C ++数组和指针

Question

I was wondering if someone could clear this up for me. It concerns pointers and arrays.

double wages[3] = {10000.0, 20000.0, 30000.0};
double * pw = wages;

In the above example one can access an element in the array the following two ways:

wages[1] or 
*(wages+ 1)

Then I stumbled upon another piece of code:

void fill(std::array<double, Seasons> * pa)
{   
    using namespace std;
    for (int i = 0; i < Seasons; i++)
    {
        cout << "Enter " << Snames[i] << " expenses: ";
        cin >> (*pa)[i];
    }
}

Why can't we write pa[i] since pa is a pointer. Isn't it the same as the example above?

Answer 1

pa is a pointer to an object, the type of which is std::array<double, Seasons> .

pa[i] will not get you the i'th double in the std::array , it will attempt to access another std::array in memory that wasn't allocated for it, and will result in undefined behavior.

(*pa) results in a reference to a std::array object, which implements operator[] .
(*pa)[i] calls operator[] of std::array on the aforementioned object.

Answer 2

Why can't we write pa[i] since pa is a pointer.

pa is pointer to array. So pa[i] does not dereference the i th element of the current array as you would expect - that syntax works as if pa is array of pointers instead.

(*pa)[i] dereference the i th element of the array pointed by pa .

Answer 3

您也可以使用pa-> at（i）而不是pa.at（i）来访问该元素，因为pa是指针。