# 8 Design and Implement for Correctness and Robustness¶

The guidelines in this section describe various software design and implementation practices that help enforce correctness and robustness and avoid mis-interpretation or confusion by others.

## Keep it simple…¶

8.1 Simplicity, clarity, ease of modification and extension should always be a main goal when writing new code or changing existing code.

8.2 Each entity (class, struct, variable, function, etc.) should embody one clear, well-defined concept.

The responsibilities of an entity may increase as it is used in new and different ways. However, changes that divert it from its original intent should be avoided. Also, large, monolithic entities that provide too much functionality or which include too many concepts tend to increase code coupling and complexity and introduce undesirable side effects. Smaller, clearly constrained objects are easier to write, test, maintain, and use correctly. Also, small, simple objects tend to get used more often and reduce code redundancy.

## Avoid global and static data¶

8.3 Global, complex, or opaque data sharing should not be used. Shared data increases coupling and contention between different parts of a code base, which makes maintenance and modification difficult.

8.4 Static or global variables of class type must not be used.

Due to indeterminate order of construction, their use may cause bugs that are very hard to find. Static or global variables that are pointers to class types may be used and must be initialized properly in a single source file.

## Avoid macros and magic numbers for constants¶

8.5 Preprocessor macros should not be used when there is a better alternative, such as an inline function or a constant variable definition.

For example, this:

const double PI = 3.1415926535897932384626433832;


is preferable to this:

#define PI (3.1415926535897932384626433832)


Macros circumvent the ability of a compiler to enforce beneficial language concepts such as scope and type safety. Macros are also context-specific and can produce errors that cannot be understood easily in a debugger. Macros should be used only when there is no better choice for a particular situation.

8.6 Hard-coded numerical constants and other “magic numbers” must not be used directly in source code. When such values are needed, they should be declared as named constants to enhance code readability and consistency.

## Avoid issues with compiler-generated class methods¶

The guidelines in this section apply to class methods that may be automatically generated by a compiler, including constructors, destructors, copy, and move methods. Developers should be aware of the conditions under which compilers will and will not generate these methods. Developers should also be aware of when compiler-generated methods suffice and when they do not. After providing some guidelines, we discuss standard C++ rules that compilers follow for generating class methods when they are not explicitly defined. See Understand standard rules for compiler-generated methods.

The most important cases to pay attention to involve the destructor, copy constructor, and copy-assignment operator. Classes that provide these methods, either explicitly or compiler-generated, are referred to as copyable. Failing to follow the rules for these methods can be damaging due to errors or unexpected behavior. Rules involving the move constructor and move-assignment operator are less important since they mostly affect efficiency and not correctness. Copy operations can be used to accomplish the same end result as move operations, just less efficiently. Move semantics are an important optimization feature of C++. The C++11 standard requires compilers to use move operations instead of copy operations when certain conditions are fulfilled. Classes that provide move operations, either explicitly or compiler-generated, are referred to as movable.

### Rule of three¶

8.7 Each class must follow the Rule of Three which states: if the destructor, copy constructor, or copy-assignment operator is explicitly defined, then the others must be defined.

Compiler-generated and explicit versions of these methods must not be mixed. If a class requires one of these methods to be implemented, it almost certainly requires all three to be implemented.

This rule helps guarantee that class resources are managed properly. C++ copies and copy-assigns objects of user-defined types in various situations (e.g., passing/returning by value, container manipulations, etc.). These special member functions will be called, if accessible. If they are not user-defined, they are implicitly-defined by the compiler.

Compiler-generated special member functions can be incorrect if a class manages a resource whose handle is an object of non-class type. Consider a class data member which is a bare pointer to an object. The compiler-generated class destructor will not free the object. Also, the compiler-generated copy constructor and copy-assignment operator will perform a “shallow copy”; i.e., they will copy the value of the pointer without duplicating the underlying resource.

