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typedef Specifier

A typedef declaration introduces a name that, within its scope, becomes a synonym for the type given by the type-declaration portion of the declaration.

typedef type-declaration synonym;

You can use typedef declarations to construct shorter or more meaningful names for types already defined by the language or for types that you have declared. Typedef names allow you to encapsulate implementation details that may change.

In contrast to the class, struct, union, and enum declarations, typedef declarations do not introduce new types — they introduce new names for existing types.

Typedef names share the name space with ordinary identifiers. Therefore, a program can have a typedef name and a local-scope identifier by the same name.

For more information, see:

// typedef_specifier1.cpp
typedef char FlagType;

int main()
{
}

void myproc( int )
{
    int FlagType;
}

When declaring a local-scope identifier by the same name as a typedef, or when declaring a member of a structure or union in the same scope or in an inner scope, the type specifier must be specified. For example:

typedef char FlagType;
const FlagType x;

To reuse the FlagType name for an identifier, a structure member, or a union member, the type must be provided:

const int FlagType;  // Type specifier required

It is not sufficient to say

const FlagType;      // Incomplete specification

because the FlagType is taken to be part of the type, not an identifier that is being redeclared. This declaration is taken to be an illegal declaration like

int;  // Illegal declaration 

You can declare any type with typedef, including pointer, function, and array types. You can declare a typedef name for a pointer to a structure or union type before you define the structure or union type, as long as the definition has the same visibility as the declaration.

Examples

One use of typedef declarations is to make declarations more uniform and compact. For example:

typedef char CHAR;          // Character type.
typedef CHAR * PSTR;        // Pointer to a string (char *).
PSTR strchr( PSTR source, CHAR target );
typedef unsigned long ulong;
ulong ul;     // Equivalent to "unsigned long ul;"

To use typedef to specify fundamental and derived types in the same declaration, you can separate declarators with commas. For example:

typedef char CHAR, *PSTR;

The following example provides the type DRAWF for a function returning no value and taking two int arguments:

typedef void DRAWF( int, int );

After the above typedef statement, the declaration

DRAWF box; 

would be equivalent to the declaration

void box( int, int );

typedef is often combined with struct to declare and name user-defined types:

// typedef_specifier2.cpp
#include <stdio.h>

typedef struct mystructtag
{
    int   i;
    double f;
} mystruct;

int main()
{
    mystruct ms;
    ms.i = 10;
    ms.f = 0.99;
    printf_s("%d   %f\n", ms.i, ms.f);
}
10   0.990000

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