支持的 Swift 字符串格式说明符有哪些

Swift中String种格式的格式说明符与Objective-C NSString格式的格式说明符相同，其本身与CFString种格式的格式说明符相同，并深深地埋藏在苹果文档的档案中(这两个页面的内容相同，最初都是2002年或更早的版本):

• https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFStrings/format标志成分.html
• https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Strings/Articles/format标志成分.html

但是这个文档页面本身是不完整的，例如没有提到flags、precision和width说明符.实际上，它声称遵循IEEE printf specifications (Issue 6, 2004 Edition)，本身符合ISO C标准.因此，这些说明符应该与我们使用Cprintf时的说明符相同，添加了%@个Objective-C对象说明符，并添加了缺乏记录的%D、%U、%O说明符和q长度修饰符.

标志成分

每个转换规范都由"%"字符或字符序列"%n$"引入.

n是参数的索引，如:

String(format: "%2$@ %1$@", "world", "Hello")

Format 标志成分

%@    Objective-C object, printed as the string returned by descriptionWithLocale: if available, or description otherwise.

实际上，您也可以使用一些Swift类型，但它们必须在标准库中定义，以符合CVARRG协议，我相信它们需要支持桥接到Objective-C对象:https://developer.apple.com/documentation/foundation/object_runtime/classes_bridged_to_swift_standard_library_value_types.

String(format: "%@", ["Hello", "world"])

%%    '%' character.

String(format: "100%% %@", true.description)

%d, %i    Signed 32-bit integer (int).

String(format: "from %d to %d", Int32.min, Int32.max)

%u, %U, %D    Unsigned 32-bit integer (unsigned int).

String(format: "from %u to %u", UInt32.min, UInt32.max)

%x    Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and lowercase a–f.

String(format: "from %x to %x", UInt32.min, UInt32.max)

%X    Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and uppercase A–F.

String(format: "from %X to %X", UInt32.min, UInt32.max)

%o, %O    Unsigned 32-bit integer (unsigned int), printed in octal.

String(format: "from %o to %o", UInt32.min, UInt32.max)

%f    64-bit floating-point number (double), printed in decimal notation. Produces "inf", "infinity", or "nan".

String(format: "from %f to %f", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%F    64-bit floating-point number (double), printed in decimal notation. Produces "INF", "INFINITY", or "NAN".

String(format: "from %F to %F", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%e    64-bit floating-point number (double), printed in scientific notation using a lowercase e to introduce the exponent.

String(format: "from %e to %e", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%E    64-bit floating-point number (double), printed in scientific notation using an uppercase E to introduce the exponent.

String(format: "from %E to %E", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%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.

String(format: "from %g to %g", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%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.

String(format: "from %G to %G", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%c    8-bit unsigned character (unsigned char).

String(format: "from %c to %c", "a".utf8.first!, "z".utf8.first!)

%C    16-bit UTF-16 code unit (unichar).

String(format: "from %C to %C", "爱".utf16.first!, "终".utf16.first!)

%s    Null-terminated array of 8-bit unsigned characters.

"Hello world".withCString {
    String(format: "%s", $0)
}

%S    Null-terminated array of 16-bit UTF-16 code units.

"Hello world".withCString(encodedAs: UTF16.self) {
    String(format: "%S", $0)
}

%p    Void pointer (void *), printed in hexadecimal with the digits 0–9 and lowercase a–f, with a leading 0x.

var hello = "world"
withUnsafePointer(to: &hello) {
    String(format: "%p", $0)
}

%n    The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions.

The 100 format specifier seems unsupported in Swift 4+

%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.

String(format: "from %a to %a", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%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.

String(format: "from %A to %A", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

旗帜

'    The integer portion of the result of a decimal conversion ( %i, %d, %u, %f, %F, %g, or %G ) shall be formatted with thousands' grouping characters. For other conversions the behavior is undefined. The non-monetary grouping character is used.

The 100 flag seems unsupported in Swift 4+

-    The result of the conversion shall be left-justified within the field. The conversion is right-justified if this flag is not specified.

String(format: "from %-12f to %-12d.", Double.leastNonzeroMagnitude, Int32.max)

+    The result of a signed conversion shall always begin with a sign ( '+' or '-' ). The conversion shall begin with a sign only when a negative value is converted if this flag is not specified.

String(format: "from %+f to %+d", Double.leastNonzeroMagnitude, Int32.max)

<space>    If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a <space> shall be prefixed to the result. This means that if the <space> and '+' flags both appear, the <space> flag shall be ignored.

String(format: "from % d to % d.", Int32.min, Int32.max)

#    Specifies that the value is to be converted to an alternative form. For o conversion, it increases the precision (if necessary) to force the first digit of the result to be zero. For x or X conversion specifiers, a non-zero result shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g , and G conversion specifiers, the result shall always contain a radix character, even if no digits follow the radix character. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. For g and G conversion specifiers, trailing zeros shall not be removed from the result as they normally are. For other conversion specifiers, the behavior is undefined.

String(format: "from %#a to %#x.", Double.leastNonzeroMagnitude, UInt32.max)

0    For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversion specifiers, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the '0' and '-' flags both appear, the '0' flag is ignored. For d, i, o, u, x, and X conversion specifiers, if a precision is specified, the '0' flag is ignored. If the '0' and '" flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behavior is undefined.

String(format: "from %012f to %012d.", Double.leastNonzeroMagnitude, Int32.max)

宽度修改器

如果转换后的值的字节数小于字段宽度，则默认情况下应在左侧填充空格；如果字段宽度有左调整标志('-')，则应在右侧填充.字段宽度采用星号("*"或十进制整数的形式.

String(format: "from %12f to %*d.", Double.leastNonzeroMagnitude, 12, Int32.max)

精度修改器

一个可选的精度，它给出了d、i、o、u、x和x转换说明符出现的最小位数；a、a、e、e、f和f转换说明符的基数字符后显示的位数；g和g转换说明符的最大有效位数；或从s和s转换说明符中的字符串打印的最大字节数.精度采用句点('.')后跟星号('*')或可选的十进制数字字符串的形式，其中空数字字符串被视为零.如果精度与任何其他转换说明符一起出现，则行为未定义.

String(format: "from %.12f to %.*d.", Double.leastNonzeroMagnitude, 12, Int32.max)

长度修饰符

h    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a short or unsigned short argument.

String(format: "from %hd to %hu", CShort.min, CUnsignedShort.max)

hh    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument.

String(format: "from %hhd to %hhu", CChar.min, CUnsignedChar.max)

l    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a long or unsigned long argument.

String(format: "from %ld to %lu", CLong.min, CUnsignedLong.max)

ll, q    长度修饰符 specifying that a following d, o, u, x, or X conversion specifier applies to a long long or unsigned long long argument.

String(format: "from %lld to %llu", CLongLong.min, CUnsignedLongLong.max)

L    Length modifier specifying that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.

I wasn't able to pass a CLongDouble argument to 100 in Swift 4+

z    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a size_t.

String(format: "from %zd to %zu", size_t.min, size_t.max)

t    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a ptrdiff_t.

String(format: "from %td to %tu", ptrdiff_t.min, ptrdiff_t.max)

j    Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a intmax_t or uintmax_t argument.

String(format: "from %jd to %ju", intmax_t.min, uintmax_t.max)