functions, global variables, etc.).
The name of a variable should not start with an underscore.
See the following examples:
Names of function parameters
The name of a function (procedure, class method) parameter should start with prefix the followed by the meaningful part of the name starting with a capital letter.
See the following examples:
Names of class member variables
The name of a class member variable should start with prefix my followed by the meaningful of the name starting with a capital letter.
See the following examples:
Names of global variables
It is strongly recommended to avoid defining any global variables. However, as soon as a global variable is necessary, its name should be prefixed by the name of a class or a package where it is defined followed with _my.
See the following examples:
Static constants within the file should be written in upper-case and begin with prefix THE_:
Names of local variables
The name of a local variable should be distinguishable from the name of a function parameter, a class member variable and a global variable.
It is preferred to prefix local variable names with a and an (or is, to and has for Boolean variables).
See the following example:
Avoid dummy names
Avoid dummy names, such as i, j, k. Such names are meaningless and easy to mix up.
The code becomes more and more complicated when such dummy names are used there multiple times with different meanings, or in cycles with different iteration ranges, etc.
See the following examples for preferred style:
Formatting rules
To improve the open source readability and, consequently, maintainability, the following set of rules is applied.
International language [MANDATORY]
All comments in all sources must be in English.
Line length
Try to stay within the limit of 120 characters per line in all sources.
C++ style comments
Prefer C++ style comments in C++ sources.
Commenting out unused code
Delete unused code instead of commenting it or using #define.
Indentation in sources [MANDATORY]
Indentation in all sources should be set to two space characters. Use of tabulation characters for indentation is disallowed.
Separating spaces
Punctuation rules follow the rules of the English language.
- C/C++ reserved words, commas, colons and semicolons should be followed by a space character if they are not at the end of a line.
- There should be no space characters after '(' and before ')'. Closing and opening brackets should be separated by a space character.
- For better readability it is also recommended to surround conventional operators by a space character. Examples:
Declaration of pointers and references
In declarations of simple pointers and references put asterisk (*) or ampersand (&) right after the type without extra space.
Since declaration of several variables with mixed pointer types contrudicts this rule, it should be avoided. Instead, declare each variable independently with fully qualified type.
Examples:
Separate logical blocks
Separate logical blocks of code with one blank line and comments.
See the following example:
Notice that multiple blank lines should be avoided.
Separate function bodies [MANDATORY]
Use function descriptive blocks to separate function bodies from each other. Each descriptive block should contain at least a function name and purpose description.
See the following example:
Block layout [MANDATORY]
Figure brackets { } and each operator (for, if, else, try, catch) should be written on a dedicated line.
In general, the layout should be as follows:
Entering a block increases and leaving a block decreases the indentation by one tabulation.
Single-line operators
Single-line conditional operators (if, while, for, etc.) can be written without brackets on the following line.
Having all code in the same line is less convenient for debugging.
Comparison expressions with constants
In comparisons, put the variable (in the current context) on the left side and constant on the right side of expression. That is, the so called "Yoda style" is to be avoided.
Alignment
Use alignment wherever it enhances the readability. See the following example:
Indentation of comments
Comments should be indented in the same way as the code to which they refer or they can be in the same line if they are short.
The text of the comment should be separated from the slash character by a single space character.
See the following example:
Early return statement
Use an early return condition rather than collect indentations.
Write like this:
Rather than:
This helps to improve readability and reduce the unnecessary indentation depth.
Trailing spaces
Trailing spaces should be removed whenever possible. Spaces at the end of a line are useless and do not affect functionality.
Headers order
Split headers into groups: system headers, headers per each framework, project headers; sort the list of includes alphabetically. Within the class source file, the class header file should be included first.
This rule improves readability, allows detecting useless multiple header inclusions and makes 3rd-party dependencies clearly visible. Inclusion of class header on top verifies consistency of the header (e.g. that header file does not use any undefined declarations due to missing includes of dependencies).
An exception to the rule is ordering system headers generating a macros declaration conflicts (like "windows.h" or "X11/Xlib.h") - these headers should be placed in the way solving the conflict.
Documentation rules
The source code is one of the most important references for documentation. The comments in the source code should be complete enough to allow understanding the corresponding code and to serve as basis for other documents.
The main reasons why the comments are regarded as documentation and should be maintained are:
- The comments are easy to reach – they are always together with the source code;
- It is easy to update a description in the comment when the source is modified;
- The source by itself is a good context to describe various details that would require much more explanations in a separate document;
- As a summary, this is the most cost-effective documentation.
The comments should be compatible with Doxygen tool for automatic documentation generation (thus should use compatible tags).
Documenting classes [MANDATORY]
Each class should be documented in its header file (.hxx). The comment should give enough details for the reader to understand the purpose of the class and the main way of work with it.
Documenting class methods [MANDATORY]
Each class or package method should be documented in the header file (.hxx).
