I have a problem in following expressions:

MyClass *myObject;

MyClass *myObject = new MyClass();

What are the differences between them?

CodePudding user response：

The difference is that:

• in MyClass *myObject;, myObject is uninitialized and thus doesn't point at anything meaningful

• in MyClass *myObject = new MyClass();, myObject is initialized to point at the object that new creates.

CodePudding user response：

The difference is that one declaration has no explicit initializer and other has an explicit initializer.

In this declaration

MyClass *myObject;

the pointer is uninitialized and has an indeterminate value if it is declared in a block scope without the storage class specifier static.

If it is declared in a namespace or with the storage class specifier static it is initialized as a null pointer. That is in this case the declaration is equivalent to

MyClass *myObject = nullptr;

In this declaration

MyClass *myObject = new MyClass();

the pointer is initialized by the address of a dynamically allocated object.

You may also rewrite the last declaration like

auto myObject = new MyClass();

because the type of the name myObject will be deduced from its initializer that is from the expression new MyClass().

Pay attention to that you may split this declaration

MyClass *myObject = new MyClass();

into two statements like

MyClass *myObject;
myObject = new MyClass();

CodePudding user response：

int a;

Does it have any meaningful value? Nothing because 'a' is not initialized.

int a = 5;

Does it have any meaningful value? Yes. 'a' has been initialized to value of 5.

In the same way

MyClass *myObject;

Does it have any meaningful value? No. myObject is not initialized. So it has some junk / random value.

MyClass *myObject = new MyClass();

Does it have any meaningful value? Yes. Sufficient memory to hold 'Myclass' has been allocated dynamically and myObject is pointing to starting of the allocated memory.