typedef

typedef is a reserved keyword in the C and C++ programming languages. It is used to create an alias name for another data type.^[1] As such, it is often used to simplify the syntax of declaring complex data structures consisting of struct and union types, but is just as common in providing specific descriptive type names for integer data types of varying lengths. The C standard library and POSIX reserve the suffix '_t', for example as in size_t and time_t.

Syntax

The syntax for a creating a typedef is: typedef typedeclaration;^[2]

Some examples:

typedef int Length;

creates Length as a synonym for int.

typedef int (*PFI)(char *, char *);

creates PFI as a synonym for a pointer to a function of two char * arguments that returns an int.

Usage examples

Indicating the meaning of a variable definition

A typedef declaration may be used to indicate the meaning of a variable within the programming context, e.g., it may be include the expression of a unit of measurement or counts. The generic declarations in the following code,

int current_speed ;
int high_score ;


void congratulate(int your_score) {
    if (your_score > high_score)
        ...

may be expressed by declaring context specific types:

typedef int km_per_hour ;
typedef int points ;

km_per_hour current_speed ;  //"km_per_hour" is synonymous with "int" here,
points high_score ;          //and thus, the compiler treats our new variables as integers.


void congratulate(points your_score) {
    if (your_score > high_score)
        ...

Both sections of code execute identically. However, the use of typedef declarations in the second code block makes it clear that the two variables, while represented by the same data type int, represent different or incompatible data. The definition in congratulate() of your_score indicates to the programmer that current_speed (or any other variable not declared as a points) should not be passed as an argument. This would not be as apparent if both were declared as variables of int datatype. However, the indication is for the programmer only; the C/C++ compiler considers both variables to be of type int and does not flag type mismatch warnings or errors for "wrong" argument types for congratulate(points your_score) in the code snippet below:

void foo() {
    km_per_hour km100 = 100;
    congratulate(km100);

Although the compiler considers km_per_hour to be equivalent to int in the above code, the two cannot be used interchangeably when the type is changed via a prefix of unsigned, signed, or long.

void foo() {
    unsigned int a;         // Okay
    unsigned km_per_hour b; // Compiler complains
    long int c;             // Okay
    long km_per_hour d;     // Compiler complains

Simplifying a definition or declaration

A typedef may be used to simplify the declaration of a compound type (struct, union) or pointer type.^[3] For example, in the following snippet:

struct MyStruct {
    int data1;
    char data2;
};

the data type struct MyStruct is defined. To declare a variable of this type in C, the struct key word is required (in C++, it may be omitted):

struct MyStruct a;

A typedef may be used to eliminate the need for the keyword struct in C. For example, the following declaration

typedef struct MyStruct newtype;

may be used to define an object of type newtype:

newtype a;

The structure declaration and definition may instead be combined into a single statement:

typedef struct MyStruct {
    int data1;
    char data2;
} newtype;

Or it may be used as follows:

typedef struct {
    int data1;
    char data2;
} newtype;

In C++, in contrast to C, the keywords struct, class, and enum are optional in variable declarations that are separate from the definitions, as long as there is no ambiguity to another identifier:

struct MyStruct x;       // This is legal
MyStruct y;              // This is also legal

As such, MyStruct can be used wherever newtype can be used. However, the reverse is not true; for instance, the constructor methods for MyStruct cannot be named newtype.

A notorious example where even C++ needs the struct keyword is POSIX' stat system call that uses a struct of the same name in its arguments:

int stat(const char *filename, struct stat *buf)
{
    // codes are here
}

Here both C as well as C++ need the struct keyword in the parameter definition.

Using typedef with pointers

We can use the typedef to define a new pointer type. For example,

typedef int *intptr;   // type name: intptr
                       // new type: int*

intptr ptr;            // same as: int *ptr

Above, intptr is a new alias with the pointer type int*. The definition, intptr ptr;, defines a variable ptr with the type int*. So, ptr is a pointer which can point to a memory with int type.

