Is not operator [] supposed to deference?

Question

Also following this topic: What does "dereferencing" a pointer mean?

assert(p[1] == 'b');

the operator [] should automatically deference the pointer, let me direct "change" the value to the pointed data.

But if I do somethings like this:

void QuadraticInterpolateArray(vector<double> *b) {

    // ...

    double step = 12.2;
    for (int j = 0; j < (*b).size(); j++) {
        b[j] = step;
    }

    // ...
}

I can't use b[j] = step; i.e. I'm trying to set 12.2 at each index of the vector.

Where am I wrong? What's different?

Change `vector *b` to `vector &b` and see what happens..! – Sean Jul 01 '16 at 10:16 — Sean, Jul 01 '16 at 10:16
Passing a pointer to vector doesn't make any sense. – Jul 01 '16 at 10:19 — , Jul 01 '16 at 10:19

LogicStuff · Accepted Answer · 2016-07-01T10:38:49.920

4

std::vector::operator[] dereferences a pointer to the beginning of vector's internal array added with the offset. You added another indirection level yourself, so you have to do (*b)[j] to call operator[] on std::vector.

b[j] however, calls operator[] on std::vector*, which gives you std::vector (valid only for j = 0 if b does not point to an array). You can't assign double to std::vector<double> like that.

And yes, you should use reference for the parameter instead, which lets you use the same syntax as with values, but with changes to the passed vector as with pointers.

void QuadraticInterpolateArray(vector<double> &b) {
    // TODO: use std::fill
    double step = 12.2;
    for (int j = 0; j < b.size(); j++) {
        b[j] = step;
    }
}

edited Jul 01 '16 at 10:38

answered Jul 01 '16 at 10:14

LogicStuff

19,397
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Yes, what I need is to change values to the passed vector. Why it works with reference and not with pointer? In both case I'm passing the address of the object :O – markzzz Jul 01 '16 at 10:36
@paizza: _"In both case I'm passing the address of the object"_ No. Think about it: if you would use pointers and references in exactly the same way, why would both things exist? :) – Lightness Races in Orbit Jul 01 '16 at 10:37
Yep, thats the question :) What's the real difference? – markzzz Jul 01 '16 at 12:38
@paizza Because you need to explicitly dereference the pointer? – LogicStuff Jul 01 '16 at 15:14

score 1 · Answer 2 · answered Jul 01 '16 at 10:15

1

In order to access each element of the vector through pointer to that vector you have to first get the vector value by dereferencing the pointer to it, and then you can use operator [] on that vector to access its elements:

(*b)[i] = step;

answered Jul 01 '16 at 10:15

Ganil

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Is not operator [] supposed to deference?

2 Answers2