GUIDELINES

This section considers the guidelines for creating safe software discussed by SoHaR [SoH95], in the context of the C++ programming language. The SoHaR report provides general programming guidelines for the assessment of software used in safety systems. It then considers these guidelines in the context of several programming languages (including C++); however, SoHaR's discussion treats C and C++ together, and was written with a strong C bias. There is a need for a true C++ assessment.

This paper partitions the SoHaR guidelines into three groups:

1. Outside.

Those guidelines outside the scope of C++. For example, control of memory paging is outside the control of a C++ programmer or the C++ compiler.

2. General.

Those guidelines that represent general advice not directly applicable to particular C++ language features. For example, minimizing the nesting level of statements.

3. Specific.

Those guidelines directly applicable to specific C++ language features. For example, minimizing dynamic memory allocation applies directly to C++'s built-in functions new and delete.

The first column of the following table indicates the group for each guideline. A further discussion of those guidelines in the general and specific categories appears after the table. Discussion of outside guidelines can be found in [SoH95].

Group

Number

Guideline

Group	Number	Guideline
	1	Reliability
	1.1	Predictability of Memory Utilization
Specific	1.1.1	Minimizing Dynamic Memory Allocation
Outside	1.1.2	Minimizing Memory Paging and Swapping
	1.2	Predictability of Control Flow
Specific	1.2.1	Maximizing Structure
Specific	1.2.2	Minimizing Control Flow Complexity
Specific	1.2.3	Initialization of Variables before Use
Specific	1.2.4	Single Entry and Exit Points in Subprograms
Specific	1.2.5	Minimizing Interface Ambiguities
Specific	1.2.6	Use of Data Typing
General	1.2.7	Precision and Accuracy
Specific	1.2.8	Use of Parentheses rather than Default Order of Precedence
Specific	1.2.9	Separating Assignment from Evaluation
Outside	1.2.10	Proper Handling of Program Instrumentation
General	1.2.11	Control of Class Library Size
General	1.2.12	Minimizing Dynamic Binding
General	1.2.13	Control of Operator Overloading
	1.3	Predictability of Timing
Outside	1.3.1	Minimizing the Use of Tasking
Outside	1.3.2	Minimizing the Use of Interrupt Driven Processing
	2	Robustness
	2.1	Controlled Use of Software Diversity
General	2.1.1	Control of Internal Diversity
Outside	2.1.2	Control of External Diversity
	2.2	Controlled Use of Exception Handling
Outside	2.2.1	Local Handling of Exceptions
Outside	2.2.2	Preservation of External Control Flow
Outside	2.2.3	Uniformity of Exception Handling
	2.3	Input and Output Checking
General	2.3.1	Input Data Checking
General	2.3.2	Output Data Checking
	3	Traceability
	3.1	Use of Built-In Functions
General	3.1.1	Controlled Use of Built-in Functions
	3.2	Use of Compiled Libraries
General	3.2.1	Controlled Use of Compiled Libraries
	4	Maintainability
	4.1	Readability
General	4.1.1	Conformance to Indentation Guidelines
General	4.1.2	Descriptive Identifier Names
General	4.1.3	Comments and Internal Documentation
General	4.1.4	Limitations on Subprogram Size
Outside	4.1.5	Minimizing Mixed Language Programming
Specific	4.1.6	Minimizing Obscure or Subtle Programming Constructs
Specific	4.1.7	Minimizing Dispersion of Related Elements
Specific	4.1.8	Minimizing Use of Literals
	4.2	Data Abstraction
Specific	4.2.1	Minimizing the Use of Global Variables
Specific	4.2.2	Minimizing the Complexity of Class and
		Function Interfaces
	4.3	Functional Cohesiveness
General	4.3.1	Single Purpose Function and Procedure
	4.4	Malleability
General	4.4.1	Isolation of Alterable Functions
	4.5	Portability
General	4.5.1	Isolation of Non-Standard Constructs

• Discussion of General Guidelines Relation to C++
• Discussion of Specific Guidelines Relation to C++