Reduce memory usage?

[Joseph Turian]

Let's say I have a vector of Base.
I want to convert it to a vector of Derived, where
Derived::Derive d(const Base&); is defined.

Is it possible to convert the vector of Base to a vector of Derived
without incurring the memory overhead of two copies of everything? If
these were vectors of the same kind, I could just use vector::swap, but
unfortunately not in this example.

Joseph

[Stefan Näwe]

Re: Reduce memory usage?

Joseph Turian wrote:[color=blue]
> Let's say I have a vector of Base.[/color]

A std::vector<Bas e*>, I presume ?
[color=blue]
> I want to convert it to a vector of Derived, where
> Derived::Derive d(const Base&); is defined.
>
> Is it possible to convert the vector of Base to a vector of Derived
> without incurring the memory overhead of two copies of everything? If
> these were vectors of the same kind, I could just use vector::swap, but
> unfortunately not in this example.[/color]

Maybe something like this:

1. Define a (template) functor that returns dynamic_cast<De rived*>(p) in operator()(cons t Base* p)
2. use std::transform( v.begin(), v.end(), v.begin(), CastingFunctor< ...>())

/S
--
Stefan Naewe
naewe.s_AT_atla s_DOT_de

[Stefan Näwe]

Re: Reduce memory usage?

Stefan Näwe wrote:[color=blue]
> Joseph Turian wrote:
>[color=green]
>>Let's say I have a vector of Base.[/color]
>
>
> A std::vector<Bas e*>, I presume ?
>
>[color=green]
>>I want to convert it to a vector of Derived, where
>>Derived::Deri ved(const Base&); is defined.
>>
>>Is it possible to convert the vector of Base to a vector of Derived
>>without incurring the memory overhead of two copies of everything? If
>>these were vectors of the same kind, I could just use vector::swap, but
>>unfortunate ly not in this example.[/color]
>
>
> Maybe something like this:
>
> 1. Define a (template) functor that returns dynamic_cast<De rived*>(p) in operator()(cons t Base* p)
> 2. use std::transform( v.begin(), v.end(), v.begin(), CastingFunctor< ...>())[/color]

This must of course be:

std::transform( v.begin(), v.end(), w.begin(), CastingFunctor< ...>())

Where w is a (large enough!) vector<Derived* >

/S
--
Stefan Naewe
naewe.s_AT_atla s_DOT_de

[Stefan Näwe]

Re: Reduce memory usage?

Stefan Näwe wrote:[color=blue]
> Stefan Näwe wrote:
>[color=green]
>>Joseph Turian wrote:
>>
>>[color=darkred]
>>>Let's say I have a vector of Base.[/color]
>>
>>
>>A std::vector<Bas e*>, I presume ?
>>
>>
>>[color=darkred]
>>>I want to convert it to a vector of Derived, where
>>>Derived::Der ived(const Base&); is defined.
>>>
>>>Is it possible to convert the vector of Base to a vector of Derived
>>>without incurring the memory overhead of two copies of everything? If
>>>these were vectors of the same kind, I could just use vector::swap, but
>>>unfortunatel y not in this example.[/color]
>>
>>
>>Maybe something like this:
>>
>>1. Define a (template) functor that returns dynamic_cast<De rived*>(p) in operator()(cons t Base* p)[/color][/color]

Oops...
This must be:
1. Define a (template) functor that returns dynamic_cast<De rived*>(p) in operator()(Base * p)
[color=blue][color=green]
>>2. use std::transform( v.begin(), v.end(), v.begin(), CastingFunctor< ...>())[/color]
>
>
> This must of course be:
>
> std::transform( v.begin(), v.end(), w.begin(), CastingFunctor< ...>())
>
> Where w is a (large enough!) vector<Derived* >[/color]

This might work for you:

<----CODE---->

#include <iostream>
#include <iterator>
#include <string>
#include <algorithm>
#include <vector>

template<typena me To>
struct CastingFunctor
{
template<typena me From>
To* operator()(From * p) const { return dynamic_cast<To *>(p); }
};

struct Base
{
virtual ~Base() { }
};

struct Derived : public Base
{
};

int main(int argc, char* argv[])
{
const size_t N = 10;
std::vector<Bas e*> v;

for(size_t i=0; i<N; ++i)
{
v.push_back(new Derived);
v.push_back(new Base);
}

std::vector<Der ived*> w(v.size());
std::transform( v.begin(), v.end(), w.begin(), CastingFunctor< Derived>());

std::copy(v.beg in(), v.end(), std::ostream_it erator<void*>(s td::cout, "\n"));
std::copy(w.beg in(), w.end(), std::ostream_it erator<void*>(s td::cout, "\n"));
return 0;
}

<----/CODE---->


/S
--
Stefan Naewe
naewe.s_AT_atla s_DOT_de