type case problem

[amit]

Hello group,

In a method of my class I have below problem in terms of type case. Can
somebody please give me insight into this?

This function returns a pointer into the array at the specified index.
Please know that the array containing image data.
void* CImageArray::El ementIndex( long iIndex) const
{
return ((unsigned char*) (void*)m_elemen ts) + ( iIndex *
m_ElementSize);

}

Consider that m_element has a type of CMemoryBlock.

*************** * Error message ***************

error C2440: 'type cast' : cannot convert from 'const class
CMemoryBlock' to 'void *'
No user-defined-conversion operator available that can perform
this conversion, or the operator cannot be called
Error executing cl.exe.



Your help will be appreciated greatly.
Thanks,
amit

[Parapura Rajkumar]

make the m_elements mutable using the mutable keyword.

Raj

[Ali Çehreli]

amit wrote:

In a method of my class I have below problem in terms of type case.
I think you mean type "cast" here.
This function returns a pointer into the array at the specified index.
Please know that the array containing image data.
void* CImageArray::El ementIndex( long iIndex) const
{
return ((unsigned char*) (void*)m_elemen ts) + ( iIndex *
m_ElementSize);

}
Don't use C-style casts in C++. You don't need them.
Consider that m_element has a type of CMemoryBlock.

*************** * Error message ***************

error C2440: 'type cast' : cannot convert from 'const class
CMemoryBlock' to 'void *'

This is because, the ElementIndex function is const, meaning that the
members of CImageArray are non-mutable in this function. You cannot return
'void*' to constant data.

You have two options:

1) Make the function non-const: remove the 'const' at the end

2) Return pointer-to-const: probably 'void const *'

In any case, use one of the C++ type conversion operators. In this case,
reinterpret_cas t:

void const * CImageArray::El ementIndex(size _t iIndex) const
{
return reinterpret_cas t<unsigned char const *>(m_elements) + iIndex *
m_ElementSize;
}

Ali

[Jay Nabonne]

On Thu, 14 Apr 2005 10:22:21 -0700, amit wrote:

Hello group,

In a method of my class I have below problem in terms of type case. Can
somebody please give me insight into this?

This function returns a pointer into the array at the specified index.
Please know that the array containing image data.
void* CImageArray::El ementIndex( long iIndex) const
{
return ((unsigned char*) (void*)m_elemen ts) + ( iIndex *
m_ElementSize);

}

Consider that m_element has a type of CMemoryBlock.

*************** * Error message ***************

error C2440: 'type cast' : cannot convert from 'const class
CMemoryBlock' to 'void *'
No user-defined-conversion operator available that can perform
this conversion, or the operator cannot be called
Error executing cl.exe.

CMemoryBlock is an object, correct? What is your expectation as far as teh
result of casting this object to a pointer? (What should it point to?)

- Jay

[amit]

I'm going to load a TIF file into an array but every TIF file could
have several pages. CImageArray is a type that I can instantiate an
object for array and CMemoryBlock is a class which should give me
methods like
free(), alloc() and realloc().

Please consider that the application I'm working on is based on libtiff
which is written in C Ansi.

Please let me know if you have any suggestions.

Thanks,
Amit,

Jay Nabonne wrote:

On Thu, 14 Apr 2005 10:22:21 -0700, amit wrote:

Hello group,

In a method of my class I have below problem in terms of type case. Can somebody please give me insight into this?

This function returns a pointer into the array at the specified index. Please know that the array containing image data.
void* CImageArray::El ementIndex( long iIndex) const
{
return ((unsigned char*) (void*)m_elemen ts) + ( iIndex *
m_ElementSize);

}

Consider that m_element has a type of CMemoryBlock.

*************** * Error message ***************

error C2440: 'type cast' : cannot convert from 'const class
CMemoryBlock' to 'void *'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
Error executing cl.exe.
CMemoryBlock is an object, correct? What is your expectation as far

as teh result of casting this object to a pointer? (What should it point to?)
- Jay

[amit]

No, mutable didn't hlep. Thanks anyway for your response.

[amit]

No, mutable didn't hlep. Thanks anyway for your response.

[amit]

I tried all steps 1 ~ 3 but still complains.

I thought solution 3 would work but it turns out with this:

error C2440: 'reinterpret_ca st' : cannot convert from 'class
suMemBlock' to 'unsigned char'
Conversion requires a constructor or user-defined-conversion
operator, which can't be used by const_cast or reinterpret_cas t
C:\Development\ testing\faxlibc onsole\ImageArr ay.cpp(135) : error C2059:
syntax error : ')'

[Jay Nabonne]

On Thu, 14 Apr 2005 12:10:09 -0700, amit wrote:

I'm going to load a TIF file into an array but every TIF file could
have several pages. CImageArray is a type that I can instantiate an
object for array and CMemoryBlock is a class which should give me
methods like
free(), alloc() and realloc().

Please consider that the application I'm working on is based on libtiff
which is written in C Ansi.

Please let me know if you have any suggestions.

Then I suspect you need to call some of the methods on the object.

Or at least answer my question about what you expected the cast of the
object to a pointer to accomplish. :)

- Jay

[amit]

Sorry, about your question:

It should return a pointer into the array at the specified index.

Jay Nabonne wrote:

On Thu, 14 Apr 2005 12:10:09 -0700, amit wrote:

I'm going to load a TIF file into an array but every TIF file could
have several pages. CImageArray is a type that I can instantiate an
object for array and CMemoryBlock is a class which should give me
methods like
free(), alloc() and realloc().

Please consider that the application I'm working on is based on libtiff which is written in C Ansi.

Please let me know if you have any suggestions.

Then I suspect you need to call some of the methods on the object.

Or at least answer my question about what you expected the cast of

the object to a pointer to accomplish. :)

- Jay