Using typedef to define a new pointer type may sometimes lead to confusion. For example:

typedef int *intptr;

intptr cliff, allen;        // both cliff and allen are int* type

intptr cliff2, *allen2;     // cliff2 is int* type, but allen2 is int** type
                            // same as: intptr cliff2;
                            //          intptr *allen2;

Above, intptr cliff, allen; means defining 2 variables with int* type for both. This is because a type defined by typedef is a type, not an expansion. In other words, intptr, which is the int* type, decorates both cliff and allen. For intptr cliff2, *allen2;, the intptr type decorates the cliff2 and *allen2. So, intptr cliff2, *allen2; is equivalent to 2 separate definitions, intptr cliff2; and intptr *allen2. intptr *allen2 means that allen2 is a pointer pointing to a memory with int* type. Shortly, allen2 has the type, int**.

Using typedef with structure pointers

Typedefs can also simplify definitions or declarations for structure pointer types. Consider this:

struct Node {
    int data;
    struct Node *nextptr;
};

Using typedef, the above code can be rewritten like this:

typedef struct Node Node;
struct Node {
    int data;
    Node *nextptr;
};

In C, one can declare multiple variables of the same type in a single statement, even mixing structure with pointer or non-pointers. However, one would need to prefix an asterisk to each variable to designate it as a pointer. In the following, a programmer might assume that errptr was indeed a Node *, but a typographical error means that errptr is a Node. This can lead to subtle syntax errors.

struct Node *startptr, *endptr, *curptr, *prevptr, errptr, *refptr;

By defining the typedef Node *, it is assured that all variables are structure pointer types, or say, that each variable is a pointer type pointing to a structure type.

typedef struct Node* NodePtr;
...
NodePtr startptr, endptr, curptr, prevptr, errptr, refptr;

Using typedef with function pointers

Consider the following code, which does not use a typedef:

int do_math(float arg1, int arg2) {
    return arg2;
}

int call_a_func(int (*call_this)(float, int)) {
    int output = call_this(5.5, 7);
    return output;
}

int final_result = call_a_func(&do_math);

This code can be rewritten with a typedef as follows:

typedef int (*MathFunc)(float, int);

int do_math(float arg1, int arg2) {
    return arg2;
}

int call_a_func(MathFunc call_this) {
    int output = call_this(5.5, 7);
    return output;
}

int final_result = call_a_func(&do_math);

Here, MathFunc is the new alias for the type. A MathFunc is a pointer to a function that returns an integer and takes as arguments a float followed by an integer.

When a function returns a function pointer, it can be even more confusing without typedef. The following is the function prototype of signal(3) from FreeBSD:

void (*signal(int sig, void (*func)(int)))(int);

The function declaration above is cryptic as it does not clearly show what the function accepts as arguments, or the type that it returns. A novice programmer may even assume that the function accepts a single int as its argument and returns nothing, but in reality it also needs a function pointer and returns another function pointer. It can be written more cleanly:

typedef void (*sighandler_t)(int);
sighandler_t signal(int sig, sighandler_t func);

Using typedef with arrays

A typedef can also be used to simplify the definition of array types. For example,

typedef char arrType[6];    // type name: arrType
                            // new type: char[6]

arrType arr={1,2,3,4,5,6};  // same as: char arr[6]={1,2,3,4,5,6}

arrType *pArr;              // same as: char (*pArr)[6];

Here, arrType is the new alias for the "char[6]" type, which is an array type with 6 elements. For arrType *pArr;, pArr is a pointer pointing to the memory of the "char[6]" type.

Using typedef with type casts

A typedef is created using type definition syntax but can be used as if it were created using type cast syntax. (Type casting changes a data type.) For instance, in each line after the first line of:

typedef int (*funcptr)(double);         // pointer to function taking a double returning int
funcptr x = (funcptr) NULL;             // C or C++
funcptr y = funcptr(NULL);              // C++ only
funcptr z = static_cast<funcptr>(NULL); // C++ only

funcptr is used on the left-hand side to declare a variable and is used on the right-hand side to cast a value. Thus, the typedef can be used by programmers who do not wish to figure out how to convert definition syntax to type cast syntax.

