C++ multiple unique pointers from same raw pointer

Question

Consider my code below. My understanding of unique pointers was that only one unique pointer can be used to reference one variable or object. In my code I have more than one unique_ptr accessing the same variable.

It's obviously not the correct way to use smart pointers i know, in that the pointer should have complete ownership from creation. But still, why is this valid and not having a compilation error? Thanks.

#include <iostream>
#include <memory>

using namespace std;

int main()
{
    int val = 0;
    int* valPtr = &val;

    unique_ptr <int> uniquePtr1(valPtr);
    unique_ptr <int> uniquePtr2(valPtr);

    *uniquePtr1 = 10;
    *uniquePtr2 = 20;

    return 0;
}

Answer 1

But still, why is this valid

It is not valid! It's undefined behaviour, because the destructor of std::unique_ptr will free an object with automatic storage duration.

Practically, your program tries to destroy the int object three times. First through uniquePtr2, then through uniquePtr1, and then through val itself.

and not having a compilation error?

Because such errors are not generally detectable at compile time:

unique_ptr <int> uniquePtr1(valPtr);
unique_ptr <int> uniquePtr2(function_with_runtime_input());

In this example, function_with_runtime_input() may perform a lot of complicated runtime operations which eventually return a pointer to the same object valPtr points to.

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If you use std::unique_ptr correctly, then you will almost always use std::make_unique, which prevents such errors.