go语言fmt占位符

• 通用

  %v the value in a default format
  when printing structs, the plus flag (%+v) adds field names
  %#v a Go-syntax representation of the value
  (floating-point infinities and NaNs print as ±Inf and NaN)
  %T a Go-syntax representation of the type of the value
  %% a literal percent sign; consumes no value

• 布尔值

  %t the word true or false

• 整型

  %b base 2
  %c the character represented by the corresponding Unicode code point
  %d base 10
  %o base 8
  %O base 8 with 0o prefix
  %q a single-quoted character literal safely escaped with Go syntax.
  %x base 16, with lower-case letters for a-f
  %X base 16, with upper-case letters for A-F
  %U Unicode format: U+1234; same as “U+%04X”

• 浮点型

  %b decimalless scientific notation with exponent a power of two,
  in the manner of strconv.FormatFloat with the ‘b’ format,
  e.g. -123456p-78
  %e scientific notation, e.g. -1.234456e+78
  %E scientific notation, e.g. -1.234456E+78
  %f decimal point but no exponent, e.g. 123.456
  %F synonym for %f
  %g %e for large exponents, %f otherwise. Precision is discussed below.
  %G %E for large exponents, %F otherwise
  %x hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
  %X upper-case hexadecimal notation, e.g. -0X1.23ABCP+20

  The exponent is always a decimal integer.
  For formats other than %b the exponent is at least two digits.

• 字符串

  %s the uninterpreted bytes of the string or slice
  %q a double-quoted string safely escaped with Go syntax
  %x base 16, lower-case, two characters per byte
  %X base 16, upper-case, two characters per byte

• 切片

  %p address of 0th element in base 16 notation, with leading 0x

• 指针

  %p base 16 notation, with leading 0x
  The %b, %d, %o, %x and %X verbs also work with pointers,
  formatting the value exactly as if it were an integer.

%v格式在不同类型下等效于：
bool: %t
int, int8 etc.: %d
uint, uint8 etc.: %d, %#x if printed with %#v
float32, complex64, etc: %g
string: %s
chan: %p
pointer: %p

对于复合对象，使用以下规则以递归方式打印元素

struct:             {field0 field1 ...}
array, slice:       [elem0 elem1 ...]
maps:               map[key1:value1 key2:value2 ...]
pointer to above:   &{}, &[], &map[]

宽度由紧接在动词前面的可选十进制数指定。如果不存在，则宽度为表示值所需的任何值。精度由（可选）宽度后面的句点和十进制数指定。如果没有句点，则使用默认精度。没有后续数字的句点指定精度为零。示例：
%f default width, default precision
%9f width 9, default precision
%.2f default width, precision 2
%9.2f width 9, precision 2
%9.f width 9, precision 0
对于字符串，宽度的单位是unicode点位而不是字节。

其它占位符

'+' always print a sign for numeric values;
    guarantee ASCII-only output for %q (%+q)
'-' pad with spaces on the right rather than the left (left-justify the field)
'#' alternate format: add leading 0b for binary (%#b), 0 for octal (%#o),
    0x or 0X for hex (%#x or %#X); suppress 0x for %p (%#p);
    for %q, print a raw (backquoted) string if [strconv.CanBackquote]
    returns true;
    always print a decimal point for %e, %E, %f, %F, %g and %G;
    do not remove trailing zeros for %g and %G;
    write e.g. U+0078 'x' if the character is printable for %U (%#U)
' ' (space) leave a space for elided sign in numbers (% d);
    put spaces between bytes printing strings or slices in hex (% x, % X)
'0' pad with leading zeros rather than spaces;
    for numbers, this moves the padding after the sign

参考：
https://pkg.go.dev/fmt

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