Note that, without the typedef, it is generally not possible to use definition syntax and cast syntax interchangeably. For example:

void *p = NULL;
int (*x)(double)  = (int (*)(double)) p; // This is legal
int (*)(double) y = (int (*)(double)) p; // Left-hand side is not legal
int (*z)(double)  = (int (*p)(double));  // Right-hand side is not legal

Usage concerns

Some people are opposed to the extensive use of typedefs. Most arguments center on the idea that typedefs simply hide the actual data type of a variable. For example, Greg Kroah-Hartman, a Linux kernel hacker and documenter, discourages their use for anything except function prototype declarations. He argues that this practice not only unnecessarily obfuscates code, it can also cause programmers to accidentally misuse large structures thinking them to be simple types.^[4]

Others argue that the use of typedefs can make code easier to maintain. K&R states that there are two reasons for using a typedef. First, it provides a means to make a program more portable. Instead of having to change a type everywhere it appears throughout the program's source files, only a single typedef statement needs to be changed. Second, a typedef can make a complex definition or declaration easier to understand.

Usage in C++

In C++ type names can be very complicated and typedef provides a mechanism to assign a simple name to the type. Consider:

std::vector<std::pair<std::string, int> > values;
for (std::vector<std::pair<std::string, int> >::const_iterator i = values.begin(); i != values.end(); ++i)
{
   std::pair<std::string, int> const & t = *i;
   // do something
}

and

typedef std::pair<std::string, int> value_t;
typedef std::vector<value_t> values_t;

values_t values;
for (values_t::const_iterator i = values.begin(); i != values.end(); ++i)
{
   value_t const & t = *i;
   // do something
}

Use with templates

C++03 does not provide templated typedefs. For instance, to have stringpair<T> represent std::pair<std::string, T> for every type T one cannot use:

template<typename T>
typedef std::pair<std::string, T> stringpair<T>; // Doesn't work

However, if one is willing to accept stringpair<T>::type in lieu of stringpair<T> then it is possible to achieve the desired result via a typedef within an otherwise unused templated class or struct:

template<typename T>
class stringpair
{
private:
    // Prevent instantiation of stringpair<T>
    stringpair();
public:
    // Make stringpair<T>::type represent std::pair<std::string, T>
    typedef std::pair<std::string, T> type;
};

// Declare a variable of type std::pair<std::string, int>
stringpair<int>::type my_pair_of_string_and_int;

In C++11, templated typedefs are added with the following syntax, which requires the using keyword rather than the typedef keyword. (See template aliases.)

template <typename T>
using stringpair = std::pair<std::string, T>;

// Declare a variable of type std::pair<std::string, int>
stringpair<int> my_pair_of_string_and_int;

Other languages

In many statically typed functional languages, like Haskell, Miranda, OCaml, etc., one can define type synonyms, which are the same as typedefs in C. An example in Haskell:

type PairOfInts = (Int, Int)

This example has defined a type synonym PairOfInts as an integer type.

In Seed7 the definition of a constant type is used to introduce a synonym for a type:

const type: myVector is array integer;

In Swift, typedef is called typealias:

typealias PairOfInts = (Int, Int)

C# also contains a feature which is similar to the typedef of C.^[5]

using newType = global::System.Runtime.Interop.Marshal;
using otherType = Enums.MyEnumType;
using StringListMap = System.Collections.Generic.Dictionary<string, System.Collections.Generic.List<string>>;

In D the keyword alias^[6] allows to create type or partial type synonyms.

struct Foo(T){}
alias FooInt = Foo!int;
alias Fun = int delegate(int);

See also

• Abstract data type
• C syntax

References