Objective-C中的字符串格式化輸出

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

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.

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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.

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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.