How to use std::unique_ptr

Unique pointers are just that. They cannot be shared. If you try to share them you will find that an assign happens instead and the ownership of the pointer moves, leaving the old unique pointer looking at nullptr.

To create a unique pointer you can either use the following which is a bit long winded

    std::unique_ptr<class Foo> uptr(new Foo("foo2"));

Or, the following which hides the use of "new" but also invokes the copy constructor. If you don't want that, use the above.

    auto uptr = std::make_unique< class Foo >(Foo("foo1"));

Note: To change the ownership of a unique pointer, use std::move

Example:

#include <iostream>
#include <memory>
#include <sstream>

class Foo
{
private:
  std::string data; // local data for foo for debugging
public:
  Foo(std::string data) : data(data) { std::cout << "new " << to_string() << std::endl; }
  Foo(const Foo &other)
  {
    std::cout << "copy constructor " << to_string() << std::endl;
    data = other.data;
  }
  ~Foo()
  {
    // Note, other.reset will be called for us
    std::cout << "delete " << to_string() << std::endl;
  }
  std::string to_string(void)
  {
    auto              address = static_cast< const void              *>(this);
    std::stringstream ss;
    ss << address;
    return "Foo(" + ss.str() + ", data=" + data + ")";
  }
};

int main(void)
{
  // NOTE: make_unique creates a new ptr and will invoke foo1's copy constructor:
  auto uptr1 = std::make_unique< class Foo >(Foo("foo1"));

  // NOTE: to avoid the copy, do this:
  std::unique_ptr< class Foo > uptr2(new Foo("foo2"));

  // As you cannot copy unique pointers, reassign it with move
  std::unique_ptr< class Foo > uptr3;
  uptr3 = std::move(uptr2);

  // Let's print all the unique ptrs now
  std::cout << "uptr1 = " << (uptr1.get() ? uptr1->to_string() : "nullptr") << std::endl;
  std::cout << "uptr2 = " << (uptr2.get() ? uptr2->to_string() : "nullptr") << std::endl;
  std::cout << "uptr3 = " << (uptr3.get() ? uptr3->to_string() : "nullptr") << std::endl;

  // Expect the unique ptr data to be destroyed now
}

To build:

cd std_unique_ptr
rm -f *.o example
clang -std=c++2a -Werror -g -O3 -fstack-protector-all -ggdb3 -Wall -c -o main.o main.cpp
clang  main.o -lstdc++  -o example
./example

Expected output:

�[31;1;4mNOTE: make_unique creates a new ptr and will invoke foo1's copy constructor:�[0m
new Foo(0x7fffce777658, data=foo1)
copy constructor Foo(0x562014a83ec0, data=)
delete Foo(0x7fffce777658, data=foo1)

�[31;1;4mNOTE: to avoid the copy, do this:�[0m
new Foo(0x562014a83ef0, data=foo2)

�[31;1;4mAs you cannot copy unique pointers, reassign it with move�[0m

�[31;1;4mLet's print all the unique ptrs now�[0m
uptr1 = Foo(0x562014a83ec0, data=foo1)
uptr2 = nullptr
uptr3 = Foo(0x562014a83ef0, data=foo2)

�[31;1;4mExpect the unique ptr data to be destroyed now�[0m
delete Foo(0x562014a83ef0, data=foo2)
delete Foo(0x562014a83ec0, data=foo1)