Struct (C programming language)

A struct in the C programming language (and many derivatives) is a complex data type declaration that defines a physically grouped list of variables to be placed under one name in a block of memory, allowing the different variables to be accessed via a single pointer, or the struct declared name which returns the same address. The struct can contain many other complex and simple data types in an association, so is a natural organizing type for records like the mixed data types in lists of directory entries reading a hard drive (file length, name, extension, physical (cylinder, disk, head indexes) address, etc.), or other mixed record type (patient names, address, telephone... insurance codes, balance, etc.).

The C struct directly corresponds to the assembly language data type of the same use, and both reference a contiguous block of physical memory, usually delimited (sized) by word-length boundaries. Language implementations which could utilize half-word or byte boundaries (giving denser packing, using less memory) were considered advanced in the mid-eighties. Being a block of contiguous memory, each variable within is located at a fixed offset from the index zero reference, the pointer. As an illustration, many BASIC interpreters once fielded a string data struct organization with one value recording string length, one indexing (cursor value of) the previous line, one pointing to the string data.

Because the contents of a struct are stored in contiguous memory, the sizeof operator must be used to get the number of bytes needed to store a particular type of struct, just as it can be used for primitives. The alignment of particular fields in the struct (with respect to word boundaries) is implementation-specific and may include padding, although modern compilers typically support the #pragma pack directive, which changes the size in bytes used for alignment.

In the C++ language, a struct is identical to a C++ class but a difference in the default visibility exists: class members are by default private, whereas struct members are by default public.

Like its C counterpart, the struct data type in C# (Structure in Visual Basic .NET) is similar to a class. The biggest difference between a struct and a class in these languages is that when a struct is passed as an argument to a function, any modifications to the struct in that function will not be reflected in the original variable (unless pass-by-reference is used).

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