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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.
When a class constructor does not explicitly call the constructor of one of its object-valued fields in its initializer list, the default constructor for the field's class is called. In the standard library, certain containers "fill in" values using the default constructor when the value is not given explicitly. E.g.
In the C programming language, struct is the keyword used to define a composite, a.k.a. record, data type – a named set of values that occupy a block of memory. It allows for the different values to be accessed via a single identifier, often a pointer. A struct can contain other data types so is used for mixed-data-type records.
On returning objects from functions or passing objects by value, the objects copy constructor will be called implicitly, unless return value optimization applies. C++ implicitly generates a default copy constructor which will call the copy constructors for all base classes and all member variables unless the programmer provides one, explicitly ...
Copy constructors are the standard way of copying objects in C++, as opposed to cloning, and have C++-specific nuances. The first argument of such a constructor is a reference to an object of the same type as is being constructed (const or non-const), which might be followed by parameters of any type (all having default values).
Before C++11, the values of variables could be used in constant expressions only if the variables are declared const, have an initializer which is a constant expression, and are of integral or enumeration type. C++11 removes the restriction that the variables must be of integral or enumeration type if they are defined with the constexpr keyword:
In C++, default constructors take no parameters, instantiating the object members with their appropriate default values, "which is normally zero for numeral fields and empty string for string fields". [11] For example, a default constructor for a restaurant bill object written in C++ might set the tip to 15%:
With the advent of C++11 the rule of three can be broadened to the rule of five (also known as "the rule of the big five" [5]) as C++11 implements move semantics, [6] allowing destination objects to grab (or steal) data from temporary objects. The following example also shows the new moving members: move constructor and move assignment operator.