Basic C++ - can you explain the difference / function of each definition ?

Lost User

This is a test in few words of plain English , can you comment on the title preferably even answer the question without RTFM Google it everybody knows that snidely remarks etc etc etc

QBluetoothLocalDevice localDevices;
QBluetoothLocalDevice \*localDevices\_new =  new QBluetoothLocalDevice();

Constructive comments , as always are appreciated. "just the facts ... ma'am...." PS I know one is using a pointer...

k5054

If you're having problems understanding that, then you really do need to take a step back, find a good resource for learning C++ and work your way through it. This is simple, basic C++ stuff that you should be able to grok, almost without thinking about it. Using more basic types

class C {
public:
int n;
C() { n = 0; } // class constructor;
};

C c; // declares an object of type C
C *pc; // declares an pointer to an object of type C.
// The pointer does not point to any object.
// Until it is instantiated, accessing *pc is undefined
C *pc2 = new C(); // declares a pointer to an object of type C
// A new object of type C is created on the stack
// and the object constructor is called
// the object will remain valid until a
// corresponding delete is performed

But these days you really should be using smart pointers instead of new/delete. See the documentation for std::shared_ptr and std::unique_ptr here: [Dynamic memory management - cppreference.com](https://en.cppreference.com/w/cpp/memory)

Keep Calm and Carry On

Graham Breach

The first one creates an instance on the stack. It will be destroyed when it goes out of scope. The second one creates an instance in heap memory*. It will not be automatically destroyed and you should call delete to destroy it. *Unless new has been overridden to do something unusual.

Lost User

Yes, I am having a problem understanding this: I can "declare " int a; a should be undefined - at it is customary to define / assign value... That is NOT the issue. if I declare class a; should a be also undefined ?? if I have

QBluetoothLocalDevice test;

"declaration of object type

QBluetoothLocalDevice

" how does "test" contains all the real Bluetooth devices data? Show me a resource which describes what I just asked ? Do I have to look thru the

QBluetoothLocalDevice

class documentation to find which method makes the "test" to contain all the hardware info?

k5054

Every C++ class has a constructor that gets called when the object is created. So if we have

class C {
public:
int n;
C() { n = -1; }
}

int main()
{
C c; // C:C() gets called here, assigning -1 to n;
std::cout << c.n << '\n'; // will print -1
}

In the class C given above, if you do not give a default constructor, then the compiler will provide one, but it will not initialize the value of C.n

class C {
public:
int n;
};

int main()
{
C c; // compiler provided constructor called
std::cout << c.n << '\n'; // This generates a warning with -Wall (gcc) that n is unititalized
}

Presumably class QBluetoothLocalDevice provides a default constructor that fills in reasonable default values for its members. If some of those members use system resources (e.g. open file handles, memory, etc), then there is also a destructor that gets called when the object goes out of scope to release the resources (e.g close open files, release memory, etc).

Keep Calm and Carry On

Lost User

1. Create an object on the stack

QBluetoothLocalDevice localDevices;

This reserves all the memory space required for a QBluetoothLocalDevice object on the local stack. It then calls the constructor of the class to initialise any parts of that memory as specified in the class (see answers by @k5054). The variable localDevices holds the address of the object (even though it does not appear to be a pointer). 2. Create an object on the dynamic heap, and return a pointer to it.

QBluetoothLocalDevice *localDevices_new = new QBluetoothLocalDevice();

In this case the memory is requested from the heap, the constructor called to initialise it, and its address returned and saved in the pointer localDevices_new. The end result is much the same in both cases, apart from the location and lifetime of the two objects. In case 1 the object only exists within the function where it is created; it is automatically destroyed when the function ends. In case 2 the object exists until it is destroyed by the delete statement, or the program terminates. But as suggested elswhere, this is basic C++, which you should have learned and understood long before you charged down this rabbit hole that you currently find yourself in.

Lost User

Member 14968771 wrote:

Do I have to look thru the QBluetoothLocalDevice class documentation to find which method makes the "test" to contain all the hardware info?

Yes, as you would need to do with any class that you are using. And here it all is: QBluetoothLocalDevice Class | Qt Bluetooth 6.4.2[^].

jschell

Member 14968771 wrote:

This is a test in few words of plain English ,

Which suggests homework. But the answer is no. The question cannot be answered in "plain english" because it require concepts that only exist in programming. So as the other answers suggest one would need to understand what a 'local variable' is and what a 'heap' is for any answer to make sense.

Lost User

It's not homework. This member is struggling with basic concepts in C++.

charlieg

Please follow up if the explanations helped.

Charlie Gilley “They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759 Has never been more appropriate.

Lost User

I will not engage in a discussion full of criticism.

charlieg

oh well, suit yourself.

Charlie Gilley “They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759 Has never been more appropriate.