The comment should explain the purpose of the method, its parameters, and returned value(s). Accepted style is:
//! Method computes the square value. //! @param theValue the input value //! @return squared value Standard_Export Standard_Real Square (Standard_Real theValue);
Documenting C/C++ sources
It is very desirable to put comments in the C/C++ sources of the package/class.
They should be detailed enough to allow any person to understand what each part of code does.
It is recommended to comment all static functions (like methods in headers), and to insert at least one comment per each 10-100 lines in the function body.
There are also some rules that define how comments should be formatted, see Formatting Rules.
Following these rules is important for good comprehension of the comments. Moreover, this approach allows automatically generating user-oriented documentation directly from the commented sources.
Application design
The following rules define the common style, which should be applied by any developer contributing to the open source.
Allow possible inheritance
Try to design general classes (objects) keeping possible inheritance in mind. This rule means that the user who makes possible extensions of your class should not encounter problems of private implementation. Try to use protected members and virtual methods wherever you expect extensions in the future.
Avoid friend declarations
Avoid using 'friend' classes or functions except for some specific cases (for example, iteration) 'Friend' declarations increase coupling.
Set/get methods
Avoid providing set/get methods for all fields of the class. Intensive set/get functions break down encapsulation.
Hiding virtual functions [MANDATORY]
Avoid hiding a base class virtual function by a redefined function with a different signature. Most of the compilers issue warning on this.
Avoid mixing error reporting strategies
Try not to mix different error indication/handling strategies (exceptions or returned values) on the same application level.
Minimize compiler warnings [MANDATORY]
When compiling the source pay attention to and try to minimize compiler warnings.
Avoid unnecessary inclusions
Try to minimize compilation dependencies by removing unnecessary inclusions.
General C/C++ rules
This section defines the rules for writing a portable and maintainable C/C++ source code.
Wrapping of global variables [MANDATORY]
Use package or class methods returning reference to wrap global variables to reduce possible name space conflicts.
Avoid private members
Use protected members instead of private wherever reasonable to enable future extensions. Use private fields if future extensions should be disabled.
Constants and inlines over defines [MANDATORY]
Use constant variables (const) and inline functions instead of defines (#define).
Avoid explicit numerical values [MANDATORY]
Avoid usage of explicit numeric values. Use named constants and enumerations instead. Numbers produce difficulties for reading and maintenance.
Three mandatory methods
If a class has a destructor, an assignment operator or a copy constructor, it usually needs the other two methods.
Virtual destructor
A class with virtual function(s) ought to have a virtual destructor.
Overriding virtual methods
Declaration of overriding method should contains specifiers "virtual" and "override" (using Standard_OVERRIDE alias for compatibility with old compilers).
This makes class definition more clear (virtual methods become highlighted).
Declaration of interface using pure virtual functions protects against incomplete inheritance at first level, but does not help when method is overridden multiple times within nested inheritance or when method in base class is intended to be optional.
And here "override" specifier introduces additional protection against situations when interface changes might be missed (class might contain old methods which will be never called).
Default parameter value
Do not redefine a default parameter value in an inherited function.
Use const modifier
Use const modifier wherever possible (functions parameters, return values, etc.)
Usage of goto [MANDATORY]
Avoid goto statement unless it is really needed.
Declaring variable in for() header
Declare a cycle variable in the header of the for() statement if not used out of cycle.
Condition statements within zero
Avoid usage of C-style comparison for non-boolean variables:
Portability issues
This chapter contains rules that are critical for cross-platform portability.
Provide code portability [MANDATORY]
The source code must be portable to all platforms listed in the official 'Technical Requirements'. The term 'portable' here means 'able to be built from source'.
The C++ source code should meet C++03 standard. Any usage of compiler-specific features or further language versions (for example, C++11, until all major compilers on all supported platforms implement all its features) should be optional (used only with appropriate preprocessor checks) and non-exclusive (an alternative implementation compatible with other compilers should be provided).
Avoid usage of global variables [MANDATORY]
Avoid usage of global variables. Usage of global variables may cause problems when accessed from another shared library.
Use global (package or class) functions that return reference to static variable local to this function instead of global variables.
Another possible problem is the order of initialization of global variables defined in various libraries that may differ depending on platform, compiler and environment.
Avoid explicit basic types
Avoid explicit usage of basic types (int, float, double, etc.), use Open CASCADE Technology types from package Standard: Standard_Integer, Standard_Real, Standard_ShortReal, Standard_Boolean, Standard_CString and others or a specific typedef instead.
Use sizeof() to calculate sizes [MANDATORY]
Do not assume sizes of types. Use sizeof() instead to calculate sizes.
Empty line at the end of file [MANDATORY]
In accordance with C++03 standard source files should be trailed by an empty line. It is recommended to follow this rule for any plain text files for consistency and for correct work of git difference tools.
