casting from raw bytes to structures.

[Josh Mcfarlane]

In an effort to get up to date with C to C++, I've ran into a problem.

I am using an 3rd-party library that returns the raw amount of bytes ot
allocate for a structure that it also uses. So I do something as
follows:

FeaturesStruct* pFeatures = (FeaturesStruct *) new BYTE[StructSize];

This works fine with the API for all intents and purposes.

However, when changing the C style cast to a static_cast, I cannot
compile it.
Using VC++ I get:

Error 1 error C2440: 'static_cast' : cannot convert from 'BYTE *' to
'FeaturesStruct *'

I was curious as to what the correct way to allocate the structure
would be in this case would be, or if I'm just stuck using C style
casts because of the way the library functions?

Thanks,
Josh McFarlane

[Tobias Blomkvist]

Re: casting from raw bytes to structures.

Josh Mcfarlane sade:[color=blue]
> In an effort to get up to date with C to C++, I've ran into a problem.
>
> I am using an 3rd-party library that returns the raw amount of bytes ot
> allocate for a structure that it also uses. So I do something as
> follows:
>
> FeaturesStruct* pFeatures = (FeaturesStruct *) new BYTE[StructSize];
>[/color]
[snip][color=blue]
>
> Error 1 error C2440: 'static_cast' : cannot convert from 'BYTE *' to
>[/color]

FeaturesStruct * pFeatures = reinterpret_cas t<FeaturesStruc t*>(new
BYTE[StructSize]);

Tobias
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[Victor Bazarov]

Re: casting from raw bytes to structures.

Josh Mcfarlane wrote:[color=blue]
> In an effort to get up to date with C to C++, I've ran into a problem.
>
> I am using an 3rd-party library that returns the raw amount of bytes ot
> allocate for a structure that it also uses. So I do something as
> follows:
>
> FeaturesStruct* pFeatures = (FeaturesStruct *) new BYTE[StructSize];
>
> This works fine with the API for all intents and purposes.
>
> However, when changing the C style cast to a static_cast, I cannot
> compile it.
> Using VC++ I get:
>
> Error 1 error C2440: 'static_cast' : cannot convert from 'BYTE *' to
> 'FeaturesStruct *'
>
> I was curious as to what the correct way to allocate the structure
> would be in this case would be, or if I'm just stuck using C style
> casts because of the way the library functions?[/color]


Use "placement new":

BYTE* space = new BYTE[sizeof(Features Struct)];
FeaturesStruct* pFeatures = new (space) FeaturesStruct;

Don't forget to destruct the object when you don't need it any more:

pFeatures->~FeaturesStruc t();

and free the memory

delete[] space;

V

[Old Wolf]

Re: casting from raw bytes to structures.

Tobias Blomkvist wrote:[color=blue]
> Josh Mcfarlane sade:[color=green]
> > In an effort to get up to date with C to C++, I've ran into a problem.[/color][/color]

Do you mean you are porting a C program to C++ ? If so , then
it's fine to leave 'malloc' and 'free' calls as they are.
(Perhaps adding in casts where necessary).
[color=blue][color=green]
>>
>> I am using an 3rd-party library that returns the raw amount of bytes ot
>> allocate for a structure that it also uses. So I do something as
>> follows:
>>
>> Error 1 error C2440: 'static_cast' : cannot convert from 'BYTE * to[/color]
>
> FeaturesStruct * pFeatures = reinterpret_cas t<FeaturesStruc t*>(new
> BYTE[StructSize]);[/color]

Remember that when you delete pFeatures, you must delete in the same
way that it was allocated:

delete [] reinterpret_cas t<BYTE *>(pFeatures);

My preferred solution (assuming FeaturesStruct is a POD and
doesn't require destruction and construction) is:

std::vector<BYT E> features_buf (StructSize);
FeaturesStruct *pFeatures = &features_bu f[0];

then you no longer have to worry about memory management.

[msalters]

Re: casting from raw bytes to structures.


Josh Mcfarlane schreef:
[color=blue]
> In an effort to get up to date with C to C++, I've ran into a problem.
>
> I am using an 3rd-party library that returns the raw amount of bytes ot
> allocate for a structure that it also uses. So I do something as
> follows:
>
> FeaturesStruct* pFeatures = (FeaturesStruct *) new BYTE[StructSize];
>
> This works fine with the API for all intents and purposes.
>
> However, when changing the C style cast to a static_cast, I cannot
> compile it.
> Using VC++ I get:
>
> Error 1 error C2440: 'static_cast' : cannot convert from 'BYTE *' to
> 'FeaturesStruct *'
>
> I was curious as to what the correct way to allocate the structure
> would be in this case would be, or if I'm just stuck using C style
> casts because of the way the library functions?[/color]

Who creates the object? You, or the library? In your code pFeatures
points to uninitialized memory suitable for allocation of a
FeaturesStruct
object.

If you use placement new:
FeaturesStruct* pFeatures = new ( new char[StructSize] )
FeaturesStruct;
you not ony get the memory needed, you get a constructed FeaturesStruct
object as well.

HTH,
Michiel Salters