boost thread code ~uintmax_t(0)

Question

bool is_sentinel() const
{
    return milliseconds==~uintmax_t(0);
}

I have found this line of code in thread_data.hpp, I am wondering why is it ~uintmax_t(0) instead of -1 ?

EDIT:

if reason is to avoid compiler warnings, why don't use :

std::numeric_limits(decltype(milliseconds)>::max()

?

[Related, but not an answer](http://stackoverflow.com/questions/809227/is-it-safe-to-use-1-to-set-all-bits-to-true) — CB Bailey, Feb 06 '12 at 10:23

score 2 · Accepted Answer · answered Feb 06 '12 at 10:35

2

One reason for using uintmax_t in the first place is that we don't know what the largest type is. Is it unsigned long or unsigned long long?

My guess is that using ~uintmax_t(0) to produce a large unsigned value simply produces the least number of warnings on the largest number of compilers.

It is common for compilers to warn if you mix signed and unsigned values, or that using a minus on an unsigned value (-1ull) surprisingly(?) gives an unsigned result.

answered Feb 06 '12 at 10:35

Bo Persson

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Why not using std::numeric_limits(decltype(milliseconds)>::max() instead of ~uintmax_t(0) ? – Guillaume Paris Feb 06 '12 at 15:17
@guillaume - Probably because that is a function and not a compile time constant (improved to `constexpr` in C++11). – Bo Persson Feb 06 '12 at 16:19

score 1 · Answer 2 · answered Feb 06 '12 at 10:45

1

~uintmax_t(0) is the simplest way to produce an all-ones value of type uintmax_t that does not produce compiler warnings.

answered Feb 06 '12 at 10:45

Anthony Williams

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I'm surprised (and mildly disappointed) that there are compilers that warn about an explicit `uintmax_t(-1)`. – CB Bailey Feb 06 '12 at 10:54
@Charles what compilers are those? MSVC? – R. Martinho Fernandes Feb 06 '12 at 11:02
@R.MartinhoFernandes: I don't know of any current ones myself. – CB Bailey Feb 06 '12 at 11:04

score 0 · Answer 3 · answered Feb 06 '12 at 10:42

0

Since milliseconds is unsigned, comparing it to -1 wouldn't make any sense.

answered Feb 06 '12 at 10:42

David Schwartz

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boost thread code ~uintmax_t(0)

3 Answers3