Why I should use '->' instead of '.' in a pointer of the object? [duplicate]

Question

I agree this might be a very beginner's question, but I have no idea why I can't use '.' to access a member of a pointer to an object.

e.g.

JMP *sum_obj = new JMP("0");
JMP a;
sum_obj->number;
a.number;

sum_obj.number; // error: request for member ‘number’ in ‘sum_obj’, which is of pointer type ‘JMP*’ (maybe you meant to use ‘->’ ?)

Here, why should I use -> for the sum_obj number member?

Rehearing

Since in C there are pointers and values so to operators are used to distinguish those too. In other languages like Java you have just references. C++ leverage this duplication of this access operators and provides smart pointers (value which has some properties accessible by . and and pointed value accessible by ->). – Barre

Because its a pointer. Because the designers of the language said so. – Linage

Because sum_obj is a pointer. x->y is the same as (*x).y. I suggest you read the chapter dealing with pointers in your beginner's C++ text book. – Indigence

Answer 1

In C there would be no technical reason. A non-technical reason is clarity - if you see a ->, you know that it's a pointer and can potentially be null, so you might need to check for null before dereferencing it.

In C++, there are classes that pretend to be pointers to some degree (std::unique_ptr, std::shared_ptr, std::optional). They support * and -> like pointers, but they also have their own member functions, accessible with .. Separating the notation this way avoids any possible member name conflicts, and also adds clarity.

Overabundance

Thanks for your good explanation. Now I know why I should use -> instead of .. – Rehearing

Good answer. However note how the OP got an error for trying to call dot operator on a pointer. You left out that p->x is identical to (*p).x (as mentioned in the comments). It seems your answer could use one introductory sentence. – Uela

@Uela They seem to already know about ->, as they mention it in the question. To me it reads like a question about why the language was designed this way. – Overabundance

Agreed. Do they know about (*p).x? – Uela

Answer 2

the arrow operator -> is used to access the member variables or member functions of an object that is pointed to by a pointer. The arrow operator is used with pointers because when we use the dot operator . to access a member variable or function, the compiler will assume that we are accessing a member of an object, not a pointer to an object.

Insight