错误C2440：“正在初始化”：无法从“类名*”转换为“类名”

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

Answer 1

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

Answer 2

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;

Answer 3

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.

Answer 4

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;

Answer 5

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() .

Answer 6

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.