Why Can't I Directly Push a Unique Pointer into a Vector in C ?

Understanding the Issue: Inserting Unique Pointers into Vectors

The provided code attempts to push a unique pointer into a vector. However, the error message indicates the reason behind the compilation failure:

error: deleted function 'std::unique_ptr<int>::unique_ptr(const std::unique_ptr<int>&)'

This error arises because it is not permissible to copy unique pointers.

The Nature of Unique Pointers

A unique pointer, as its name suggests, establishes exclusive ownership of a dynamic memory location. It guarantees that there will be only one unique pointer container with ownership of the held pointer. Consequentially, creating a copy of a unique pointer becomes impermissible. Copying would result in two unique pointers claiming ownership of the same dynamic memory, which violates the inherent rule of unique ownership.

Instead, when one needs to transfer ownership of a unique pointer to another container or function, the move semantics of C come into play.

Resolving the Issue: Utilizing Move Semantics

The solution to the compilation error and the problem of pushing unique pointers into vectors lies in leveraging move semantics. Move semantics, implemented using the std::move function, transfer the ownership of the unique pointer without creating a copy.

In this scenario, the corrected code snippet would be:

int main()
{
    std::vector<std::unique_ptr<int>> vec;

    int x(1);
    std::unique_ptr<int> ptr2x(new int(x)); // Allocate the object dynamically
    vec.push_back(std::move(ptr2x)); // Transfer ownership with move semantics

    return 0;
}

By using std::move, the ownership of ptr2x is transferred to vec, and ptr2x is left with a nullptr.

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