将数组分配给std :: vector会给出错误的输出

Question

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

int p[5] = {1,2,3,4,5};
vector<int> x(p[0], p[4]);

class Polynomial {    
};

int main(){
    Polynomial Poly;

    unsigned int i;

    for(i = 0;i<=4; i++)
        cout<< x[i]<<endl;

    return 0;


}

The above code outputs:

5
-1073741824
0
-1073741824
-2018993448

Whereas I expect it to output

1
2
3
4
5

Why is the output incorrect, and what am I doing wrong?

Answer 1

The access x[i] is out of bounds when i is greater than 0, since x is a vector containing one (= p[0] ) element of value 5 (= p[4] ).

This is because vector<int> v(a, b) creates a vector with a elements with value b each.

To achieve what you want ( x being the same as p ), you need:

#include <iterator>

vector<int> x(std::begin(p), std::end(p))

Or, guessing the size yourself:

vector<int> x(p, p + 5);

Or perhaps somewhat C-like:

vector<int> x(p, p + sizeof(p)/sizeof *p);

Answer 2

vector<int> x(p[0], p[4]); doesn't do what you expect. It constructs a vector with p[0] elements of value p[4] .

This overload of std::vector::vector will be called here.

explicit vector( size_type count,
                 const T& value,
                 const Allocator& alloc = Allocator());

If you try cout << x.size(); you'll find the size of vector is 1 , cout << x[i] when i > 0 is out of bound and leads to UB.

You might want to call this overload:

template< class InputIt >
vector( InputIt first, InputIt last,
        const Allocator& alloc = Allocator() );

Then change it to vector<int> x(&p[0], &p[4]); . Note this will constructor vector with 4 elements. vector<int> x(&p[0], &p[5]); will constructor vector with all the 5 elements from p .