Why can I not assign values to a shared pointer array? - c++

Question

This small example is giving the error message
Error C2440 '=': cannot convert from '_Ux (*const )' to 'int *' Templ 1266
Error C3692 non-scalar type 'int []' cannot be used in a pseudo-destructor expression
What is _Ux(*const)?
This is the program:
#include <memory>
int main()
{
shared_ptr<int[]> as = make_shared<int[]>(10);
for (int i = 0; i < 10; i++) {
as[i] = i + 100;
}
}

Answer 1

The code as shown does not compile, because shared_ptr and make_shared() are in the std namespace, but there is no using namespace std; statement, or at least using std::shared_ptr; and using std::make_shared; statements.
However, fixing that, make sure you are compiling this code for C++20, as std::make_shared() does not support the creation of arrays in earlier versions, which can lead to the error you are seeing.
In C++17, you will have to construct the array manually instead, but std::shared_ptr will free the array correctly, eg:
std::shared_ptr<int[]> as( new int[10] );
Live Demo
But, std::shared_ptr does not support arrays at all in C++11 and C++14, so you will have to use a custom deleter to free the array properly, and use std::shared_ptr::get() instead of std::shared_ptr::operator[] to access the array elements, eg:
#include <memory>
int main()
{
std::shared_ptr<int> as( new int[10], [](int *p) { delete[] p; } );
for (int i = 0; i < 10; i++) {
as.get()[i] = i + 100;
}
}
For dynamic arrays, you should consider using std::vector instead, eg:
#include <vector>
int main()
{
std::vector<int> as(10);
for (int i = 0; i < 10; i++) {
as[i] = i + 100;
}
}