On Tue, 25 Mar 2008 19:47:39 +0100, Nicolas Alvarez  
<nic### [at] gmailisthebestcom> wrote:

>> On Tue, 25 Mar 2008 17:34:14 +0100, Nicolas Alvarez  
>> <nic### [at] gmailisthebestcom> wrote:
>>> C++ STL vector template class has a specialization for 'bool' that  
>>> uses 1 bit per element too. That is, a vector<bool> takes 1 bit per  
>>> element.
>>  Yes, but that specialisation is largely considered a mistake because  
>> it breaks the interface associated with the generic std::vector.
>
> What exactly is broken in the interface?

Well, it does not satisfy the requirements of an STL container, making it  
incompatible for use with some algorithms and other containers.

For a concrete example, consider this code:

// Code begins
template < typename T >
void func()
{
	std::vector< T > v( 10 );
	T& ref = v[0];
}
// Code ends

It works with any instantiation of the generic std::vector - the standard  
guarantees it - but fails with std::vector<bool>.

Also, things like 'std::stack< bool, std::vector<bool> >' do not work  
either.

As someone else once put it: std::vector<bool>, while listed under  
"containers" in the standard, is not a container, nor does it contain  
bools.


If you really need a compact set of booleans, use std::bitset or  
boost::dynamic_bitset instead.


-- 
FE

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