Neglecting to free a pointer or perform a deep copy when those operations are expected can result in serious logic errors. Following the Rule of Three guards against such errors. On the rare occasion these actions are intentional, a programmer-written destructor, copy constructor, and copy-assignment operator are ideal places to document intent of design decisions.

### Restrict copying of non-copyable resources¶

8.8 A class that manages non-copyable resources through non-copyable handles, such as pointers, should declare the copy methods private and and leave them unimplemented.

When the intent is that such methods should never be called, this is a good way to help a compiler to catch unintended usage. For example:

class MyClass
{
// ...

private:
DISABLE_DEFAULT_CTOR(MyClass);
DISABLE_COPY_AND_ASSIGNMENT(MyClass);

// ...
};


When code does not have access to the private members of a class tries to use such a method, a compile-time error will result. If a class does have private access and tries to use one of these methods an link-time error will result.

This is another application of the “Rule of Three”.

Please see 10 Common Code Development Macros, Types, etc. for more information about the macros used in this example to disable compiler-generated methods.

Note

Exception: If a class inherits from a base class that declares these methods private, the subclass need not declare the methods private. Including comments in the derived class header indicating that the the parent class enforces the non-copyable properties of the class is helpful.

### Rely on compiler-generated methods when appropriate¶

8.9 When the compiler-generated methods are appropriate (i.e., correct and sufficiently fast), the default constructor, copy constructor, destructor, and copy assignment may be left undeclared. In this case, it is often helpful to add comments to the class header file indicating that the compiler-generated versions of these methods will be used.

8.10 If a class is default-constructable and has POD (“plain old data”) or pointer data members, a default constructor should be provided explicitly and its data members must be initialized explicitly if a default constructor is provided. A compiler-generated default constructor will not initialize such members, in general, and so will leave a constructed object in an undefined state.

For example, the following class should provide a default constructor and initialize its data members in it:

class MyClass
{
MyClass();

// ...

private:
double* m_dvals;
int[]   m_ivals;

};


### Functors should always be copyable¶

8.11 By convention, a functor class should have a copy constructor and copy-assignment operator.

Typically, the compiler-generated versions are sufficient when the class has no state or non-POD data members. Since such classes are usually small and simple, the compiler-generated versions of these methods may be used without documenting the use of default value semantics in the functor definition.

For example:

class MyFunctor
{
// Compiler-generated copy ctor and copy assignment sufficient

private:
DIABLE_DEFAULT_CTOR(MyFunctor); // prevent default construction

// ...
};


Note that in this example, the default constructor is disabled to prevent default construction. This can help prevent programming errors when object state must be fully initialialized on construction. For more information about common Axom macros, see 10 Common Code Development Macros, Types, etc..

## Understand standard rules for compiler-generated methods¶

This section provides some background information related to the guidelines in the previous section. There, we provide guidelines that help to decide when to define class methods that may be generated automatically by a compiler and when relying on compiler-generated versions suffices. Here, we describe the conditions under which compilers generate methods automatically.

Consider the following simple class:

class MyClass
{
public:
int x;
};


How many methods does it have? None?

Actually, MyClass may have as many as six methods depending on how it is used: a default constructor, destructor, copy constructor, copy-assignment operator, move constructor, and move-assignment operator. Any of these may be generated by a compiler.

Note

See 11 Portability, Compilation, and Dependencies for discussion about using C++11 features such as move semantics.

C++ compiler rules for generating class member functions are:

• The parameter-less default constructor is generated if a class does not define any constructor and all base classes and data members are default-constructable. This means that once you declare a copy constructor (perhaps to disable the automatically provided one), the compiler will not supply a default constructor.
• The destructor is automatically supplied if possible, based on the members and the base classes.
• A copy constructor is generated if all base classes and members are copy-constructable. Note that reference members are copy-constructable.
• The copy-assignment operator is generated if all base classes and members are copy-assignable. For this purpose, reference members are not considered copy-assignable.
• A move constructor is supplied unless the class has any of the following: a user-defined copy constructor, copy-assignment operator, move-assignment operator, or destructor. If the move constructor cannot be implemented because not all base classes or members are move-constructable, the supplied move constructor will be defined as deleted.
• A move-assignment operator is generated under the same conditions as the move constructor.

