Re: Why do they cast it twice?
ManicQin <ManicQin@gmail.com> wrote:
Hi, I've browsed the STL code a bit and stumble upon the next line (in
the operator << overload of both long and short outputs - the line is
for checking does the input is a manipulator)
long _Tmp = (_Bfl == ios_base::oct || _Bfl == ios_base::hex)
? (long)(unsigned short)_Val //<-This Line
: (long)_Val;
Is there any profound reason why they "Double" casting the _Val?
Thanks.
Let's pretend that _Val is a short and sizeof(short) is 2 and
sizeof(long) is 4...
If _Bfl == oct or hex, and _Val == 0xFFFF (-1), then _Val will be cast
to (unsigned short)0xFFFF (65535) and then to (long)0xFFFF (65535).
whereas if _Bfl is not oct or hex and _Val == 0xFFFF (-1), then _Val
will be cast to (long)0xFFFFFFFF (-1).
As you can see, the double cast is important if you want to maintain the
literal bit pattern, but the single cast is important if you want to
maintain the logical decimal value.
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