Typedef

typedef is a reserved keyword in the programming languages C, C++, and Objective-C. It is used to create an additional name (alias) for another data type, but does not create a new type, except in the obscure case of a qualified typedef of an array type where the typedef qualifiers are transferred to the array element type. As such, it is often used to simplify the syntax of declaring complex data structures consisting of struct and union types, although it is also commonly used to provide specific descriptive type names for integer data types of varying sizes.

Syntax
A typedef declaration follows the same syntax as declaring any other C identifier. The keyword  itself is a specifier which means that while it typically appears at the start of the declaration, it can also appear after the type specifiers or between two of them.

In the C standard library and in POSIX specifications, the identifier for the typedef definition is often suffixed with _t, such as in  and. This is practiced in other coding systems, although POSIX explicitly reserves this practice for POSIX data types.

Examples
This creates the type length as a synonym of the type int:

Documentation use
A typedef declaration may be used as documentation by indicating the meaning of a variable within the programming context, e.g., it may include the expression of a unit of measurement or counts. The generic declarations,

may be expressed by declaring context specific types:

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 representing the same data type int, store 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:

Declaration simplification
A typedef may be used to simplify the declarations of objects having types with verbose names, such as struct, union, or pointer types. For example,

defines the data type struct MyStruct. In C, absent a typedef, the full type name must be used to declare variables of that type:

A typedef declaration can provide a simpler type name that does not include struct. For example, with the type declaration

the variable declaration can be reduced to:

The structure declaration and typedef may also be combined into a single declaration:

, including when no tag is declared:

In C++, unlike in C, tags of class, struct, union, and enum types (such as the "MyStruct" above) automatically can be used by themselves as aliases for the full type names, very much as if there were explicit typedefs declaring so at the point of the type declaration:

Indeed, for class, struct, and union types, C++ calls the tag a class name.

The correspondence between this C++ feature and typedef is very strong, extending to the fact that it is possible to shadow the simple type name in a nested scope by declaring it as the identifier of another kind of entity. In such a case, the C-style full type name (an "elaborated type specifier") can still be used to refer to the class or enum type.

In C++, then, MyStruct can be used anywhere that newtype can be used, as long as there are no other declarations of these identifiers. The reverse is not true, however, for C++ requires the class name for some purposes, such as the names of constructor methods.

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

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

Pointers
The typedef may be used to define a new pointer type.

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 variable of type int.

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

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**.

Constant pointers
Again, because typedef defines a type, not an expansion, declarations that use the const qualifier can yield unexpected or unintuitive results. The following example declares a constant pointer to an integer type, not a pointer to a constant integer:

Since it is a constant pointer, it must be initialized in the declaration.

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

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

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.

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.

Function pointers
The preceding code may be rewritten with typedef specifications:

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:

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:

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

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.

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:

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.

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

Usage in C++
In C++ type names can be complex, and typedef provides a mechanism to assign a simple name to the type.

and

C++11 introduced the possibility to express typedefs with using instead of typedef. For example, the above two typedefs could equivalently be written as

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

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

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

Other languages
In SystemVerilog, typedef behaves exactly the way it does in C and C++.

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:

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, one uses the typealias keyword to create a typedef:

C# contains a feature which is similar to the typedef or the using syntax of C++.

In D the keyword alias allows to create type or partial type synonyms.

Usage concerns
Kernighan and Ritchie stated two reasons for using a typedef. First, it provides a means to make a program more portable or easier to maintain. Instead of having to change a type in every appearance throughout the program's source files, only a single typedef statement needs to be changed. and  in   are such typedef names. Second, a typedef can make a complex definition or declaration easier to understand.

Some programmers 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.