问题
In the following C code are octal literals used for all these defines? Even if they start with multiple zeros?
#define TCL_REG_BASIC 000000 /* BREs (convenience). */
#define TCL_REG_EXTENDED 000001 /* EREs. */
#define TCL_REG_ADVF 000002 /* Advanced features in EREs. */
#define TCL_REG_ADVANCED 000003 /* AREs (which are also EREs). */
#define TCL_REG_QUOTE 000004 /* No special characters, none. */
#define TCL_REG_NOCASE 000010 /* Ignore case. */
#define TCL_REG_NOSUB 000020 /* Don't care about subexpressions. */
#define TCL_REG_EXPANDED 000040 /* Expanded format, white space & comments. */
#define TCL_REG_NLSTOP 000100 /* \n doesn't match . or [^ ] */
#define TCL_REG_NLANCH 000200 /* ^ matches after \n, $ before. */
#define TCL_REG_NEWLINE 000300 /* Newlines are line terminators. */
#define TCL_REG_CANMATCH 001000 /* Report details on partial/limited * matches. */
回答1:
From C Standard, 6.4.4.1 Paragraph 3:
An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only
回答2:
Yes You are right.
C11, 6.4.4.1 Integer constants:
An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only.
回答3:
Yes.
http://www.cplusplus.com/doc/tutorial/constants/
In addition to decimal numbers (those that most of us use every day), C++ allows the use of octal numbers (base 8) and hexadecimal numbers (base 16) as literal constants. For octal literals, the digits are preceded with a 0 (zero) character. And for hexadecimal, they are preceded by the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:
75 // decimal
0113 // octal
0x4b // hexadecimal
回答4:
OP's examples are all octal as they begin with a 0. Multiple leading zeros or the presence of an 8 or 9 does not change it away from being octal. An 8 or 9 should case an error.
FYI: Some additional occurrences (though maybe not a literal) in C where decimal, octal and hexadecimal interpretation occur.
Although not a literal, in a format string for printf(), specifiers like "%016llx" are not an octal 016 width, but a flag '0' and a decimal value of 16.
With printf(), output using specifier "%a", the output is in the style "[−]0xh.hhhh p±d," where the significand is in hexadecimal and the exponent is in decimal.
In escape sequences there is no decimal specification. Some samples follow:
\' \" \? \\ \a \b \f \n \r \t \v
\0 (octal)
\01 (octal)
\012 (octal)
\0123 (bad - only up to 3)
\x0 (hexadecimal)
\x01 (hexadecimal)
\x012 (hexadecimal)
\x0123 (hexadecimal)
\x01234… (hexadecimal)
\u1234 (hexadecimal)
\U00012345 (hexadecimal)
来源：https://stackoverflow.com/questions/20621405/using-c-am-i-right-in-thinking-that-literals-beginning-with-multiple-zeros-are-c