Declaring an object using the "new" keyword

The following line of code won't compile (I copied it from MSDN)
myMetafile = new Metafile(L"MyDiskFile.emf", hdc);

I get this error:
error C2065 'myMetafile' : undeclared identifier

I've never declared anything with new before, so I don't have any ideas as to how to fix this.

Any ideas?

Quote:

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Originally Posted by mmscg

The following line of code won't compile (I copied it from MSDN)
myMetafile = new Metafile(L"MyDiskFile.emf", hdc);

I get this error:
error C2065 'myMetafile' : undeclared identifier

I've never declared anything with new before, so I don't have any ideas as to how to fix this.

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What does your C++ book say about "new"? Not only that, what does it say about "delete"?

You can't just copy stuff out of MSDN without understanding basic C++.

The "new" returns a pointer, therefore that variable must be a pointer.

Secondly, you have to call "delete" to deallocate the memory or you get a memory leak.

Third, programmers try to avoid "new", since it creates programs that are harder to maintain.

So to be honest, if you don't know what "new" means, I suggest you not use it, or learn why it is used and the drawbacks if used too much.

Regards,

Paul McKenzie

Metafile sounds like a GDI+ class.
Isn't it?

I am trying to use GDI+ to create the metafile (file that contains all instructions to render a vector image).

Well then you have to include Gdiplus.h, use the namespace Gdiplus, link to Gdiplus.lib, initialize Gdiplus library, and finally... DECLARE myMetafile:

Code:

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  using namespace Gdiplus;
  Metafile* myMetafile = new Metafile(L"c:\\MyDiskFile.emf", hdc);

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And of course, as Paul already stated, it's good to avoid dynamic allocation with new when it's not necessary. So, better is:

Code:

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  Metafile myMetafile(L"c:\\MyDiskFile.emf", hdc);

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Thanks ovidiucucu, both of your methods work... code successfully produces a metafile. :D

While we are on the subject of new, when might it be necessary to declare something this way?

Also, I have never come across using a namespace.
After a quick search I sort of understand what it is... why does it have to be used here?

Quote:

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Originally Posted by mmscg

Thanks ovidiucucu, both of your methods work... code successfully produces a metafile. :D

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And again, without "delete", you have a memory leak, so if you forgot to call "delete", your program really isn't working as you think it is.

Quote:

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While we are on the subject of new, when might it be necessary to declare something this way?

Also, I have never come across using a namespace.
After a quick search I sort of understand what it is... why does it have to be used here?

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These are questions that are best answered by C++ books, as all of these issues are part of basic C++ programming. What C++ books are you using?

Code:

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#include <iostream>

using namespace std;

int main()
{
  cout << "Hello World";
}

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This is a "Hello World" program in C++. You have never seen this before?

Regards,

Paul McKenzie

Yes I used delete with the new method of declaration...
don't know if I used it correctly, but there were no compile errors or warnings.

Yes I've seen that Hello World program before.

With namespaces I'd expect to see std::<some function or constant>, but I don't.

Quote:

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Originally Posted by mmscg

Yes I used delete with the new method of declaration...
don't know if I used it correctly, but there were no compile errors or warnings.

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The problem is that wrong usage of delete (if you are deleting a pointer) is not flagged by the compiler. It is a runtime issue, so you only know if you did something wrong if your program doesn't behave correctly.

Code:

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int main()
{
  char *ptr;
  delete ptr;
}

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This code compiles, but it is wrong. The overall message is that without learning C++ properly, you will not write programs correctly, or you may think you have a working program when it really is just working by chance or luck. C++ isn't a language where you can cherry pick what you want to use and use it properly. It doesn't work that way.

That's why I asked what books are you using to learn C++.

Quote:

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Yes I've seen that Hello World program before.

With namespaces I'd expect to see std::<some function or constant>, but I don't.

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So I gather you aren't using C++ books. "using namespace xxx" is another way to specify that a namespace is in use. This clause is usually introduced on the first few pages of any good C++ book, so there is a big gulf somewhere in what you're learning with respect to C++.

Regards,

Paul McKenzie