std::shared_ptr

Spirit: Not Unique, But Shared (i.e. Reference Counted)

Ownership is not always clear …

  • Rare occasions where shared ownership is the right design choice

  • … laziness, mostly

  • If in doubt, say std::shared_ptr

#include <memory>
std::shared_ptr<MyClass> ptr(
    new MyClass(666));
../../../../../_images/sharedptr.svg

Methods

Object of type std::shared_ptr behave like pointers in any respect (->, *, copy, …), except that they have methods:

Method

Description

std::shared_ptr()

Initializes to nullptr

std::shared_ptr(T* pointer)

Initializes to pointer (but see std::make_shared instead); refcount is 1

std::shared_ptr(std::shared_ptr&& from)

Move constructor; from is empty afterwards (see Move Semantics, Rvalue References, Perfect Forwarding)

reset(T* pointer)

Replaces the managed object if any, possibly deleting it if refcount reaches zero. pointer can be nullptr

get()

Pointer to currently managed object

std::shared_ptr: Copy

Copying is what shared pointer are there for

shared_ptr<MyClass> ptr(
    new MyClass(666));
shared_ptr<MyClass> copy1 = ptr;
shared_ptr<MyClass> copy2 = copy1;
../../../../../_images/sharedptr-copy.svg

std::shared_ptr vs. std::unique_ptr

How do std::shared_ptr and std::unique_ptr compare?

  • std::unique_ptr

    • Small - size of a pointer

    • Operations compile away entirely

    • No excuse not to use it

    • Have to think more though

  • std::shared_ptr

    • Size of two pointers

    • Copying manipulates the resource count

    • Copying manipulates non-adjacent memory locations

    • Usage is very easy (no std::move and such)

    Attention

    • Cyclic references possible!

    • No garbage collection as in Java

    • ⟶ Leak!!

    See below …

std::shared_ptr: Object Lifetime

How long does the pointed-to object live?

  • Reference count is used to track share ownership

  • When reference count drops to zero, the object is not referenced anymore

  • ⟶ deleted

Examining the reference count

shared_ptr<MyClass> ptr(new MyClass(666));
auto refcount = ptr->use_count();

Attention

Do not make any decisions based on it - at least not when the pointer is shared among multiple threads!

std::shared_ptr: Juggling

Clearing: reset()

shared_ptr<MyClass> ptr(
    new MyClass(666));
auto copy = ptr;
ptr.reset();
  • Decrements reference count

  • Only if it becomes zero, object is deleted

Filling: reset()

shared_ptr<MyClass> ptr;
ptr.reset(new MyClass(666));
  • Makes an empty pointer the initial reference

Demo: Basic Usage

#include "sensors.h"

#include <gtest/gtest.h>
#include <memory>

TEST(shared_ptr_suite, basic)
{
    std::map<std::string, std::shared_ptr<Sensor>> sensors;
    
    std::shared_ptr<Sensor> rs1{new RandomSensor{20, 40}};
    std::shared_ptr<Sensor> rs2{new RandomSensor{10, 30}};
    std::shared_ptr<Sensor> cs{new ConstantSensor{36.5}};

    auto temp = rs1->get_temperature();
    ASSERT_TRUE(temp>=20 && temp<=40);

    ASSERT_EQ(rs1.use_count(), 1);

    sensors["rand1"] = rs1;
    sensors["rand2"] = rs2;
    sensors["const"] = cs;

    ASSERT_EQ(rs1.use_count(), 2);

    // shared ownership: cann access sensors from map *and* from
    // outside references
    ASSERT_DOUBLE_EQ(cs->get_temperature(), 36.5);
    ASSERT_DOUBLE_EQ(sensors["const"]->get_temperature(), 36.5);
}

Demo: std::make_shared

#include "sensors.h"

#include <gtest/gtest.h>
#include <memory>

TEST(shared_ptr_suite, basic_make_shared)
{
    std::map<std::string, std::shared_ptr<Sensor>> sensors;
    
    auto rs1 = std::make_shared<RandomSensor>(20, 40);
    auto rs2 = std::make_shared<RandomSensor>(10, 30);
    auto cs = std::make_shared<ConstantSensor>(36.5);

    auto temp = rs1->get_temperature();
    ASSERT_TRUE(temp>=20 && temp<=40);

    sensors["rand1"] = rs1;
    sensors["rand2"] = rs2;
    sensors["const"] = cs;

    // shared ownership: cann access sensors from map *and* from
    // outside references
    ASSERT_DOUBLE_EQ(cs->get_temperature(), 36.5);
    ASSERT_DOUBLE_EQ(sensors["const"]->get_temperature(), 36.5);
}

Demo: Cyclic References

#include "sensors.h"

#include <gtest/gtest.h>
#include <memory>

class NonSense
{
public:
    void set_reference(std::shared_ptr<NonSense> ref)
    {
        _ref = ref;
    }

private: 
    std::shared_ptr<NonSense> _ref;
};

TEST(shared_ptr_suite, cyclic_reference)
{
    std::shared_ptr<NonSense> cycle(new NonSense);
    cycle->set_reference(cycle);
}
$ valgrind ./c++11-smartptr --gtest_filter=shared_ptr_suite.cyclic_reference
==303549== Memcheck, a memory error detector
==303549== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==303549== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==303549== Command: ./c++11-smartptr --gtest_filter=shared_ptr_suite.cyclic_reference
==303549==
Running main() from /home/jfasch/work/jfasch-home/googletest/googletest/src/gtest_main.cc
Note: Google Test filter = shared_ptr_suite.cyclic_reference
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from shared_ptr_suite
[ RUN      ] shared_ptr_suite.cyclic_reference
[       OK ] shared_ptr_suite.cyclic_reference (8 ms)
[----------] 1 test from shared_ptr_suite (13 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (38 ms total)
[  PASSED  ] 1 test.
==303549==
==303549== HEAP SUMMARY:
==303549==     in use at exit: 40 bytes in 2 blocks
==303549==   total heap usage: 515 allocs, 513 frees, 134,791 bytes allocated
==303549==
==303549== LEAK SUMMARY:
==303549==    definitely lost: 16 bytes in 1 blocks
==303549==    indirectly lost: 24 bytes in 1 blocks
==303549==      possibly lost: 0 bytes in 0 blocks
==303549==    still reachable: 0 bytes in 0 blocks
==303549==         suppressed: 0 bytes in 0 blocks
==303549== Rerun with --leak-check=full to see details of leaked memory
==303549==
==303549== For lists of detected and suppressed errors, rerun with: -s
==303549== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)