effective-c++-3rd-edition.markdown

Effective C++, Third Edition

by Scott Meyers

I, Michael Parker, own this book and took these notes to further my own learning. If you enjoy these notes, please purchase the book!

Chapter 1: Accustoming Yourself to C++

Item 1: View C++ as a federation of languages

• There is C, object-oriented C++, templates and template metaprogramming, and the STL.

Item 2: Prefer const, enum, and inline to #define

• Unlike when using a #define, if an error occurs when using a constant, then its name is included in the error message.
• For a constant pointer in the header file, declare both the pointer and the data it points to as const.
• In-class initialization is allowed only for integral types, and only for constants.
• When using a macro, remember to parenthesize all the arguments, and beware expressions being evaluated multiple times if used as arguments.

Item 3: Use const whenever possible

• Using const is wonderful because it allows the compiler to enforce a semantic constraint.
• Declaring an iterator const means it isn't allowed to point to something different, but whatever it points to may be modified.
• One of the hallmarks of good user-defined types is that they avoid gratuitous incompatibilities with built-in types.
• A const member function can overload a non-const member function, and the former will be used on const objects.
• Bitwise const is C++'s definition of const. Logical const is when bits in the object are changed, but in ways that the client cannot detect.
• The mutable keyword frees non-static data members from the constraints of bitwise const.
• By calling static_cast to add const to this, and then const_cast to remove const from the return value, the overloaded non-const function can call the const one.

Item 4: Make sure that objects are initialized before they're used

• Reading uninitialized values yields undefined behavior, so always initialize objects before you use them.
• Always listing every data member on the initialization list avoids having to remember which data members may go uninitialized.
• Within a class, data members are initialized in the order they're declared, and not in their order on the initialization list.
• If a non-local static object in one translation unit uses a non-local static object in another translation unit, it may be uninitialized, because their initialization order is undefined.
• Using static objects defined in functions eliminates this problem, and if you never call such a function, you don't even construct its static object.

Chapter 2: Constructors, Destructors, and Assignment Operators

Item 5: Know what functions C++ silently writes and calls

• A compiler declares a copy constructor, copy assignment operator, and destructor if you don't declare them yourself, as well as a default constructor if you declare none.
• The generated destructor is not virtual unless the class inherits from a base class with a virtual destructor.
• You must define the copy constructor yourself if the class contains a reference member or a const member.

Item 6: Explicitly disallow the use of compiler-generated functions you do not want

• Declare the copy constructor and the copy assignment operator private to prevent the compiler from generating its own version.
• To stop member and friend functions from still calling them, don't actually define them; this generates an error during the linking stage.

Item 7: Declare destructors virtual in polymorphic base classes

• When a derived class object is deleted through a pointer to the base class with a non-virtual destructor, results are undefined, but typically the derived part isn't destroyed.
• If the class isn't intended to be a base class, making the destructor virtual increases its size, as this adds a vptr (virtual table pointer) and vtbl (virtual table).
• The string class and STL container types (vector, list, set, etc.) lack virtual destructors, and so should never be inherited from.

Item 8: Prevent exceptions from leaving destructors

• Depending on the circumstances, if two destructors simultaneously emit exceptions, program execution either terminates or yields undefined behavior.
• One option is to terminate the program in the destructor, thereby preventing any undefined behavior.
• The second option is to swallow the exception, but only if the program can reliably continue after the exception was ignored.
• Good practice is to try and move the operation that can generate an exception to outside the destructor.

Item 9: Never call virtual functions during construction or destruction

• During base class construction, virtual function calls never go down into a derived class, because an object is not of a derived class until its constructor begins execution.
• The same reasoning applies during destruction.
• You must ensure that your constructors or destructors don't call virtual functions, and that all the functions they call obey the same constraint.

Item 10: Have assignment operators return a reference to *this

• This is what all built-in types do, thereby allowing chained assignments, and it applies to all assignment operators (such as operator+=).

Item 11: Handle assignment to self in operator=

• Code that operates on references or pointers to multiple objects of the same type must consider that the objects might be the same.
• Guard against self-assignment by checking the argument's address against this at the top of operator+=.
• In many cases, a careful ordering of statements can yield code that guards against both exceptions and self-assignment.
• Another alternative that guards against both exceptions and self-assignment is the "copy and swap" technique, which is covered in Item 29.

