Step 3 of 5

Returning ownership

A function that creates an object on the heap should return a unique_ptr. Then the return type itself documents "you own this now", and the caller can't forget to delete it. In old C and C++ code, the same function would return a raw pointer and a comment saying "caller must free", which people forget.

A unique_ptr can also be empty. Returning nullptr is a clean way to say "I couldn't make one".

#include <iostream>
#include <memory>
#include <string>

struct Shape {
    std::string kind;
    double area;
};

std::unique_ptr<Shape> make_shape(const std::string& kind, double size) {
    if (kind == "square") return std::make_unique<Shape>(Shape{kind, size * size});
    if (kind == "circle") return std::make_unique<Shape>(Shape{kind, 3.14 * size * size});
    return nullptr;                           // unknown kind: nothing to own
}

int main() {
    for (const char* k : {"square", "hexagon", "circle"}) {
        auto s = make_shape(k, 2);
        if (s) {
            std::cout << s->kind << " " << s->area << "\n";
        } else {
            std::cout << "can't make a " << k << "\n";
        }
    }
}

square 4
can't make a hexagon
circle 12.56

How it works

• Returning a unique_ptr by value moves it to the caller automatically. No std::move needed on return.
• return nullptr; converts to an empty unique_ptr.
• The caller must check if (s) before using s->.... Dereferencing an empty unique_ptr is undefined behavior, just like a null raw pointer.

Building the string

Your function returns a pointer to a new std::string. std::make_unique<std::string>("bomb " + site) constructs it from the concatenated text.

Your turn: write std::unique_ptr<std::string> make_callout(const std::string& site) that returns a new string "bomb " + site for sites A and B, and nullptr for anything else.

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