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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.
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.
C++ uses the three modifiers called public, protected, and private. [3] C# has the modifiers public, protected,internal, private, protected internal, private protected, and file. [4] Java has public, package, protected, and private; package is the default, used if no other access modifier keyword is specified. The meaning of these modifiers may ...
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 C, declaring a new type with the same name as an existing struct, union or enum is valid, but it is invalid in C++, because in C, struct, union, and enum types must be indicated as such whenever the type is referenced whereas in C++, all declarations of such types carry the typedef implicitly.
C++ allows more than one constructor. The other constructors must have different parameters. Additionally constructors which contain parameters which are given default values, must adhere to the restriction that not all parameters are given a default value. This is a situation which only matters if there is a default constructor.
Struct and union return types with sizes of two pointers or fewer are returned in RAX and RDX (or XMM0 and XMM1). When an oversized struct return is needed, another pointer to a caller-provided space is prepended as the first argument, shifting all other arguments to the right by one place, and the value of this pointer is returned in RAX.
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).