Question about vector<bool>

[continuation.nomad]

suppose I have a large byte array which is actually a bit array. I
want to put this into a vector<bool>. What is the most efficient
way? Internally I know a vector<bool> is represented as a byte array,
so I really would just like a function that says "here's the bytes,
just copy the data". It could even do a memcpy for efficiency. Is
this possible? I've been toying around with it a bit, and it seems
like it might not be possible, and that I actually have to loop
through every single item setting bits. I've tried memcpying to &vec
[0] as well, still no luck.

 

[Zeppe]

suppose I have a large byte array which is actually a bit array. I
want to put this into a vector<bool>. What is the most efficient
way? Internally I know a vector<bool> is represented as a byte array,
so I really would just like a function that says "here's the bytes,
just copy the data". It could even do a memcpy for efficiency. Is
this possible? I've been toying around with it a bit, and it seems
like it might not be possible, and that I actually have to loop
through every single item setting bits. I've tried memcpying to &vec
[0] as well, still no luck.

memcopying to &vec[0] should work if properly done(i.e., if the command
is correct and the vector buffer is correctly allocated), however there
are two correct methods that I would use and that should also be the
fastest ones:

1) construct from iterators
byte foo[128]; // your byte array
std::vector vec(foo, foo+128);

2) std::copy
byte foo[128]; // your byte array
std::vector vec;
vec.reserve(128);
std::copy(foo, foo+128, vec.begin());

Best wishes,

Zeppe

 

[Juha Nieminen]

memcopying to &vec[0] should work if properly done

Except that vec[0] doesn't return a reference to the element with a
std::vector<bool>.

 

[Zeppe]

memcopying to &vec[0] should work if properly done

Except that vec[0] doesn't return a reference to the element with a
std::vector<bool>.

smart! That's why we should stick to the rules

Best,

Zeppe

 

[Zachary Turner]

memcopying to &vec[0] should work if properly done

  Except that vec[0] doesn't return a reference to the element with a
std::vector<bool>.

smart! That's why we should stick to the rules

Best,

Zeppe

After some more experimentation, I found that I could do (&vec[0])-

_Getptr(). However I seriously doubt this method is guaranteed by

the standard, and as such I ultimately decided not to use it and to
roll my own class. What was odd is that is the buffer seems to be a
reverse buffer, so if I copied 100101010 into the buffer, then using
an iterator would give me 010101001, and a revsere_iterator gave me
the correct sequence. Either way, the class seems to limited to be of
much practical use I guess.

 

[Bo Persson]

Zeppe wrote:
memcopying to &vec[0] should work if properly done

Except that vec[0] doesn't return a reference to the element with
a std::vector<bool>.

smart! That's why we should stick to the rules

Best,

Zeppe

After some more experimentation, I found that I could do (&vec[0])-

_Getptr(). However I seriously doubt this method is guaranteed by

the standard, and as such I ultimately decided not to use it and to
roll my own class.

Good idea. Anything starting with underscore uppercase, or containing
double underscores, are implementation details that are subject to
change without notice, and very likely to differ between different
compilers.

What was odd is that is the buffer seems to be a
reverse buffer, so if I copied 100101010 into the buffer, then using
an iterator would give me 010101001, and a revsere_iterator gave me
the correct sequence. Either way, the class seems to limited to be
of much practical use I guess.

The internal representation isn't standardized either. There are
several ways to number the bits in a machine word.


Bo Persson