Section
std::unique_ptr
std::unique_ptr is a smart pointer that owns (is responsible for) and manages another object via a pointer and subsequently disposes of that object when the unique_ptr goes out of scope.
# Declarations
template<
class T,
class Deleter = std::default_delete<T>
> class unique_ptr;
(since C++11)
template <
class T,
class Deleter
> class unique_ptr<T[], Deleter>;
(since C++11)
# Notes
Only non-const unique_ptr can transfer the ownership of the managed object to another unique_ptr. If an object’s lifetime is managed by a const std::unique_ptr, it is limited to the scope in which the pointer was created.
std::unique_ptr is commonly used to manage the lifetime of objects, including:
std::unique_ptr may be constructed for an incomplete type T, such as to facilitate the use as a handle in the pImpl idiom. If the default deleter is used, T must be complete at the point in code where the deleter is invoked, which happens in the destructor, move assignment operator, and reset member function of std::unique_ptr. (In contrast, std::shared_ptr can’t be constructed from a raw pointer to incomplete type, but can be destroyed where T is incomplete). Note that if T is a class template specialization, use of unique_ptr as an operand, e.g. !p requires T’s parameters to be complete due to ADL.
If T is a derived class of some base B, then std::unique_ptr
Unlike std::shared_ptr, std::unique_ptr may manage an object through any custom handle type that satisfies NullablePointer. This allows, for example, managing objects located in shared memory, by supplying a Deleter that defines typedef boost::offset_ptr pointer; or another fancy pointer.
# Example
#include <cassert>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <locale>
#include <memory>
#include <stdexcept>
// helper class for runtime polymorphism demo below
struct B
{
virtual ~B() = default;
virtual void bar() { std::cout << "B::bar\n"; }
};
struct D : B
{
D() { std::cout << "D::D\n"; }
~D() { std::cout << "D::~D\n"; }
void bar() override { std::cout << "D::bar\n"; }
};
// a function consuming a unique_ptr can take it by value or by rvalue reference
std::unique_ptr<D> pass_through(std::unique_ptr<D> p)
{
p->bar();
return p;
}
// helper function for the custom deleter demo below
void close_file(std::FILE* fp)
{
std::fclose(fp);
}
// unique_ptr-based linked list demo
struct List
{
struct Node
{
int data;
std::unique_ptr<Node> next;
};
std::unique_ptr<Node> head;
~List()
{
// destroy list nodes sequentially in a loop, the default destructor
// would have invoked its `next`'s destructor recursively, which would
// cause stack overflow for sufficiently large lists.
while (head)
{
auto next = std::move(head->next);
head = std::move(next);
}
}
void push(int data)
{
head = std::unique_ptr<Node>(new Node{data, std::move(head)});
}
};
int main()
{
std::cout << "1) Unique ownership semantics demo\n";
{
// Create a (uniquely owned) resource
std::unique_ptr<D> p = std::make_unique<D>();
// Transfer ownership to `pass_through`,
// which in turn transfers ownership back through the return value
std::unique_ptr<D> q = pass_through(std::move(p));
// p is now in a moved-from 'empty' state, equal to nullptr
assert(!p);
}
std::cout << "\n" "2) Runtime polymorphism demo\n";
{
// Create a derived resource and point to it via base type
std::unique_ptr<B> p = std::make_unique<D>();
// Dynamic dispatch works as expected
p->bar();
}
std::cout << "\n" "3) Custom deleter demo\n";
std::ofstream("demo.txt") << 'x'; // prepare the file to read
{
using unique_file_t = std::unique_ptr<std::FILE, decltype(&close_file)>;
unique_file_t fp(std::fopen("demo.txt", "r"), &close_file);
if (fp)
std::cout << char(std::fgetc(fp.get())) << '\n';
} // `close_file()` called here (if `fp` is not null)
std::cout << "\n" "4) Custom lambda-expression deleter and exception safety demo\n";
try
{
std::unique_ptr<D, void(*)(D*)> p(new D, [](D* ptr)
{
std::cout << "destroying from a custom deleter...\n";
delete ptr;
});
throw std::runtime_error(""); // `p` would leak here if it were a plain pointer
}
catch (const std::exception&)
{
std::cout << "Caught exception\n";
}
std::cout << "\n" "5) Array form of unique_ptr demo\n";
{
std::unique_ptr<D[]> p(new D[3]);
} // `D::~D()` is called 3 times
std::cout << "\n" "6) Linked list demo\n";
{
List wall;
const int enough{1'000'000};
for (int beer = 0; beer != enough; ++beer)
wall.push(beer);
std::cout.imbue(std::locale("en_US.UTF-8"));
std::cout << enough << " bottles of beer on the wall...\n";
} // destroys all the beers
}