Smart pointers: std::unique_ptr

Worked example

Read it before you run it

#include <memory>
// Header for unique_ptr, shared_ptr and weak_ptr.
struct MotorDriver { void start() {} };
// A stand-in driver class.
auto driver = std::make_unique<MotorDriver>();
// Create the driver on the heap with one owner; no new or delete.
driver->start();
// -> reaches the object, exactly like a raw pointer.
auto other = std::move(driver);
// Transfer ownership; driver is now empty (nullptr).
if (!driver) std::cout << "moved out\n";
// A moved-from unique_ptr tests false.
std::unique_ptr<MotorDriver> make() { return std::make_unique<MotorDriver>(); }
// A factory that returns ownership to the caller.
auto fresh = make();
// The returned pointer is moved into fresh automatically.
// other and fresh are deleted automatically at the end of the scope.
// No delete anywhere: RAII does the cleanup.

Practice code

Now type it yourself

Copy the starter into practice.cpp and replace every TODO with your own code. Build and run it: g++ -std=c++17 -Wall -Wextra -Wpedantic practice.cpp -o practice -pthread && ./practice

#include <iostream>
#include <memory>

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

int main() {
  // TODO: make a unique_ptr<Driver> named a with make_unique, then move it into b
  std::cout << (a ? "a owns it" : "a empty") << '\n';
  return 0;
}

created
a empty
destroyed

#include <iostream>
#include <memory>

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

int main() {
  auto a = std::make_unique<Driver>();
  auto b = std::move(a);
  std::cout << (a ? "a owns it" : "a empty") << '\n';
  return 0;
}