Item 12: Copy all parts of an object

• When you add new data members to a class, be sure to update its copy constructor and copy assignment operator accordingly.
• A copying function should copy all local data members, and also invoke the appropriate copying function in all base classes.

Chapter 3: Resource Management

Item 13: Use objects to manage resources

• A thrown exception or a premature return, continue, or goto statement might preclude execution from eaching a delete statement.
• By putting resources inside objects like auto_ptr, we can rely on C++'s destructor invocation to ensure that the resources are released.
• Resource Acquisition Is Initialization, or RAII, means acquiring a resource and initializing its managing object happen in the same statement.
• Copying an auto_ptr sets it to null. While this enforces an object being managed by only one auto_ptr, you cannot use them in STL containers.
• There is no auto_ptr or shared_ptr for dynamically allocated arrays because you should be using vector instead.

Item 14: Think carefully about copying behavior in resource-managing classes

• For resources that are not heap-based, smart pointers like auto_ptr and shared_ptr are inappropriate as resource handlers.
• Policies for copying an RAII object include prohibiting copying, and reference counting, copying, and transferring ownership of the underlying resource.

Item 15: Provide access to raw resources in resource-managing classes

• An implicit conversion function on the RAII class can make access to the raw resource easier, but this increases the chance of errors.
• Often an explicit conversion function simply named get is preferable, even if clients must explicitly call it each time.
• Returning the raw resource violates encapsulation, but RAII classes don't exist simply to encapsulate it, but to ensure that it is released.

Item 16: Use the same form in corresponding uses of new and delete

• Using the expression delete when delete [] should be used results in undefined behavior.
• The memory for an array usually includes the size of the array, so that the delete operator knows how many destructors to call.
• Use the same form of new in all constructors that initialize a pointer member, or else you may use the wrong form of delete.
• The string and vector classes largely eliminate the need to dynamically allocate an array.

Item 17: Store new objects in smart pointers in standalone statements

• Compilers are given less leeway in reordering operations across statements than within them.

Chapter 4: Designs and Declarations

Item 18: Make interfaces easy to use correctly and hard to use incorrectly

• The type system is your primary ally in preventing undesirable code from compiling.
• Restrict what can be done with a type. A common way to impose restrictions is to add const wherever you can.
• Avoid gratuitous incompatibilities with the built-in types so that interfaces behave consistently, thereby reducing mental friction.
• Any interface that requires clients to remember to do something is prone to incorrect use, because clients can forget to do it.
• In many applications, the additional runtime costs of resource managers are unnoticeable, but the reduction in client errors will be noticed by everyone.

Item 19: Treat class design as type design

• Approach class design with the same care that language designers lavish on the design of built-in types.
• Good types have a natural syntax, intuitive semantics, and one or more efficient implementations.
• If you inherit from existing classes, you are constrained by their design, particularly by whether their functions are virtual or not.
• Guarantees made with respect to performance, exception safety, and resource usage impose constraints on your implementation.
• If you're defining a whole family of types, you don't want to define a new class, you want to define a new class template.
• If you're only subclassing so you can add functionality to an existing class, consider non-member functions or templates instead.

Item 20: Prefer pass-by-reference-to-const to pass-by-value

• Passing by reference eliminates the slicing problem, where passing a derived class object to a function that accepts a base class object calls the base class copy constructor.
• References are typically implemented as pointers, so passing built-in types like int by value is usually more efficient.
• Implementers of iterators and function objects ensure that they are efficient to copy and are not subject to the slicing problem.
• Avoid passing a user-defined type by value because while its size may be small now, that is subject to change with its implementation.

Item 21: Don't try to return a reference when you must return an object

• A function should never return a reference or pointer to a local object that is destroyed when the function exits.
• Returning a reference from a function like operator* is incorrect. Such a function must return a new object.
• In some cases the construction and destruction of such a return value can be safely eliminated by the compiler.

Item 22: Declare data members private

• Many data members should be hidden, and rarely does every data member require a getter and a setter.
• Only functional interfaces makes it easy to notify other objects when variables are read or written, to verify class invariants and function pre- and postconditions, to perform synchronization in threaded environments, etc.
• Public means unencapsulated, which means an unchangeable implementation, especially for classes that are widely used.
• Protected data is as unencapsulated as public data, since changing such a data member could break all derived classes that use it, which is an unknowably large amount of code.

