Manage a resource with unique_ptr

Description: Make a class Resource whose constructor and destructor print a message. Create it with std::make_unique, move it into a function which takes ownership, and see when it is destroyed.

Hint:

• A unique_ptr can not be copied, only moved with std::move.

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Solution

#include <iostream>
#include <memory>

struct Resource {
    Resource()  { std::cout << "created\n"; }
    ~Resource() { std::cout << "destroyed\n"; }
};

void consume(std::unique_ptr<Resource> r) {
    std::cout << "consume has the resource\n";
}   // r goes out of scope here: the resource is destroyed

int main() {
    auto r = std::make_unique<Resource>();
    consume(std::move(r));
    std::cout << "after consume, empty: " << (r == nullptr) << '\n';
}

How it works

• std::move gives the ownership to the function. After that, r in main is empty.
• The destructor ran at the end of consume, with no delete in our code.

Read Smart Pointers.

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