error C2440: 'initializing' : cannot convert from 'classname *' to 'classname'

Question

I have a class defined called extBlock.

I then make an instance of that class with this

extBlock mainBlock = new extBlock(1, 1024);

I get this error: error C2440: 'initializing' : cannot convert from 'extBlock *' to 'extBlock'

Can anyone help me with why I am getting this error.

I have seen examples online of declaring it like this with a pointer

extBlock *mainBlock = new extBlock(1, 1024);

But if I do it this way it does not let me call the functions of mainBlock

You should put your code on a new line and indent every line with four spaces. — Chris Cooper, Apr 16 '10 at 16:25
You might consult one of the introductory C++ books listed in this question: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — James McNellis, Apr 16 '10 at 16:26
possible duplicate of http://stackoverflow.com/questions/2647660/creation-of-an-object-in-c — Nemanja Trifunovic, Apr 16 '10 at 16:28

score 13 · Answer 1 · answered Apr 16 '10 at 16:26

Read up on your C++ syntax:

extBlock mainBlock(1, 1024); // create a class instance (object) on the stack
mainBlock.memberFunction(); // call a member function of a stack object

extBlock * ptrBlock = new extBlock(1, 1024); // create an object on the heap
ptrBlock->memberFunctions(); // member access through pointers has different syntax
delete ptrBlock; // must deallocate memory when you're done with a heap object

kennytm · Answer 2 · 2010-04-16T17:24:43.327

Switching from Java/C#?

In C++, to initialize an object on stack, you just need to use

extBlock mainBlock (1, 1024);
...
mainBlock.call_func(1,2,4,7,1);

The new operator creates an object on heap, and return the pointer to it. To access functions from a pointer, you need to dereference it with *:

extBlock* mainBlock = new extBlock(1,1024);
...
(*mainBlock).call_func(1,2,4,7,1);

In C and C++, a->b can be used in place of (*a).b:

mainBlock->call_func(1,2,4,7,1);

Also, C++ doesn't have Garbage Collection by default, so you need to deallocate with delete explicitly:

delete mainBlock;

score 7 · Accepted Answer · answered Apr 16 '10 at 16:25

This isn't C#: new extBlock returns a pointer to an extBlock, and you're trying to assign that pointer to a value type (which would be an incompatible cast).

What you want to write here is

extBlock mainBlock(1, 1024);

And the reason you couldn't call methods on the second code snippet was probably because you were using the . operator instead of the -> (arrow) operator needed to dereference a pointer.

score 1 · Answer 4 · answered Apr 16 '10 at 16:26

1

You want this, like you had:

extBlock *mainBlock = new extBlock(1, 1024);

but then you call functions using -> instead of ., like this:

mainBlock->FunctionOnIt(...);

Don't forget to delete it when it's no longer needed.

delete mainBlock;

answered Apr 16 '10 at 16:26

Stephen

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score 1 · Answer 5 · answered Apr 16 '10 at 16:28

new returns a pointer to the allocated memory, where the constructor has initialized your object. Thus you need to use extBlock *mainBlock = new extBlock(1, 1024);. You can call the methods afterwards via mainBlock->someMethod() or (*mainBlock).someMethod().

score 0 · Answer 6 · answered Apr 16 '10 at 16:28

The new keyword always assigns to a pointer. What you need to do is something like this:

extBlock *mainBlock = new extBlock(1, 1024);
mainBlock->functionName();

Since mainBlock is now a pointer, the . operator won't work anymore to reference fields or methods and the -> operator must be used in its place.

error C2440: 'initializing' : cannot convert from 'classname *' to 'classname'

6 Answers6