Objective-C中的字串格式化輸出

在使用諸如NSLog, [NSString stringWithFormat:]之類的函數時,都是基於c/c++風格的字串格式化工作的.

本來c/c++就沒怎麼用過,到iphone開發開發時摻合上NS系的對象,格式化輸出更是一頭的亂.
看了一下Programming Guide for Cocoa的文檔,還是有比較詳盡的說明的,整理出來備查.

格式定義
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

定義	說明
%@	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

平台依賴
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

類型	定義	建議
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.

1 2	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.

1 2 3 4

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.