Re: std::vector anomally

From:

Norbert Unterberg <nunterberg@newsgroups.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 14 Dec 2007 22:56:46 +0100

Message-ID:

<OKLv5wpPIHA.5988@TK2MSFTNGP02.phx.gbl>

John Keenan schrieb:

I have stumbled across an interesting anomally (well... I find it
interesting). In VS8 the following gives a compiler error that method
Xxx::readOnly is attempting to return a reference to a local value (see
"problem here" below).

class Xxx{
private:
   std::vector< bool >* _readOnly;
public:
   Xxx();
   virtual ~Xxx();
   const bool& readOnly( const unsigned int& i_ ) const;
};

Xxx::Xxx(){ _readOnly = new std::vector< bool >; }
Xxx::~Xxx(){ delete _readOnly; };

const bool&
Xxx::readOnly( const unsigned int& i_ ) const{
   return _readOnly->at( i_ ); // problem here.
};

If I change the three occurances of bool to double or std::string the
problem disappears. When I move it to VS6 the problem with bool does not
occur. Is there something about bool that would cause std::vector::at to not
return a referenced? Is there some project and/or solution setting that I
may have changed that would cause this anomally with bool?

Yes, there is something different about the bool vector.

The std C++ library provides a template specialization for vector<bool> that
allows implementors to pack the bool values into a bit array to make a compact
array. Since you can not create a reference or a pointer to a single bit,
vector<bool>::at() does not return a bool reference.

Visual Studio provides a nested vector<bool>::reference class that provides
references to the bool values of vector<bool>.
See http://msdn2.microsoft.com/en-us/library/t0723a54(VS.80).aspx
and http://msdn2.microsoft.com/en-us/library/t248k97e(VS.80).aspx

Norbert