The C programming language 2nd -- Notes -- 6.7

6.7 The type definition

        C Yes typedef function , Used to create a new data type name . Such as , Statement

typedef int Length;

take Length Defined as and int Names with the same meaning . type Length Can be used for type declarations 、 Type conversion, etc , It and type int Exactly the same , Such as ：

Length len, maxlen;
Length *Lengths[];

Similarly , Statement

typedef char* String;/* chinese */

typedef char *String;/* english */

take String Defined as char * Or character pointer , It can then be used in type declarations and type conversions String, for example ：

String p, lineptr[MAXLINES], alloc(int);
int strcmp(String, String);
p = (String) malloc(100);

         Be careful ,  typedef The type declared in the variable name appears in the variable name position , Not immediately after the keyword typedef after . typedef It is similar to storage class in syntax extern、 static etc. . Use capital letters here typedef The first letter of the name of the defined type , In order to show the difference between .

        Take a more complicated example ： use typedef Define the tree nodes described earlier in this chapter . as follows ：

typedef struct tnode *Treeptr;

typedef struct tnode {  /* The tree node：*/
    char *word;         /* Points to the text*/
    int count;          /* Number of the occurrences*/
    struct tnode *left  /* Left child*/
    struct tnode *right /* Right child*/
} treenode;

The above type definition creates two new type keywords ： Treenode（ A structure ） and   Treeptr（ A pointer to the structure ）. such , function talloc It can be modified as ：

Treeptr talloc(void)
{
    return (treeptr) malloc(sizeof(treenode));
}

emphasize , typedef The declaration does not create a new type , It just adds a new name to an existing type .typedef The declaration does not add any new semantics ： Variables declared in this way have exactly the same properties as variables declared in the normal way . actually ,typedef  Be similar to #define sentence , But because of typedef Is interpreted by the compiler , Therefore, its text replacement function exceeds the capacity of the preprocessor . for example ：

typedef int (*PFI) (char *, char *); 

This statement defines the type PFI yes “ A pointer to a function , This function has two char * Parameters of type , The return value type is int”, It can be used in certain contexts , for example , Can be used on page 5 In the sorting program in Chapter , as follows ：

PFI strcmmp, numcpm;

        In addition to more concise expression , Use typedef There are two other important reasons . First , It can parameterize the program , To improve the portability of the program . If typedef The declared data type is machine dependent , When the program is transplanted to other machines , Only need to change typedef Just define the type . A frequently used case is , For shaping values of various sizes , All through typedef Defined type name , then , Select a set of appropriate hosts for each host short、int、 and long Type size is enough . There are some examples in the standard library , for example size_t and ptrdiff_t etc. .

        typedef Of the 2 One function is to provide better explanation for the program ——Treeptr A type is obviously easier to understand than a pointer declared to a complex structure .