Structs and Unions

Structs

Struct Declaration

A struct is a type consisting of a sequence of members whose storage is allocated in an ordered sequence (as opposed to union, which is a type consisting of a sequence of members whose storage overlaps).

The type specifier for a struct is identical to the  union type specifier except for the keyword used:

Syntax

1) Struct definition: introduces the new type struct name and defines its meaning

2) If used on a line of its own, as in  struct name  ;,  declares but doesn't define the struct  name (see forward declaration below). In other contexts, names the previously-declared struct, and attr-spec-seq is not allowed.

Explanation

Within a struct object, addresses of its elements (and the addresses of the bit field allocation units) increase in order in which the members were defined. A pointer to a struct can be cast to a pointer to its first member (or, if the member is a bit field, to its allocation unit). Likewise, a pointer to the first member of a struct can be cast to a pointer to the enclosing struct. There may be unnamed padding between any two members of a struct or after the last member, but not before the first member. The size of a struct is at least as large as the sum of the sizes of its members.

struct s { int n; double d[]; }; // s.d is a flexible array member 
 
    struct s t1 = { 0 };         // OK, d is as if double d[1], but UB to access
    struct s t2 = { 1, { 4.2 } }; // error: initialization ignores flexible array
 
    // if sizeof (double) == 8
    struct s *s1 = malloc(sizeof (struct s) + 64); // as if d was double d[8]
    struct s *s2 = malloc(sizeof (struct s) + 40); // as if d was double d[5]
 
    s1 = malloc(sizeof (struct s) + 10); // now as if d was double d[1]. Two bytes excess.
    double *dp = &(s1->d[0]);    // OK
    *dp = 42;                    // OK
    s1->d[1]++;                  // Undefined behavior. 2 excess bytes can't be accessed 
                                 // as double.
 
    s2 = malloc(sizeof (struct s) + 6);  // same, but UB to access because 2 bytes are
                                         // missing to complete 1 double
    dp = &(s2->d[0]);            //  OK, can take address just fine
    *dp = 42;                    //  undefined behavior
 
    *s1 = *s2; // only copies s.n, not any element of s.d
               // except those caught in sizeof (struct s)

struct v {
   union { // anonymous union
      struct { int i, j; }; // anonymous structure
      struct { long k, l; } w;
   };
   int m;
} v1;
 
v1.i = 2;   // valid
v1.k = 3;   // invalid: inner structure is not anonymous
v1.w.k = 5; // valid

Forward declaration

A declaration of the following form

hides any previously declared meaning for the name name in the tag name space and declares name as a new struct name in current scope, which will be defined later. Until the definition appears, this struct name has  incomplete type.

This allows structs that refer to each other:

struct y;
struct x {struct y *p;/* ... */};
struct y {struct x *q;/* ... */};

Note that a new struct name may also be introduced just by using a struct tag within another declaration, but if a previously declared struct with the same name exists in the tag name space, the tag would refer to that name

struct s* p = NULL; // tag naming an unknown struct declares it
struct s { int a; }; // definition for the struct pointed to by p
void g(void)
{
    struct s; // forward declaration of a new, local struct s
              // this hides global struct s until the end of this block
    struct s *p;  // pointer to local struct s
                  // without the forward declaration above,
                  // this would point at the file-scope s
    struct s { char* p; }; // definitions of the local struct s
}

Keywords

struct

Notes

Because members of incomplete type are not allowed, and a struct type is not complete until the end of the definition, a struct cannot have a member of its own type. A pointer to its own type is allowed, and is commonly used to implement nodes in linked lists or trees.

Because a struct declaration does not establish scope, nested types, enumerations and enumerators introduced by declarations within struct-declaration-list are visible in the surrounding scope where the struct is defined.

Example

#include <stddef.h>
#include <stdio.h>
 
int main(void)
{
    struct car { char *make; char *model; int year; }; // declares the struct type
    // declares and initializes an object of a previously-declared struct type
    struct car c = {.year=1923, .make="Nash", .model="48 Sports Touring Car"};
    printf("car: %d %s %s\n", c.year, c.make, c.model);
 
    // declares a struct type, an object of that type, and a pointer to it
    struct spaceship { char *make; char *model; char *year; }
    ship = {"Incom Corporation", "T-65 X-wing starfighter", "128 ABY"},
    *pship = &ship;
    printf("spaceship: %s %s %s\n", ship.year, ship.make, ship.model);
 
    // addresses increase in order of definition
    // padding may be inserted
    struct A { char a; double b; char c;};
    printf("offset of char a = %zu\noffset of double b = %zu\noffset of char c = %zu\n"
           "sizeof(struct A) = %zu\n", offsetof(struct A, a), offsetof(struct A, b),
           offsetof(struct A, c), sizeof(struct A));
    struct B { char a; char b; double c;};
    printf("offset of char a = %zu\noffset of char b = %zu\noffset of double c = %zu\n"
           "sizeof(struct B) = %zu\n", offsetof(struct B, a), offsetof(struct B, b),
           offsetof(struct B, c), sizeof(struct B));
 
    // A pointer to a struct can be cast to a pointer to its first member and vice versa
    char* pmake = (char*)pship;
    pship = (struct spaceship *)pmake;
}

car: 1923 Nash 48 Sports Touring Car
spaceship: 128 ABY Incom Corporation T-65 X-wing starfighter
offset of char a = 0
offset of double b = 8
offset of char c = 16
sizeof(struct A) = 24
offset of char a = 0
offset of char b = 1
offset of double c = 8
sizeof(struct B) = 16

Unions

Unions are similar to structures but only one member within the union can be used at a time, due to it having a shared memory size for all members.

union identifier {
     char desc[50] ;
     float price ;
     int devs ;
     } ;

Unions are used when just one condition will be applied and only one variable is required.

#include <iostream>
using namespace std ;
 
union opSysName {
    char desc[50];
    float price;
    int devs;
} ;
 
int main() {
 
    union
    opSysName linux = {"fun, free and better!"} ;
    cout << "The best OS might just possibly be " << linux.desc ;
 
    /*set price - remembering that a union can only hold one variable at a time*/
  
     linux.price = 0.01 ;
    cout << " It costs less than $" << linux.price ;
 
    linux.devs = 1200 ;
    cout << " and has about " << linux.devs << "
    developers on each release!" << endl << endl ;
 
    union opSysName windows = {"restrictive, proprietary and monopolistic!"} ;
 
    cout << "A certain other consumer OS is generally
    " << windows.desc ;
 
    windows.price = 99.99 ;
 
    cout << " It costs too much, i.e £" << windows.price ;
 
    windows.devs = 1138 ;
 
  
     cout << ", and has around " << windows.devs << " developers trying to make it work!" ;
 
    return 0;
}

Output:

The best OS might just possibly be fun, free and better! It costs less than $0.01 and has about 1200 developers on each release!
                
A certain other consumer OS is generally restrictive, proprietary and monopolistic! 
It costs too much, i.e £99.99, and has around 1138 developers trying to make it work!

Source: Derrick Robinson, http://cpp.tech-academy.co.uk/unions/
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.

Source: C++ Reference, https://en.cppreference.com/w/c/language/struct
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 License.