iPhone开发--字符串格式化字符的意义

今天看见一段代码，有个%p，我当时就迷惑了，不知道代表什么意思。之后找了网络和帮助文档，最后知道是格式化指针。哈，把帮助文档原文复制如下:

String Format Specifiers

This article summarizes the format specifiers supported by string formatting methods and functions.

Format Specifiers

The format specifiers supported by the 

NSString

 formatting methods and CFString formatting functions follow the IEEE
printf specification; the specifiers are summarized in Table
1. Note that you can also use the “

n$

” positional specifiers such as 

%1$@
%2$s

. For more details, see the IEEE
printf specification. You can also use these format specifiers with the 

NSLog

 function.

Table 1  Format specifiers supported by
the 

NSString

 formatting methods and CFString formatting functions

Specifier	Description
%@	Objective-C object, printed as the string returned by  descriptionWithLocale:  if available, or  description  otherwise. Also works with  CFTypeRef  objects, returning the result of the  CFCopyDescription  function.
%%	'%'  character
%d ,  %D , %i	Signed 32-bit integer ( int )
%u ,  %U	Unsigned 32-bit integer ( unsigned int )
%hi	Signed 16-bit integer ( short )
%hu	Unsigned 16-bit integer ( unsigned short )
%qi	Signed 64-bit integer ( long long )
%qu	Unsigned 64-bit integer ( unsigned long long )
%x	Unsigned 32-bit integer ( unsigned int ), printed in hexadecimal using the digits 0–9 and lowercase a–f
%X	Unsigned 32-bit integer ( unsigned int ), printed in hexadecimal using the digits 0–9 and uppercase A–F
%qx	Unsigned 64-bit integer ( unsigned long long ), printed in hexadecimal using the digits 0–9 and lowercase a–f
%qX	Unsigned 64-bit integer ( unsigned long long ), printed in hexadecimal using the digits 0–9 and uppercase A–F
%o ,  %O	Unsigned 32-bit integer ( unsigned int ), printed in octal
%f	64-bit floating-point number ( double )
%e	64-bit floating-point number ( double ), printed in scientific notation using a lowercase e to introduce the exponent
%E	64-bit floating-point number ( double ), printed in scientific notation using an uppercase E to introduce the exponent
%g	64-bit floating-point number ( double ), printed in the style of  %e  if the exponent is less than –4 or greater than or equal to the precision, in the style of  %f  otherwise
%G	64-bit floating-point number ( double ), printed in the style of  %E  if the exponent is less than –4 or greater than or equal to the precision, in the style of  %f  otherwise
%c	8-bit unsigned character ( unsigned char ), printed by  NSLog()  as an ASCII character, or, if not an ASCII character, in the octal format  \\ddd  or the Unicode hexadecimal format  \\udddd , where  d  is a digit
%C	16-bit Unicode character ( unichar ), printed by  NSLog()  as an ASCII character, or, if not an ASCII character, in the octal format \\ddd  or the Unicode hexadecimal format  \\udddd , where  d  is a digit
%s	Null-terminated array of 8-bit unsigned characters.  %s  interprets its input in the system encoding rather than, for example, UTF-8.
%S	Null-terminated array of 16-bit Unicode characters
%p	Void pointer ( void * ), printed in hexadecimal with the digits 0–9 and lowercase a–f, with a leading  0x
%L	Length modifier specifying that a following  a ,  A ,  e ,  E ,  f ,  F ,  g , or  G  conversion specifier applies to a  long double  argument
%a	64-bit floating-point number ( double ), printed in scientific notation with a leading  0x  and one hexadecimal digit before the decimal point using a lowercase  p  to introduce the exponent
%A	64-bit floating-point number ( double ), printed in scientific notation with a leading  0X  and one hexadecimal digit before the decimal point using a uppercase  P  to introduce the exponent
%F	64-bit floating-point number ( double ), printed in decimal notation
%z	Length modifier specifying that a following  d ,  i ,  o ,  u ,  x , or  X  conversion specifier applies to a  size_t  or the corresponding signed integer type argument
%t	Length modifier specifying that a following  d ,  i ,  o ,  u ,  x , or  X  conversion specifier applies to a  ptrdiff_t  or the corresponding unsigned integer type argument
%j	Length modifier specifying that a following  d ,  i ,  o ,  u ,  x , or  X  conversion specifier applies to a  intmax_t  or  uintmax_t  argument

Platform Dependencies

Mac OS X uses several data types—

NSInteger

, 

NSUInteger

,

CGFloat

,
and 

CFIndex

—to provide a consistent means of representing values in 32- and 64-bit environments. In a 32-bit environment, 

NSInteger

 and 

NSUInteger

 are
defined as 

int

 and 

unsigned int

, respectively. In 64-bit
environments, 

NSInteger

 and 

NSUInteger

 are defined as 

long

 and 

unsigned
long

, respectively. To avoid the need to use different printf-style type specifiers depending on the platform, you can use the specifiers shown in Table
2. Note that in some cases you may have to cast the value.

Table 2  Format specifiers for data types

Type	Format specifier	Considerations
NSInteger	%ld  or  %lx	Cast the value to  long
NSUInteger	%lu  or  %lx	Cast the value to  unsigned long
CGFloat	%f  or  %g	%f  works for floats and doubles when formatting; but see below warning when scanning
CFIndex	%ld  or  %lx	The same as  NSInteger
pointer	%p	%p  adds  0x  to the beginning of the output. If you don't want that, use  %lx  and cast to  long .
long long	%lld  or  %llx	long long  is 64-bit on both 32- and 64-bit platforms
unsigned long long	%llu  or  %llx	unsigned long long  is 64-bit on both 32- and 64-bit platforms

The following example illustrates the use of 

%ld

 to format an 

NSInteger

 and
the use of a cast.

NSInteger i = 42;

printf("%ld\n", (long)i);

In addition to the considerations mentioned in Table
2, there is one extra case with scanning: you must distinguish the types for 

float

 and

double

.
You should use 

%f

 for float, 

%lf

 for double. If you need
to use 

scanf

 (or a variant thereof) with 

CGFloat

, switch
to 

double

 instead, and copy the 

double

 to 

CGFloat

.

CGFloat imageWidth;

double tmp;

sscanf (str, "%lf", &tmp);

imageWidth = tmp;

It is important to remember that 

%lf

 does not represent 

CGFloat

 correctly
on either 32- or 64-bit platforms. This is unlike 

%ld

, which works for

long

 in
all cases.