Item 23: Prefer non-member non-friend functions to member functions

• The more functions that can access data, the less the data is encapsulated, and the harder it is to change the characteristics of the data.
• Unlike a member function, a non-member non-friend function doesn't increase the count of functions that can access the private parts of a class.
• Consider putting the non-member function in the same namespace as the class it operates on.
• Partitioning functionality in a namespace across multiple files promotes clean organization, and clients can freely add more functions to the namespace.

Item 24: Declare non-member functions when type conversions should apply to all parameters

• For overloaded operators, compilers will also look at non-member functions accepting two parameters in the namespace or global scope.
• Parameters are eligible for implicit type conversion only if they are listed on the parameter list. The object on which the member function is invoked is never eligible.
• To support mixed mode arithmetic with operator overloading, make operators non-member functions accepting both operands as arguments.
• The opposite of a member function is a non-member function, not a friend function. Avoid friend functions when you can.

Item 25: Consider support for a non-throwing swap

• If using the "pimp idiom," define a member function named swap that does the actual swapping, then specialize std::swap to call that member function.
• It's okay to totally specialize templates in std, adding new templates, classes, functions, or anything else to std results in undefined behavior.
• In addition to the specialization of std::swap, write a non-member version of swap in the same namespace of your class.
• By prefacing a call to swap with using std::swap, compilers will look for the namespace definition first, then the specialization in std, and finally the general form.

Chapter 5: Implementations

Item 26: Postpone variable definitions as long as possible

• Postponing declaring a variable until you have its initialization arguments avoids unnecessary default constructions.
• Assigning a variable defined outside a loop is more efficient than initializing it on ever iteration, because it avoids destructing when each iteration completes.

Item 27: Minimize casting

• Casts can subvert the type system, which is there to ensure that you're not trying to perform any unsafe or nonsensical operations on objects.
• Only use reinterpret_cast for low-level casts in low-level code, such as from a pointer to an int. It yields unportable results.
• Use static_cast to force implicit conversions as well as the reverse of such conversions, with the exception of const to non-const.
• Prefer the explicit, new-style casts. They are easier to search for, and their narrow purpose makes it possible for compilers to diagnose usage errors.
• Type conversions of any kind, either explicit via casts or implicit by compilers, often lead to additional code that is executed at runtime.
• Avoid making assumptions about how things are laid out in C++. Making casts based on such assumptions leads to undefined behavior.
• A dynamic_cast can have a significant runtime cost. If you need to perform derived class operations through a pointer or reference to the base class, explore alternative designs.
• Casts should be isolated as much as possible, typically hidden inside functions whose interfaces shield callers from the work done inside.

Item 28: Avoid returning "handles" to object internals

• A data member is only as encapsulated as the most accessible function returning a reference to it.
• Returning const references to data members can still lead to dangling handles, which refer to parts of objects that don't exist any longer.
• Functions that return a handle to an object internal are the exception and not the rule.

Item 29: Strive for exception-safe code

• Exception-safe functions don't leak resources, and don't allow data structures or objects to enter a corrupted state.
• The basic guarantee ensures that the program remains in a valid state if an exception is thrown, but that exact state may ot be predictable. The strong guarantee ensures that the state is unchanged.
• Often you can't guarantee that no exceptions are thrown, because anything that dynamically allocates memory can throw a bad_alloc exception.
• When functions have side-effects on non-local data instead of operating exclusively on local state, it's much harder to offer the strong guarantee.
• If a system has even a single function that's not exception-safe, then the system as a whole is not exception-safe.
• A function's exception-safety guarantee is a visible part of its interface, and you should choose it as deliberately as you choose all other interface aspects.

Item 30: Understand the ins and outs of inlining

• Compiler optimizations are typically designed for stretches of code that lack function calls, so inlining allows more optimizations.
• Inlined code can also lead to additional paging, a reduced instruction cache hit rate, and their accompanying performance penalties.
• The inline keyword is a request that compilers may ignore, and only the most trivial functions that are not virtual may be inlined.
• Even empty constructors and destructors are unlikely to be inlined, as they implicitly call the constructors of base classes and data members.
• Debuggers have trouble with inlined functions. For example, you can't set a breakpoint in a function that isn't there.