The importance of understanding these rules and applying the guidelines in the previous section is underscored by the fact that compiler-generated methods may have different behaviors depending on how they are used. Here we provide some examples based on MyClass defined above.

If MyClass has a user-defined constructor, then

MyClass item1;


and

MyClass item2 = MyClass();


will both call the user-defined default constructor “MyClass()” and there is only one behavior.

However, if MyClass relies on the compiler-generated constructor

MyClass item1;


performs default initialization, while

MyClass item2 = MyClass();


performs value initialization.

Default initialization calls the constructors of any base classes, and nothing else. Since constructors for intrinsic types do not do anything, that means all member variables will have garbage values; specifically, whatever values happen to reside in the corresponding addresses.

Value initialization also calls the constructors of any base classes. Then, one of two things happens:

• If MyClass is a POD class (all member variables are either intrinsic types or classes that only contain intrinsic types and have no user-defined constructor/destructor), all data is initialized to 0.
• If MyClass is not a POD class, the constructor does not touch any data, which is the same as default initialization (so member variables have garbage values unless explicitly constructed otherwise).

Other points worth noting:

• Intrinsic types, such as int, float, bool, pointers, etc. have constructors that do nothing (not even initialize to zero), destructors that do nothing, and copy constructors and copy assignment-ers that blindly copy bytes.
• Comparison operators, such as “==” or “!=” are never automatically generated by a compiler, even if all base classes and members are comparable.

## Initializing and copying class members¶

### Initialize all members at construction¶

8.12 Class type variables should be defined using direct initialization instead of copy initialization to avoid unwanted and spurious type conversions and constructor calls that may be generated by compilers.

For example, use:

std::string name("Bill");


std::string name = "Bill";


or:

std::string name = std::string("Bill");


8.13 Each class data member must be initialized (using default values when appropriate) in every class constructor. That is, an initializer or initialization must be provided for each class data member so that every object is in a well-defined state upon construction.

Generally, this requires a user-defined default constructor when a class has POD members. Do not assume that a compiler-generated default constructor will leave any member variable in a well-defined state.

Note

Exception: A class that has no data members, including one that is derived from a base class with a default constructor that provides full member initialization, does not require a user-defined default constructor since the compiler-generated version will suffice.

### Know when to use initialization vs. assignment¶

8.14 Data member initialization should be used instead of assignment in constructors, especially for small classes. Initialization prevents needless run-time work and is often faster.

8.15 When using initialization instead of assignment to set data member values in a constructor, data members should always be initialized in the order in which they appear in the class definition.

Compilers adhere to this order regardless of the order that members appear in the class initialization list. So you may as well agree with the compiler rules and avoid potential errors that could result when one member depends on the state of another.

8.16 For classes with complex data members, assignment within the body of the constructor may be preferable.

If the initialization process is sufficiently complex, it may be better to initialize (i.e., assign) member objects in a method that is called after object creation, such as “init()”.

### Use the copy-and-swap idiom¶

8.17 A user-supplied implementation of a class copy-assignment operator should check for assignment to self, must copy all data members from the object passed to operator, and must return a reference to “*this”.

The copy-and-swap idiom should be used.

### Initializing, copying, and inheritance¶

8.18 A constructor must not call a virtual function on any data member object since an overridden method defined in a subclass cannot be called until the object is fully constructed.

There is no general guarantee that data members are fully-created before a constructor exits.

8.19 All constructors and copy operations for a derived class must call the necessary constructors and copy operations for each of its base classes to insure that each object is properly allocated and initialized.

## Prefer composition to inheritance¶

8.20 Class composition should be used instead of inheritance to extend behavior.

Looser coupling between objects is typically more flexible and easier to maintain and refactor.

## Keep inheritance relationships simple¶

8.21 Class hierarchies should be designed so that subclasses inherit from abstract interfaces; i.e., pure virtual base classes.

