Is this codepiece legal c++? and/or portable?

[DonBot]

Hello Group,

I want to use this peace of code as
a core functionality for a polymorphic iterator.
Since i don't want to use plain new to create a
polymorphic iterator, i thought about this "workaround".
The actual storage is allocated on the stack
then inplace new constructs the object...

please tell me if the TEmbed<> is legal/valid/portable/c++?
and/or if there are other ways to do this...
By "portable" i mean: does this work on Win32(MSVC)
and Linux (GCC). I compiled it with MSVC7.1
and it worked.

template
<
typename Value,
size_t ValueSize = sizeof( Value )class TEmbed
{
public:
typedef Value& reference;
typedef Value* pointer;

~TEmbed()
{
std::_Destroy( this->operator->() );
}

explicit TEmbed( const Value& value )
{
ASSERT( sizeof( value ) <= ValueSize );
std::_Construct( this->operator->(), value );
}

pointer operator->()
{
return reinterpret_cast< Value* >( bytes_ );
}

reference operator*()
{
return *reinterpret_cast< Value* >( bytes_ );
}

private:
char bytes_[ ValueSize ];
TEmbed();

};

// ----


struct Foo
{
Foo( int i = 0 ) : t_( i ) { }
int i_;
};

void bar()
{
typedef std::vector< Foo*> VecFooPtr;
typedef VecIFooPtr::iterator VecFooPtrIter;
typedef TEmbed< VecIFooPtrIter > EmbeddedIter;

Foo f0( 0 );
Foo f1( 1 );
Foo f2( 2 );

VecIFooPtr vec;
vec.push_back( &f0 );
vec.push_back( &f1 );
vec.push_back( &f2 );

EmbeddedIter embIterBeg( vec.begin() );
EmbeddedIter embIterEnd( vec.end() );
}


many thanks
DonBot

 

[Peter Koch Larsen]

Hello Group,

I want to use this peace of code as
a core functionality for a polymorphic iterator.
Since i don't want to use plain new to create a
polymorphic iterator, i thought about this "workaround".
The actual storage is allocated on the stack
then inplace new constructs the object...

please tell me if the TEmbed<> is legal/valid/portable/c++?
and/or if there are other ways to do this...
By "portable" i mean: does this work on Win32(MSVC)
and Linux (GCC). I compiled it with MSVC7.1
and it worked.

I see two problems:
One is the use of std::_Destroy, which I do not know. I doubt that this
function is part of the official standard, but I could of course be wrong.
Anyway why not just call the destructor directly?
The other problem is one of alignment. Your TEmbed<> class will not be
properly aligned.

/Peter

template
<
typename Value,
size_t ValueSize = sizeof( Value )class TEmbed
{
public:
typedef Value& reference;
typedef Value* pointer;

~TEmbed()
{
std::_Destroy( this->operator->() );
}

explicit TEmbed( const Value& value )
{
ASSERT( sizeof( value ) <= ValueSize );
std::_Construct( this->operator->(), value );
}

pointer operator->()
{
return reinterpret_cast< Value* >( bytes_ );
}

reference operator*()
{
return *reinterpret_cast< Value* >( bytes_ );
}

private:
char bytes_[ ValueSize ];
TEmbed();

};

// ----


struct Foo
{
Foo( int i = 0 ) : t_( i ) { }
int i_;
};

void bar()
{
typedef std::vector< Foo*> VecFooPtr;
typedef VecIFooPtr::iterator VecFooPtrIter;
typedef TEmbed< VecIFooPtrIter > EmbeddedIter;

Foo f0( 0 );
Foo f1( 1 );
Foo f2( 2 );

VecIFooPtr vec;
vec.push_back( &f0 );
vec.push_back( &f1 );
vec.push_back( &f2 );

EmbeddedIter embIterBeg( vec.begin() );
EmbeddedIter embIterEnd( vec.end() );
}


many thanks
DonBot