Item 31: Minimize compilation dependencies between files

• Instead of trying to forward-declare parts of the standard library, use the proper #include statements and be done with it.
• You never need a class definition to declare a function using that class. Forward declare the class, and shift the burden of including its definition to clients that call the function.
• A class that employs the pimpl idiom is often called a handle class.
• If a function is declared as pure virtual in an interface class, then there is no need to include the keyword virtual in its declaration in the subclass.
• Handle classes and interface classes decouple interfaces from implementations and thereby reduce compilation dependencies between files.

Chapter 6: Inheritance and Object-Oriented Design

Item 32: Make sure public inheritance models "is-a"

• The most important rule in object-oriented programming in C++ is that public inheritance means "is-a".
• There is no one ideal design for all software. The best design depends on what the system is expected to do, both now and in the future.
• Good interfaces prevent invalid code from compiling. Prefer design that rejects operations during compilation than one that rejects at runtime.
• A class named Square extending Rectangle is a classic example of the fragile nature of class hierarchies.

Item 33: Avoid hiding inherited names

• If a function in a derived class has the same name as a function in the base class, it hides all overloaded forms of that function in the base class.
• To preserve the is-a relationship of inheritance, the derived class must include a using declaration to inherit all overloaded forms of a function with a given name.

Item 34: Differentiate between inheritance of interface and inheritance of implementation

• Pure virtual functions must be redeclared by any concrete class that inherits them.
• A simple virtual function allows a derived class to inherit a function interface as well as an implementation.
• A pure virtual function can still have an implementations of its own. A subclass must redeclare it, but it can call down to this "default" implementation.
• A non-virtual function serves a mandatory implementation, and should never be redefined in a derived class.
• Don't blindly declare all member functions virtual, and don't blindly declare all member functions non-virtual. Consider each one individually.

Item 35: Consider alternatives to virtual functions

• When a non-virtual member function calls a private virtual member function, subclasses can redefine the latter. This is a form of the template method design pattern.
• The only way to resolve the need for non-member functions to access the non-public parts of a class is to weaken its encapsulation.
• The tr1::function class can refer to anything that acts like a function and returns a type convertible to the specified type.
• The "standard" strategy pattern offers the possibility that an existing strategy can be tweaked via defining a subclass.

Item 36: Never redefine an inherited non-virtual function

• Non-virtual functions are statically bound, so calling one on a base class pointer or reference uses the base class implementation, and not any derived class implementation.
• A non-virtual function is an invariant over specialization for the base class, and so no derived class should try to redefine it.
• Item 7, which warns against not specifying a virtual destructor in a base class, is a special case of this item.

Item 37: Never redefine a function's inherited default parameter value

• An object's dynamic type is determined by the object to which it currently refers, which in turn determines how it will behave.
• Default parameters are statically bound, so invoking a virtual function defined in a derived class uses a default parameter value from the base class.
• To avoid duplication of the default parameter, use the non-virtual interface idiom, where the default parameter is only defined once in the public non-virtual function.

Item 38: Model "has-a" or "is-implemented-in-terms-of" through composition

• Composition in the application domain expresses a has-a relationship, while in the implementation domain it expresses an is-implemented-in-terms-of relationship.
• Inappropriate subclassing violates the is-a principle, and should be replaced with composition.

Item 39: Use private inheritance judiciously

• With private inheritance, you cannot assign a derived object to a base class pointer, and protected and public members in the base class are private in the derived class.
• Use composition whenever you can, and use private inheritance whenever you must.
• Use private inheritance if two classes don't have an is-a relationship, but one needs to access the protected members or redefine a virtual function in the other.
• If a class privately inherits from another and defines a virtual function, its own subclasses can redefine that function, even if they can't call it.

Item 40: Use multiple inheritance judiciously

• If a function is defined in two base classes, then a call in the derived class to that function is always ambiguous, even if only one definition is accessible.
• Virtual inheritance prevents replicating data in the base class when multiple inheritance is used, but it's slower and creates larger objects.
• The one reasonable of multiple inheritance is to combine public inheritance of an interface with private inheritance of an implementation.
• If the only design you can come up with involves multiple inheritance, you should think a little harder.