Compile error

[sethjackson]

Hi I get this error

error: incompatible types in assignment of `int' to `unsigned char*[((unsigned int)((int)imageSize))]'

with this code......

Code (cpp) Select Expand


if (image.HasAlpha())
{
        unsigned long imageSize = image.GetWidth() * image.GetHeight() * 4;
        unsigned char* imageData = image.GetData();
        unsigned char* imageAlpha = image.GetAlpha();
        unsigned char* data[imageSize];

        for (unsigned long index = 0; index < imageSize; index += 4)
        {
            memcpy(data, imageData, 3);

            imageData += 3;

            memcpy(data + 3, imageAlpha, 1);

            imageAlpha++;

            data += 4;
        }

        gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGBA, image.GetWidth(), image.GetHeight(),
                          GL_RGB, GL_UNSIGNED_BYTE, data);
}



Help me please...... Sorry for the beginner question? I just don't get it I used to do stuff like increaseing the ptr with += all the time.....

EDIT:

Let me say that it fails on the line

Code (cpp) Select Expand


data += 4;


[sethjackson]

Ok I fixed it now could someone tell me why this works???

Code (cpp) Select Expand


*(data) += 4;


[polygon7]

Hi,
this should work

Code Select Expand

if (image.HasAlpha())
{
        unsigned long imageSize = (image.GetWidth() * image.GetHeight())  << 2;
        unsigned char* imageData = image.GetData();
        unsigned char* imageAlpha = image.GetAlpha();
        unsigned char* data[imageSize];

        for (unsigned long index = 0; index < imageSize; index += 4)
        {
            memcpy(data, imageData, 3);

            imageData += 3;

            memcpy(data + 3, imageAlpha, 1);

            ++imageAlpha;

           (*data) += 4;
        }

        gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGBA, image.GetWidth(), image.GetHeight(),
                          GL_RGB, GL_UNSIGNED_BYTE, data);
}
// Edit: Ah, you're faster

Code Select Expand

char* array[size] is the same as

Code Select Expand

char** array

[sethjackson]

 :lol: :lol: Read my post above. Exactly why does it work though? Thanks. 

[Michael]

:lol: :lol: Read my post above. Exactly why does it work though? Thanks. 

You tried to assign an int to an unsigned char*. This does not work for sure. What do did is to de-reference the pointer. Then it works.

Why you cast before int and then unsigned int?

Anyway, do not use C-cast style. This is not a good idea in C++. You should use C++ cast style or better no cast at all (if possible).

Best wishes,
Michael

[sethjackson]

@polygon7

Why the

Code (cpp) Select Expand

<< 2 instead of

Code (cpp) Select Expand

* 4?

This line

Code (cpp) Select Expand


unsigned long imageSize = (image.GetWidth() * image.GetHeight())  << 2;


@Michael How to use a C++ style cast in this case??? Yes I know static_cast, dynamic_cast etc. How to use it here???

[polygon7]

@polygon7

Why the

Code (cpp) Select Expand

<< 2 instead of

Code (cpp) Select Expand

* 4?

This line

Code (cpp) Select Expand


unsigned long imageSize = (image.GetWidth() * image.GetHeight())  << 2;


"<<" is shifting bits operator (shift left, SHL in assembly). SHL can be used for multiply by power of two (SHR, shift right for dividing by power of two).

x*2 == x <<1, x*4 == x <<2 ...

Bit shifting usually is faster then normal multiply.

[Michael]

@Michael How to use a C++ style cast in this case??? Yes I know static_cast, dynamic_cast etc. How to use it here???

In your case and if I am not wrong, I would use the static_cast Operator:

static_cast<T> (expr)
To convert the expression expr to type T. Such conversions rely on static (compile-time) type information.

Anyway, I am not an expert in C++ cast as usually I used C cast until I discovered that are not so good (cast is already ugly, but C-cast is uglier). You can get some useful info here:

• http://www.research.att.com/~bs/bs_faq2.html#static-cast
• http://en.wikipedia.org/wiki/Cast_(computer_science)#in_C.2FC.2B.2B
• http://www.coding-zone.co.uk/cpp/articles/050101casting.shtml

You can also have a look at this topic.

Michael

[sethjackson]

Ok guys thanks. I learnt something today.

@Michael yeah I always use C++ casts instead of C casts. I couldn't figure out how to get it to work here. What type do I need to static_cast to?? I tried static_cast, but I couldn't figure out which type to cast to. 

[Michael]

@Michael yeah I always use C++ casts instead of C casts. I couldn't figure out how to get it to work here. What type do I need to static_cast to?? I tried static_cast, but I couldn't figure out which type to cast to. 

For example, you can use it to cast a double to an int:

Code Select Expand


double myDouble = 3.0;
int myInt = static_cast<int>(myDouble);


Anyway, be careful because static casts could be dangerous.

Have a look at the links I have suggested. There is a lot of examples and useful bla bla .

Michael

[sethjackson]

Ok I'll check it out tomorrow hopefully.  :lol:

[sethjackson]

I tried

Code (cpp) Select Expand


static_cast<unsigned char* []>(data) += 4;


and it didn't work.

Code Select Expand


error: invalid static_cast from type `unsigned char*[((unsigned int)((int)imageSize))]' to type `unsigned char*[]'


What type (int, char, etc.) do I need to cast to???

[sethjackson]

Nevermind. This works, and better too I think.

Code (cpp) Select Expand


if (image.HasAlpha())
{
        unsigned long imageSize = image.GetWidth() * image.GetHeight() * 4;
        unsigned char* imageData = image.GetData();
        unsigned char* imageAlpha = image.GetAlpha();
        unsigned char* data = new unsigned char[imageSize];

        for (unsigned long index = 0; index < imageSize; index += 4)
        {
            memcpy(data, imageData, 3);

            imageData += 3;

            memcpy(data + 3, imageAlpha, 1);

            ++imageAlpha;

            data += 4;
        }

        gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGBA, image.GetWidth(), image.GetHeight(),
                          GL_RGB, GL_UNSIGNED_BYTE, data);

        delete[] data;
}


[Michael]

Hello,

When you use new you should check if the memory has been reserved or not. If I remember correctly the new sends a bad_allocation exception if it fails. Another version of new just a null pointer (which could or not be a better alternative ).

Best wishes,
Michael

[killerbot]

Hello,

When you use new you should check if the memory has been reserved or not. If I remember correctly the new sends a bad_allocation exception if it fails. Another version of new just a null pointer (which could or not be a better alternative ).

Best wishes,
Michael

Normally when new fails, it throws -> catch it.
There exist a no_throw new. I think it is not portable, the gurus (Sutter, ..) give the advice not to use it. So just forget you ever heard about it ;-)