1. Structures, Unions, and T
ypedefs
CS-2303, C-Term 2010 1
Structures, Unions, and Typedefs
CS-2303
System Programming Concepts
(Slides include materials from The C Programming Language, 2nd
edition, by Kernighan and Ritchie and
from C: How to Program, 5th
and 6th
editions, by Deitel and Deitel)
2. Structures, Unions, and T
ypedefs
CS-2303, C-Term 2010 2
Reading Assignment
• Chapter 6 of Kernighan & Ritchie
Chapter 10 of Deitel & Deitel
3. Structures, Unions, and T
ypedefs
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Structures and Unions
• The last major language facility in C to be
introduced in this course.
• Essential for building up “interesting” data
structures — e.g.,
• Data structures of multiple values of different kinds
• Data structures of indeterminate size
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ypedefs
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Definition — Structure
• A collection of one or more variables,
typically of different types, grouped
together under a single name for convenient
handling
• Known as struct in C and C++
Like a class in Java
but with no methods
In C++, a struct is a
class with no methods
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struct
• Defines a new type
• I.e., a new kind of data type that compiler regards as
a unit
• E.g.,
struct motor {
float volts; //voltage of the motor
float amps; //amperage of the motor
int phases; //# of phases of the motor
float rpm; //rotational speed of motor
}; //struct motor
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struct
• Defines a new type
• E.g.,
struct motor {
float volts;
float amps;
int phases;
float rpm;
}; //struct motor
Name of the type
Note:– name of type is optional if
you are just declaring a single
struct (middle p. 128 of K&R)
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struct
• Defines a new type
• E.g.,
struct motor {
float volts;
float amps;
int phases;
float rpm;
}; //struct motor
Members of the
struct
A member of a struct is analogous
to a field of a class in Java
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ypedefs
CS-2303, C-Term 2010 8
Declaring struct variables
struct motor p, q, r;
• Declares and sets aside storage for three variables –
p, q, and r – each of type struct motor
struct motor M[25];
• Declares a 25-element array of struct motor;
allocates 25 units of storage, each one big enough to
hold the data of one motor
struct motor *m;
• Declares a pointer to an object of type struct
motor
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ypedefs
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Accessing Members of a struct
• Let
struct motor p;
struct motor q[10];
• Then
p.volts — is the voltage
p.amps — is the amperage
p.phases — is the number of phases
p.rpm — is the rotational speed
q[i].volts — is the voltage of the ith motor
q[i].rpm — is the speed of the ith motor
Like Java!
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ypedefs
CS-2303, C-Term 2010 10
Accessing Members of a struct (continued)
• Let
struct motor *p;
• Then
(*p).volts — is the voltage of the motor pointed
to by p
(*p).phases — is the number of phases of the
motor pointed to by p
Why the parentheses?
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Accessing Members of a struct (continued)
• Let
struct motor *p;
• Then
(*p).volts — is the voltage of the motor pointed
to by p
(*p).phases — is the number of phases of the
motor pointed to by p
Because '.' operator
has higher precedence
than unary '*'
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ypedefs
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Accessing Members of a struct (continued)
• Let
struct motor *p;
• Then
(*p).volts — is the voltage of the motor pointed
to by p
(*p).phases — is the number of phases of the
motor pointed to by p
Reason:– you really want the expression
m.volts * m.amps
to mean what you think it should mean!
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ypedefs
CS-2303, C-Term 2010 13
Accessing Members of a struct (continued)
• The (*p).member notation is a nuisance
• Clumsy to type; need to match ( )
• Too many keystrokes
• This construct is so widely used that a
special notation was invented, i.e.,
– p->member, where p is a pointer to the
structure
• Ubiquitous in C and C++
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Previous Example Becomes …
• Let
struct motor *p;
• Then
p -> volts — is the voltage of the motor pointed
to by p
p -> phases — is the number of phases of the
motor pointed to by p
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ypedefs
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Operations on struct
• Copy/assign
struct motor p, q;
p = q;
• Get address
struct motor p;
struct motor *s
s = &p;
• Access members
p.volts;
s -> amps;
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Operations on struct (continued)
• Remember:–
– Passing an argument by value is an instance of copying
or assignment
– Passing a return value from a function to the caller is an
instance of copying or assignment
• E.g,:–
struct motor f(struct motor g) {
struct motor h = g;
...;
return h;
}
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Assigning to a struct
• K & R say (p. 131)
– “If a large structure is to be passed to a function, it is
generally more efficient to pass a pointer than to copy
the whole structure”
• I disagree:–
– Copying is very fast on modern computers
– Creating an object with malloc() and assigning a
pointer is not as fast
– Esp. if you want the object passed or returned by value
– In real life situations, it is a judgment call
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Initialization of a struct
• Let struct motor {
float volts;
float amps;
int phases;
float rpm;
}; //struct motor
• Then
struct motor m = {208, 20, 3,
1800};
initializes the struct
• See also p. 133 of K&R for initializing arrays of
structs
C99 introduces a new
way of initializing a
struct – truly ugly!
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ypedefs
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Why structs?
