Operator += applied to std::valarray<int*> - c++

Question

Here is the example:
#include <iostream>
#include <string>
#include <valarray>
int main()
{
std::valarray<std::string> vs(2);
// vs[0] += "hello"; // works
// vs[1] += "hello"; // works
vs += "hello"; // works
std::cout << vs[0] << std::endl;
std::cout << vs[1] << std::endl;
std::valarray<int*> vi(2);
vi[0] = new int[2];
vi[0][0] = 0;
vi[0][1] = 1;
vi[1] = new int[2];
vi[1][0] = 2;
vi[1][1] = 3;
std::cout << vi[0][0] << std::endl;
std::cout << vi[1][0] << std::endl;
// vi[0] += 1; // works
// vi[1] += 1; // works
vi += 1; // error: invalid operands of types 'int*' and 'int*' to binary 'operator+'
std::cout << vi[0][0] << std::endl;
std::cout << vi[1][0] << std::endl;
}
I don't understand this error, if someone may explain this to me.
Is there a workaround?
Best regards,

Answer 1

std::valarray doesn't have overloads for heterogeneous binary operations, but it does have a catch-all for other functions, apply.
vi.apply([](int * p){ return p + 1; });

Answer 2

You can see operator+ overloads for valarray here. As you can see no overload is suitable for what you're trying to do (add an int and a pointer).

Answer 3

My last dabble in C++ has been a while, so please excuse any slipups in terms of terminology/details.
What your error boils down to is that += on a valarray attempts to perform the operation on each element of the valarray. And by default (if my memory doesn't deceive me), there's no + operation for integer pointers, which are stored in your valarray. You'd need to specify an overridden + operator for int pointers first.
Also, for an explanation of why the other operations work and vi += 1 doesn't:
vi[0] += 1; // works
vi[1] += 1; // works
These work, because (and I don't know if that's what you intended to do) you've placed integers in these vararray fields (new int(n) will create an int with a value of n), each with a value of 2. So vi[0] is 2, as is vi[0]. You could as well have written
vi[0] = 2;
vi[1] = 2;
I'm assuming you were trying to instead have an integer array stored in vi[0]/vi[1], which would've been
vi[0] = new int[2];
vi[1] = new int[2];
The problem is that per documentation, vi[0][0] = 0, etc... simply prompt the valarray to create these fields if they don't exist already, so of course
std::cout << vi[0][0] << std::endl;
std::cout << vi[1][0] << std::endl;
will work.