A class in C++ is a user-defined type or data structure declared with any of the keywords class, struct or union (the first two are collectively referred to as non-union classes) that has data and functions (also called member variables and member functions) as its members whose access is governed by the three access specifiers private, protected or public.
A POD-struct (Plain Old Data Structure) is a non-union aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined assignment operator and no user-defined destructor.
These members include variables (including other structures and classes), functions (specific identifiers or overloaded operators) known as member functions, constructors and destructors.
Either code will define objects of type Person as having two public data members, name and age.
This is commonly implemented by passing the address of the object as an implicit first argument to the function.
The interface, needed by the user, is kept in a header and the implementation is kept separately in either source or compiled form.
A properly written C++ program shouldn't make any assumptions about the layout of inherited fields, in any case.
Whenever the compiler needs to convert a pointer from the D type to either P or C, the compiler will provide an automatic conversion from the address of the derived class to the address of the base class fields (typically, this is a simple offset calculation).
[14][15] final is not a reserved word in C++, and is instead defined as a contextual keyword, in order to not conflict with uses of the identifier 'final' in existing codebases.
By convention, overloaded operators should behave nearly the same as they do in built-in datatypes (int, float, etc.
For clearer presentation (although this could decrease efficiency of the program if the compiler cannot optimize the statement into the equivalent one above), the above code can be rewritten as: Programmers can also put a prototype of the operator in the struct declaration and define the function of the operator in the global scope: i above represents the sender's own member variable, while k.i represents the member variable from the argument variable k. The const keyword appears twice in the above code.
The second incidence at the end of the declaration promises the compiler that the sender would not be changed by the function run.
Examples are the negative sign (when nothing is put on the left of it) and the "logical NOT" (exclamation mark, !
Replace return_type with the datatype of the return value (int, bool, structures etc.)
The int parameter essentially means nothing but a convention to show that the sender is on the left of the operator.
Like the assignment (=) operator, they are also overloaded by default if no specific declaration is made.
Sometimes programmers may want their variables to take a default or specific value upon declaration.
In that case an initialized Person type variable can be thought of as the return value: An alternate syntax that does the same thing as the above example is Specific program actions, which may or may not relate to the variable, can be added as part of the constructor.
It is called when an instance of a class is destroyed, e.g. when an object of a class created in a block (set of curly braces "{}") is deleted after the closing brace, then the destructor is called automatically.
Destructors can be used to release resources, such as heap-allocated memory and opened files when an instance of that class is destroyed.
However, the compiler may add padding between the variables or at the end of the structure to ensure proper data alignment for a given computer architecture, often padding variables to be 32-bit aligned.
Bit fields are used to define the class members that can occupy less storage than an integral type.
This field is applicable only for integral types (int, char, short, long, etc.)
Unions are also allowed to have bit-field members: Many programmers prefer to use the ampersand (&) to declare the arguments of a function involving structures.
This is because by using the dereferencing ampersand only one word (typically 4 bytes on a 32 bit machine, 8 bytes on a 64 bit machine) is required to be passed into the function, namely the memory location to the variable.
Otherwise, if pass-by-value is used, the argument needs to be copied every time the function is called, which is costly with large structures.
To facilitate classes' ability to reference themselves, C++ implements the this keyword for all member functions.