• Open-ended data structures
– E.g., structures that may grow during
processing
– Avoids the need for realloc() and a lot of
copying
• Self-referential data structures
– Lists, trees, etc.
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Example
struct item {
char *s;
struct item *next;
}
• I.e., an item can point to another item
• … which can point to another item
• … which can point to yet another item
• … etc.
Thereby forming a list of items
Yes! This is legal!
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A note about structs and pointers
• The following is legal:–
/* in a .c or .h file */
struct item;
struct item *p, *q;
… /* In another file */
struct item {
int member1;
float member2;
struct item *member3;
};
Called an opaque type!
Program can use pointers to items
but cannot see into items.
Cannot define any items, cannot
malloc any items, etc.
Implementer of item can change
the definition without forcing
users of pointers to change their
code!
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ypedefs
CS-2303, C-Term 2010 22
Another note about structs
• The following is not legal:–
struct motor {
float volts;
float amps;
float rpm;
unsigned int phases;
}; //struct motor
motor m;
motor *p;
You must write
struct motor m;
struct motor *p;
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ypedefs
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Typedef
• Definition:– a typedef is a way of
renaming a type
– See §6.7
• E.g.,
typedef struct motor Motor;
Motor m, n;
Motor *p, r[25];
Motor function(const Motor m; …);
E.g., typedef, lets you
leave out the word “struct”
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typedef (continued)
• typedef may be used to rename any type
– Convenience in naming
– Clarifies purpose of the type
– Cleaner, more readable code
– Portability across platforms
• E.g.,
– typedef char *String;
• E.g.,
– typedef int size_t;
– typedef long int32;
– typedef long long int64;
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CS-2303, C-Term 2010 25
typedef (continued)
• typedef may be used to rename any type
– Convenience in naming
– Clarifies purpose of the type
– Cleaner, more readable code
– Portability across platforms
• E.g.,
– typedef char *String;
• E.g.,
– typedef int size_t;
– typedef long int32;
– typedef long long int64;
These three may change from
platform to platform
Defined once in a .h file!
Very common in C and C++
Esp. for portable code!
26. Structures, Unions, and T
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CS-2303, C-Term 2010 26
Revisit note about structs and pointers
• The following is legal:–
/* in a .c or .h file */
typedef struct _item Item;
Item *p, *q;
… /* In another file */
struct _item {
char *info;
Item *nextItem;
};
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Questions about structs and pointers?
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Unions
• A union is like a struct, but only one of its
members is stored, not all
• I.e., a single variable may hold different types at different
times
• Storage is enough to hold largest member
• Members are overlaid on top of each other
• E.g.,
union {
int ival;
float fval;
char *sval;
} u;
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Unions (continued)
• It is programmer’s responsibility to keep track of
which type is stored in a union at any given time!
• E.g., (p. 148)
struct taggedItem {
enum {iType, fType, cType} tag;
union {
int ival;
float fval;
char *sval;
} u;
};
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Unions (continued)
• It is programmer’s responsibility to keep track of
which type is stored in a union at any given time!
• E.g., (p. 148)
struct taggedItem {
enum {iType, fType, cType} tag;
union {
int ival;
float fval;
char *sval;
} u;
};
Members of struct are:–
enum tag;
union u;
Value of tag says which
member of u to use
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ypedefs
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Unions (continued)
• unions are used much less frequently than
structs — mostly
• in the inner details of operating system
• in device drivers
• in embedded systems where you have to access
registers defined by the hardware
![Structures, Unions, and T
ypedefs
CS-2303, C-Term 2010 8
Declaring struct variables
struct motor p, q, r;
• Declares and sets aside storage for three variables –
p, q, and r – each of type struct motor
struct motor M[25];
• Declares a 25-element array of struct motor;
allocates 25 units of storage, each one big enough to
hold the data of one motor
struct motor *m;
• Declares a pointer to an object of type struct
motor](https://image.slidesharecdn.com/week2structuresetc-250415111153-ecda5596/85/Week2_StructuresEtcgghjjvfyiigggbjkyt-ppt-8-320.jpg)
![Structures, Unions, and T
ypedefs
CS-2303, C-Term 2010 9
Accessing Members of a struct
• Let
struct motor p;
struct motor q[10];
• Then
p.volts — is the voltage
p.amps — is the amperage
p.phases — is the number of phases
p.rpm — is the rotational speed
q[i].volts — is the voltage of the ith motor
q[i].rpm — is the speed of the ith motor
Like Java!](https://image.slidesharecdn.com/week2structuresetc-250415111153-ecda5596/85/Week2_StructuresEtcgghjjvfyiigggbjkyt-ppt-9-320.jpg)
![Structures, Unions, and T
ypedefs
CS-2303, C-Term 2010 23
Typedef
• Definition:– a typedef is a way of
renaming a type
– See §6.7
• E.g.,
typedef struct motor Motor;
Motor m, n;
Motor *p, r[25];
Motor function(const Motor m; …);
E.g., typedef, lets you
leave out the word “struct”](https://image.slidesharecdn.com/week2structuresetc-250415111153-ecda5596/85/Week2_StructuresEtcgghjjvfyiigggbjkyt-ppt-23-320.jpg)
