Using a pointer or not

Question

Will there be a difference between the following two ways of calling a function from an instance? Which is better?

    Motor M;
    M.moveToPosition(Speed, TargetPosition);

    Motor *M;
    M->moveToPosition(Speed, TargetPosition);

Thanks!

Did you try the second?! It's UB to dereference an uninitialized pointer and I'd be surprised if this runs without problems. — Niklas B., Mar 22 '12 at 14:09
@Marwan Dessouki it does compile indeed, but it won't run as expected. — , Mar 22 '12 at 14:13
@daknok_t: It *could* run as expected, but that's very unlikely. — Niklas B., Mar 22 '12 at 14:18

score 4 · Accepted Answer · answered Mar 22 '12 at 14:09

4

Your second version has Undefined Behavior, because the pointer is uninitialized!

You can dynamically allocate an object of type Motor(i.e. allocate it on the heap):

Motor *M = new Motor;
M->moveToPosition(Speed, TargetPosition);
...
delete M;

Smart Pointers will help you avoid to have to remember to delete objects allocated on the heap.

As for which is better, I am afraid it depends a lot on the context. Usually in C++ you should prefer automatic objects (on the stack) to dynamic objects (on the heap) unless you have a definite reason for the contrary.

answered Mar 22 '12 at 14:09

Armen Tsirunyan

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It would be better to point to M via a smart pointer (std::unique_ptr). – Marcin Hoppe Mar 22 '12 at 14:11
@MarcinHoppe: I was on my way of adding that piece (and more) while you were commenting :) – Armen Tsirunyan Mar 22 '12 at 14:13

score 2 · Answer 2 · answered Mar 22 '12 at 14:09

The first method will call the Motor constructor; the second method won't and the pointer will be uninitialized.

In C++, stay away from pointers where possible. Use the first method here.

If you want to dynamically allocate the motor, use this:

std::unique_ptr<Motor> M(new Motor);
M->moveToPosition(...);

score 0 · Answer 3 · answered Mar 22 '12 at 14:11

0

It's the same, but using pointers you'd be able to dynamically create and destroy objects (i.e. in loops), perform operations on arbitrary number of objects, take advantage of the polymorphism, and so on.

answered Mar 22 '12 at 14:11

Alexander

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... and get some really cool memory leaks if you're not very cautious! :) – hochl Mar 22 '12 at 15:08
".. come great memory leaks?" ^^ – hochl Mar 22 '12 at 15:20

Using a pointer or not

3 Answers3