Inheritance is often done to reuse code that exists in a base class. However, there are usually better design choices to achieve reuse. Good object-oriented use of inheritance is to reuse existing calling code by exploiting base class interfaces using polymorphism. Put another way, “interface inheritance” should be used instead of “implementation inheritance”.

8.22 Deep inheritance hierarchies; i.e., more than 2 or 3 levels, should be avoided.

8.23 Multiple inheritance should be restricted so that only one base class contains methods that are not “pure virtual”.

8.24 “Private” and “protected” inheritance must not be used unless you absolutely understand the ramifications of such a choice and are sure that it will not create design and implementation problems.

Such a choice must be reviewed with team members. There almost always exist better alternatives.

## Design for/against inheritance¶

8.25 One should not inherit from a class that was not designed to be a base class; e.g., if it does not have a virtual destructor.

Doing so is bad practice and can cause problems that may not be reported by a compiler; e.g., hiding base class members. To add functionality, one should employ class composition rather than by “tweaking” an existing class.

8.26 The destructor of a class that is designed to be a base class must be declared “virtual”.

However, sometimes a destructor should not be declared virtual, such as when deletion through a pointer to a base class object should be disallowed.

## Use virtual functions responsibly¶

8.27 Virtual functions should be overridden responsibly. That is, the pre- and post-conditions, default arguments, etc. of the virtual functions should be preserved.

Also, the behavior of an overridden virtual function should not deviate from the intent of the base class. Remember that derived classes are subsets, not supersets, of their base classes.

8.28 Inherited non-virtual methods must not be overloaded or hidden.

8.29 A virtual function in a base class should only be implemented in the base class if its behavior is always valid default behavior for any derived class.

8.30 If a method in a base class is not expected to be overridden in any derived class, then the method should not be declared virtual.

8.31 If each derived class has to provide specific behavior for a base class virtual function, then it should be declared pure virtual.

8.32 Virtual functions must not be called in a class constructor or destructor. Doing so is undefined behavior. Even if it seems to work correctly, it is fragile and potentially non-portable.

## Inline functions¶

Function inlining is a compile time operation and the full definition of an inline function must be seen wherever it is called. Thus, the implementation of every function to be inlined must be provided in a header file.

Whether or not a function implemented in a header file is explicitly declared inline using the “inline” keyword, the compiler decides if the function will be inlined. A compiler will not inline a function that it considers too long or too complex (e.g., if it contains complicated conditional logic). When a compiler inlines a function, it replaces the function call with the body of the function. Most modern compilers do a good job of deciding when inlining is a good choice.

It is possible to specify function attributes and compiler flags that can force a compiler to inline a function. Such options should be applied with care to prevent excessive inlining that may cause executable code bloat and/or may make debugging difficult.

Note

When in doubt, don’t use the “inline” keyword and let the compiler decide whether to inline a function.

### Inline short, simple functions¶

8.33 Simple, short frequently called functions, such as accessors, that will almost certainly be inlined by most compilers should be implemented inline in header files.

### Only inline a class constructor when it makes sense¶

8.34 Class constructors should not be inlined in most cases.

A class constructor implicitly calls the constructors for its base classes and initializes some or all of its data members, potentially calling more constructors. If a constructor is inlined, the construction and initialization needed for its members and bases will appear at every object declaration.

Note

Exception: A class/struct that has only POD members, is not a subclass, and does not explicitly declare a destructor, can have its constructor safely inlined in most cases.

### Do not inline virtual methods¶

8.35 Virtual functions must not be inlined due to polymorphism.

For example, do not declare a virtual class member function as:

inline virtual void foo( ) { }


In most circumstances, a virtual method cannot be inlined because a compiler must do runtime dispatch on a virtual method when it doesn’t know the complete type at compile time.

Note

Exception: It is safe to define an empty destructor inline in an abstract base class with no data members.

