# Struct (C programming language)

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. For example a bank customer struct might contains fields: name, address, telephone, balance.
A struct occupies a contiguous block of memory, usually delimited (sized) by word-length boundaries. It corresponds to the similarly named feature available in some assemblers for Intel processors. Being a block of contiguous memory, each field within a struct is located at a certain fixed offset from the start.
The sizeof operator results in the number of bytes needed to store a particular struct, just as it does for a primitive data type. The alignment of particular fields in the struct (with respect to word boundaries) is implementation-specific and may include padding. Modern compilers typically support the #pragma pack directive, which sets the size in bytes for alignment.
The C struct feature was derived from the same-named concept in ALGOL 68.

## Declaration

The syntax for a struct declaration is shown by this simple example:
The tag_name is optional in some contexts.

## Typedef

Via the keyword typedef, a struct type can be referenced without using the struct keyword. However, some programming style guides advise against this, claiming that it can obfuscate the type.
For example:
In C++ code, typedef is not needed because types defined via struct are part of the regular namespace, so the type can be referred to as either struct thing_t or thing_t.

## Initialization

There are three ways to initialize a structure.
For the type:
C89-style initializers are used when contiguous members may be given. For example:
For non contiguous or out of order members list, designated initializer style may be used. For example:
If an initializer is given or if the object is statically allocated, omitted elements are initialized to 0.
A third way of initializing a structure is to copy the value of an existing object of the same type. For example:

## Copy

The state of a struct can be copied to another instance. A compiler might use memcpy to copy the bytes of the memory block.

## Pointers

Pointers can be used to refer to a struct by its address. This is useful for passing a struct to a function to avoid the overhead of copying the struct. The -> operator dereferences the pointer (left operand) and accesses the value of a struct member (right operand).

## In other languages
### C++

In C++, struct is essentially the same as for C. Further, a class is the same as a struct but with different default visibility: class members are private by default, whereas struct members are public by default.

### .NET

.NET languages have a feature similar to struct in C – called struct in C# and Structure in Visual Basic .NET). This construct provides many features of a class, but acts as a value type instead of a reference type. For example, when passing a .NET struct to a function, the value is copied so that changes to the input parameter do not affect the value passed in.