指向指向对象

Question

Dereferencing pointers can make the code very hard to read. What I usually do is putting a reference to the pointed object and working with the reference. Example:

shared_ptr<std::vector<int> > sp = get_sp_to_vector();
std::vector<int>& vec = *sp;
...
vec.push_back(5);

I wonder if it's a good practice. Does it have any drawback?

Update: To complete the example, I'd define get_sp_to_vector() the following way:

shared_ptr<std::vector<int> >  get_sp_to_vector()
{
    // create a vector and send back a shared pointer pointing at it
    shared_ptr<std::vector<int> >  sp(new std::vector<int>);
    sp->push_back(1);  sp->push_back(3);
    return sp;
}

Answer 1

I wouldn't consider this a good practice. Pointers are a main-stay of C/C++ development and b/c of that, C/C++ coders are comfortable with the de-reference syntax needed by pointers. I do try and avoid the use of pointers when possible (not for syntactical reasons though), but sometimes it's just the best tool for the job. If you have code that is gnarly b/c you're using a pointer, I would typically de-reference it into a function that passes the object by reference which essentially does what you're doing, but in my opinion it does so in a more elegant way. So instead of:

shared_ptr<std::vector<int> > sp = get_sp_to_vector();
std::vector<int>& vec = *sp;
...ugly stuff...
vec.push_back(5);

I would do:

void func(std::vector<int> &vec)
{
     ... previously ugly stuff...
     vec.push_back(5);
}

shared_ptr<std::vector<int> > sp = get_sp_to_vector();
func(*sp);

EDIT:
This isn't to say that you should create a function simply to create nicer syntax for pointers, as it is to state that if you don't like pointer syntax and your code uses clean and concise functions, you can simply make the functions take references and de-reference your pointer when calling the function. For situations where you have only a few calls to *p or p->x, it seems silly to create a function with a reference or to create a reference in order to call px Just use the pointer syntax in these cases as this is C/C++ syntax.

Others have brought up using references within loops where a pointer may have to be de-referenced many times. I do agree that in these cases a reference would be beneficial.

Answer 2

Using local references is common, especially inside loop bodies, but there is one requirement: only do it when the reference will obviously live as long as the target object.

This usually isn't hard to guarantee, but here are some bad examples:

shared_ptr<X> p = get_p();
X& r = *p;
p.reset(); // might result in destroying r
use(r); // oops

// or:
shared_ptr<X> p = get_p();
X& r = *p;
p = get_some_other_p(); // might result in destroying r
use(r); // oops

Answer 3

IMHO, I think this is an acceptable practice (with some caveats - See Roger Pate's post). If you're adding a line of code just for the push_back() call, then I don't think that is acceptable.

However, if you find you are dereferencing the pointer many times, then not only is dereferencing it once into a reference object acceptable, it could be a performance win (depending on your compiler, the code in question, the phases of the moon, etc).

Answer 4

If I am actually doing pointer manipulation, then I leave it a pointer. If my pointer is simply a nilable reference, then I leave it a pointer. If I am assured or wanting to assure that the pointer will always be non-zero (Ie if I am are returning the non-nil object from a method) then I make this explicit by using a reference.

Answer 5

Working back from your proposed code, I don't agree that:

shared_ptr<std::vector<int> > sp = get_sp_to_vector();
...
sp->push_back(5);

is "very hard to read". It's also shorter than your code. So although I don't think there's much wrong with defining and using a local reference, I also don't think you should make a rule to always do so.

Perhaps if your pointer code is very hard to read, it's because you're missing some trick which would make it simpler. Maybe if you ask another question with an example of bad pointer code that you'd currently fix with a reference, SOers would find alternative fixes.