Important

### There’s a fine line between clever and…¶

8.36 Operator overloading must not be used to be clever to the point of obfuscation and cause others to think too hard about an operation. Specifically, an overloaded operator must preserve “natural” semantics by appealing to common conventions and must have meaning similar to non-overloaded operators of the same name.

Overloading operators can be beneficial, but should not be overused or abused. Operator overloading is essentially “syntactic sugar” and an overloaded operator is just a function like any other function. An important benefit of overloading is that it often allows more appropriate syntax that more easily communicates the meaning of an operation. The resulting code can be easier to write, maintain, and understand, and it may be more efficient since it may allow the compiler to take advantage of longer expressions than it could otherwise.

8.37 Function overloading must not be used to define functions that do conceptually different things.

Someone reading declarations of overloaded functions should be able to assume (and rightfully so!) that functions with the same name do something very similar.

8.38 If an overloaded virtual method in a base class is overridden in a derived class, all overloaded methods with the same name in the base class must be overridden in the derived class.

This prevents unexpected behavior when calling such member functions. Remember that when a virtual function is overridden, the overloads of that function in the base class are not visible to the derived class.

### Common operators¶

8.39 Both boolean operators “==” and “!=” should be implemented if one of them is.

For consistency and correctness, the “!=” operator should be implemented using the “==” operator implementation. For example:

bool MyClass::operator!= (const MyClass& rhs)
{
return !(this == rhs);
}


8.40 Standard operators, such as “&&”, “||”, and “,” (i.e., comma), must not be overloaded.

Built-in versions of these operators are typically treated specially by a compiler. Thus, programmers cannot implement their full semantics. This can cause confusion. For example, the order of operand evaluation cannot be guaranteed when overloading operators “&&” or “||”. This may cause problems as someone may write code that assumes that evaluation order is the same as the built-in versions.

## Function arguments¶

### Consistent argument order makes interfaces easier to use¶

8.41 Function arguments must be ordered similarly for all routines in an Axom component.

Common conventions are either to put all input arguments first, then outputs, or vice versa. Input and output arguments must not be mixed in a function signature. Parameters that are both input and output can make the best choice unclear. Conventions consistent with related functions must always be followed. When adding a new parameter to an existing method, the established ordering convention must be followed.

Note

When adding an argument to an existing method, do not just stick it at the end of the argument list.

### Pointer and reference arguments and const¶

8.42 Each function argument that is not a built-in type (i.e., int, double, char, etc.) should be passed either by reference or as a pointer to avoid unnecessary copies.

8.43 Each function reference or pointer argument that is not changed by the function must be declared “const”.

### Always name function arguments¶

8.44 Each argument in a function declaration must be given a name that exactly matches the function implementation.

For example, use:

void computeSomething(int op_count, int mode);


not:

void computeSomething(int, int);


## Function return points¶

8.45 Each function should have exactly one return point to make control logic clear.

Functions with multiple return points tend to be a source of errors when trying to understand or modify code, especially if there are multiple return points within a scope. Such code can always be refactored to have a single return point by using local scope boolean variables and/or different control logic.

A function may have two return points if the first return statement is associated with error condition check, for example. In this case, the error check should be performed at the start of the function body before other statements are reached. For example, the following is a reasonable use of two function return points because the error condition check and the return value for successful completion are clearly visible:

int computeSomething(int in_val)
{
if (in_val < 0) { return -1; }

// ...rest of function implementation...

return 0;
}


Note

Exception. If multiple return points actually fit well into the logical structure of some code, they may be used. For example, a routine may contain extended if/else conditional logic with several “if-else” clauses. If needed, the code may be more clear if each clause contains a return point.

## Proper type usage¶

8.46 The “bool” type should be used instead of “int” for boolean true/false values.

8.47 The “string” type should be used instead of “char*”.

The string type supports and optimizes many character string manipulation operations which can be error-prone and less efficient if implemented explicitly using “char*” and standard C library functions. Note that “string” and “char*” types are easily interchangeable, which allows C++ string data to be used when interacting with C routines.

