Smart pointers

Use case

Automatic memory management for parsed binary structures.

Explanation

unique_ptr has exclusive ownership (cannot copy, only move). shared_ptr allows multiple owners with reference counting. A shared_ptr holds the managed object and a reference counter. Accessing the reference counter is thread safe but manipulating the managed object is not thread-safe. Both auto-delete when the last owner goes out of scope (no manual delete needed).

Code

#include <cstdint>
#include <iostream>
#include <memory>

struct LoadCommand {
  uint32_t cmd;
  LoadCommand(uint32_t c) : cmd(c) { std::cout << "  Created.\n"; }
  ~LoadCommand() { std::cout << "  Destroyed.\n"; }
};

int main() {
  std::cout << "unique_ptr\n";
  {
    // https://en.wikipedia.org/wiki/Mach-O
    std::unique_ptr<LoadCommand> lc(new LoadCommand(0x19));
    // function "std::__1::unique_ptr<_Tp, _Dp>::unique_ptr(const std::__1::unique_ptr<LoadCommand, std::__1::default_delete<LoadCommand>> &) [with _Tp=LoadCommand, _Dp=std::__1::default_delete<LoadCommand>]" (declared implicitly) cannot be referenced -- it is a deleted function
    // auto copy = lc;
    auto moved = std::move(lc);
  }

  std::cout << "shared_ptr\n";
  {
    // https://en.wikipedia.org/wiki/Mach-O
    auto a = std::make_shared<LoadCommand>(0x02);
    auto b = a;
    std::cout << "  Owners: " << a.use_count() << "\n";
  }

  return 0;
}

View on GitHub.

Output

$ ./src/c++11/build/smart-pointers
unique_ptr
  Created.
  Destroyed.
shared_ptr
  Created.
  Owners: 2
  Destroyed.