Sessions on advanced C++ programming

A.1. Constant magic

Topics

• Arrays and string literals
  Be carefull the difference between character arrays and string literals. Arrays are alwais allocated in the user data area, and they are mutable, string literals, however, has type const char array, and they are frequently allocated in read-only areas. Modification attempt of such literals can cause run-time errors.
• When constant is stored in memory
  The fact, that a C++ object is constant, doesn't mean memory is not allocated to store it. There is a different concept here: whether the value of the constant is known at compilation time. Value known in compilation time is called constant expression.
• Using constants to detect errors
  Constants could be used to map semantic information to the language syntax. Therefore the compiler at least has some chance to detect the problem.
• Scope rules for constants
  Constants have static linkage by default - similarly to typedef. This could allow confusion between source files. However, it is possible to declare a const as extern, so one can define the value of a constant in exactly one source file, and declare it in the other files.
• Constant class members
  Class members could be - and frequently should be - constants. Constant class attributes (data members) should be initialized in the initializer list of a constructor. Initializer list also used to initialize subobjects without default constructors and reference members.
• Static const
  One can define a static const member in a class, and can use similar way to the non-const static members: this members must be defined outside of the class definition: tipically in the .cpp file. However, there is an exceptional rule for integral data types: those could be initialized inside the class definition.
• Constant memberfunctions
  Class methods (member functions) could be defined as constant. This qualifier means two things: first this functions are allowed to be called on constant objects, second: there is a compilation time guarantee, that such methods do not alter their objects. The this pointer for constant members are declared as pointer to const
• Const_iterator
  Iterators to constant: const_iterator is used in the STL library. It has similar role to the pointer to constants for ordinary types. Operator* and Operator-> for const_iterator return with const reference, so they are not left values. Be care: const_iterator and const iterator is not the same!