Why can't we use square brackets in case of dynamic vectors?-CodePudding

#include <iostream>
using namespace std;
#include <vector>
#include <queue>

int main(){
   vector<int> *v = new vector<int>;
    v -> push_back(1);
    
    //min priority queue
    priority_queue<int, vector<int>, greater<int>> pq;

     pq.push(v[0]); //Able to do pq.push(v -> at(0))
}

So why is this giving an error? Am i not pushing an integer inside my priority queue?

CodePudding user response：

A std vector already manages its buffer dynamically.

In 99.9% of cases, new std::vector is a mistake.

vector<int> v;
v . push_back(1);

//min priority queue
priority_queue<int, vector<int>, greater<int>> pq;

 pq.push(v[0]);

this works.

If you are really in the 0.1% of cases where new on a vector makes sense, change v[0] to (*v)[0].

What v[0] does is treat the pointer v as a pointer to an array of vectors. It then picks the 0th one. It does not pick the 0th element of the 0th one. v[0][0] would also work, but would be dumb.

So C inherits an array pointer duality from C. When you have a pointer, you can use square brackets to treat it as an array as if it was the pointer to the first element of the array.

C also has objects, and those objects can overload operators like square brackets. The overloading happens on the objects not on pointers to those objects.

So vector[] fakes being an array and finds the elements owned by the vector. ptr_to_vector[] insteat treats it as an array of vectors starting at *ptr_to_vector, almost never what you want.

C is relatively unique in having full fledged objects that can be values. Few languages do. This can be confusing if you are coming from other languages. Embrance value semantics in C when you can.

CodePudding user response：

This will fix your compile

pq.push((*v)[0]);

Since v is pointer to a vector<int>.

In this case pq.push will accept only an int as input. v[0] will return a vector<int> object. If you really want to use [] index operator you can go with pq.push(v[0][0])

Reworked code:

#include <iostream>
using namespace std;
#include <vector>
#include <queue>

int main(){
    vector<int> *v = new vector<int>;
    v -> push_back(1);
    
    //min priority queue
    priority_queue<int, vector<int>, greater<int>> pq;

    pq.push((*v)[0]);
    delete v; // don't forget.
}