Stability issues
The rules listed in this chapter are important for stability of the programs that use Open CASCADE Technology libraries.
Use OSD::SetSignal() to catch exceptions
When using Open CASCADE Technology in an application, call OSD::SetSignal() function when the application is initialized.
This will install C handlers for run-time interrupt signals and exceptions, so that low-level exceptions (such as access violation, division by zero, etc.) will be redirected to C++ exceptions that use try {...} catch (Standard_Failure) {...} blocks.
The above rule is especially important for robustness of modeling algorithms.
Cross-referenced handles
Take care about cycling of handled references to avoid chains, which will never be freed. For this purpose, use a pointer at one (subordinate) side.
See the following example:
C++ memory allocation
In C++ use new and delete operators instead of malloc() and free(). Try not to mix different memory allocation techniques.
Match new and delete [MANDATORY]
Use the same form of new and delete.
Methods managing dynamical allocation [MANDATORY]
Define a destructor, a copy constructor and an assignment operator for classes with dynamically allocated memory.
Uninitialized variables [MANDATORY]
Every variable should be initialized.
Uninitialized variables might be kept only within performance-sensitive code blocks and only when their initialization is guaranteed by subsequent code.
Do not hide global new
Avoid hiding the global new operator.
Assignment operator
In operator=() assign to all data members and check for assignment to self.
Float comparison
Don't check floats for equality or non-equality; check for GT, GE, LT or LE.
Package Precision provides standard values for SI units and widely adopted by existing modeling algorithms:
- Precision::Confusion() for lengths in meters;
- Precision::Angular() for angles in radians.
as well as definition of infinite values within normal range of double precision:
- Precision::Infinite()
- Precision::IsInfinite()
- Precision::IsPositiveInfinite()
- Precision::IsNegativeInfinite()
Non-indexed iteration
Avoid usage of iteration over non-indexed collections of objects. If such iteration is used, make sure that the result of the algorithm does not depend on the order of iterated items.
Since the order of iteration is unpredictable in case of a non-indexed collection of objects, it frequently leads to different behavior of the application from one run to another, thus embarrassing the debugging process.
It mostly concerns mapped objects for which pointers are involved in calculating the hash function. For example, the hash function of TopoDS_Shape involves the address of TopoDS_TShape object. Thus the order of the same shape in the TopTools_MapOfShape will vary in different sessions of the application.
Do not throw in destructors
Do not throw from within a destructor.
Assigning to reference [MANDATORY]
Avoid the assignment of a temporary object to a reference. This results in a different behavior for different compilers on different platforms.
Performance issues
These rules define the ways of avoiding possible loss of performance caused by ineffective programming.
Class fields alignment
Declare fields of a class in the decreasing order of their size for better alignment. Generally, try to reduce misaligned accesses since they impact the performance (for example, on Intel machines).
Fields initialization order [MANDATORY]
List class data members in the constructor's initialization list in the order they are declared.
Initialization over assignment
Prefer initialization over assignment in class constructors.
Optimize caching
When programming procedures with extensive memory access, try to optimize them in terms of cache behavior. Here is an example of how the cache behavior can be impacted:
On x86 this code
is more efficient then
since linear access does not invalidate cache too often.
Draw Harness command
Draw Harness provides TCL interface for OCCT algorithms.
There is no TCL wrapper over OCCT C++ classes, instead interface is provided through the set of TCL commands implemented in C++.
There is a list of common rules which should be followed to implement well-formed Draw Harness command.
Return value
Command should return 0 in most cases even if the executed algorithm has failed. Returning 1 would lead to a TCL exception, thus should be used in case of a command line syntax error and similar issues.
Validate input parameters
Command arguments should be validated before usage. The user should see a human-readable error description instead of a runtime exception from the executed algorithm.
Validate the number of input parameters
Command should warn the user about unknown arguments, including cases when extra parameters have been pushed for the command with a fixed number of arguments.
Message printing
Informative messages should be printed into standard output std::cout, whilst command results (if any) – into Draw Interpreter.
Information printed into Draw Interpreter should be well-structured to allow usage in TCL script.
Long list of arguments
Any command with a long list of obligatory parameters should be considered as ill-formed by design. Optional parameters should start with flag name (with '-' prefix) and followed by its values:
Arguments parser
- Integer values should be read using Draw::Atoi() function.
- Real values should be read using Draw::Atof() function.
- Flags names should be checked in case insensitive manner.
Functions Draw::Atof() and Draw::Atoi() support expressions and read values in C-locale.
Examples
Sample documented class
class Package_Class
{
public: //! @name public methods
//! Method computes the square value.
//! @param theValue the input value
//! @return squared value
Standard_Export Standard_Real Square (const Standard_Real theValue);
private: //! \@name private methods
//! Auxiliary method
void increment();
private: //! \@name private fields
Standard_Integer myCounter; //!< usage counter
};TCL script for Draw Harness
GLSL program:
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