8.48 An enumeration type should be used instead of macro definitions or “int” data for sets of related constant values.

Since C++ enums are distinct types with a compile-time specified set of values, there values cannot be implicitly cast to integers or vice versa – a “static_cast” operator must be used to make the conversion explicit. Thus, enums provide type and value safety and scoping benefits.

In many cases, the C++11 enum class construct should be used since it provides stronger type safety and better scoping than regular enum types.

## Templates¶

8.49 A class or function should only be made a template when its implementation is independent of the template type parameter.

Note that class member templates (e.g., member functions that are templates of a class that is not a template) are often useful to reduce code redundancy.

8.50 Generic templates that have external linkage must be defined in the header file where they are declared since template instantiation is a compile time operation. Implementations of class templates and member templates that are non-trivial should be placed in the class header file after the class definition.

## Use const to enforce correct usage¶

8.51 The “const” qualifier should be used for variables and methods when appropriate to clearly indicate usage and to take advantage of compiler-based error-checking. For example, any class member function that does not change the state of the object on which it is called should be declared “const”

Constant declarations can make code safer and less error-prone since they enforce intent at compile time. They also improve code understanding because a constant declaration clearly indicates that the state of a variable or object will not change in the scope in which the declaration appears.

8.52 Any class member function that does not change a data member of the associated class must be declared “const”.

This enables the compiler to detect unintended usage.

8.53 Any class member function that returns a class data member that should not be changed by the caller must be declared “const” and must return the data member as a “const” reference or pointer.

Often, both “const” and non-“const” versions of member access functions are needed so that callers may declare the variable that holds the return value with the appropriate “const-ness”.

## Casts and type conversions¶

### Avoid C-style casts, const_cast, and reinterpret_cast¶

8.54 C-style casts must not be used.

All type conversions must be done explicitly using the named C++ casting operators; i.e., “static_cast”, “const_cast”, “dynamic_cast”, “reinterpret_cast”.

8.55 The “const_cast” operator should be avoided.

Casting away “const-ness” is usually a poor programming decision and can introduce errors.

Note

Exception: It may be necessary in some circumstances to cast away const-ness, such as when calling const-incorrect APIs.

8.56 The “reinterpret_cast” must not be used unless absolutely necessary.

This operator was designed to perform a low-level reinterpretation of the bit pattern of an operand. This is needed only in special circumstances and circumvents type safety.

### Use the explicit qualifier to avoid unwanted conversions¶

8.57 A class constructor that takes a single non-default argument, or a single argument with a default value, must be declared “explicit”.

This prevents compilers from performing unexpected (and, in many cases, unwanted!) implicit type conversions. For example:

class MyClass
{
public:
explicit MyClass(int i, double x = 0.0);
};


Note that, without the explicit declaration, an implicit conversion from an integer to an object of type MyClass could be allowed. For example:

MyClass mc = 2;


Clearly, this is confusing. The “explicit” keyword forces the following usage pattern:

MyClass mc(2);


to get the same result, which is much more clear.

## Memory management¶

### Allocate and deallocate memory in the same scope¶

8.58 Memory should be deallocated in the same scope in which it is allocated.

8.59 All memory allocated in a class constructor should be deallocated in the class destructor.

Note that the intent of constructors is to acquire resources and the intent of destructors is to free those resources.

8.60 Pointers should be set to null explicitly when memory is deallocated. This makes it easy to check pointers for “null-ness” when needed.

### Use new/delete consistently¶

8.61 Data managed exclusively within C++ code must be allocated and deallocated using the “new” and “delete” operators.

The operator “new” is type-safe, simpler to use, and less error-prone than the “malloc” family of C functions. C++ new/delete operators must not be combined with C malloc/free functions.

8.62 Every C++ array deallocation statement must include “[ ]” (i.e., “delete[ ]”) to avoid memory leaks.

The rule of thumb is: when “[ ]” appears in the allocation, then “[ ]” must appear in the corresponding deallocation statement.