【例子驱动学习】shared_ptr

用例子来学习shared_ptr
写这个是为了熟悉shared_ptr的接口，注意它与其他智能指针的区别。本测试主要来自cplusplus。

代码

/* * --------------------------------------------------- *  Copyright (c) 2017 josan All rights reserved. * --------------------------------------------------- * *               创建者： Josan *             创建时间： 2017/9/20 10:26:42 */#define _CRTDBG_MAP_ALLOC#include <cstdlib>#include <crtdbg.h>#include <memory>   //shared_ptr#include <iostream>#include <set>using namespace std;//---------------测试shared_ptr的各种成员函数  测试用例来自cplusplus.com-------struct C{    int* data;};static void testCtor(){    //default constructor     std::shared_ptr<int> p1;    std::shared_ptr<int> p2(nullptr);    //ctor from null pointer     std::shared_ptr<int> p3(new int);    //ctor pointer + deleter     std::shared_ptr<int> p4(new int, std::default_delete<int>());    //ctor from pointer + deleter + allocator     std::shared_ptr<int> p5(new int, [](int* p) {delete p; }, std::allocator<int>());    //copy constructors      /*     * shared_ptr (const shared_ptr& x) noexcept;     * template <class U> shared_ptr (const shared_ptr<U>& x) noexcept;   //注意，这是（左）引用，区分unique_ptr     */    std::shared_ptr<int> p6(p5);     //此时，p6.use_count()  == 2    //move constructors  p6失去指针的所有权    std::shared_ptr<int> p7(std::move(p6));    //move from other types of managed pointers    也可以是auto_ptr  注意这里面的传参是右值引用    std::shared_ptr<int> p8(std::unique_ptr<int>(new int));    //aliasing ctor    /*     * template <class U> shared_ptr (const shared_ptr<U>& x, element_type* p) noexcept;     */    std::shared_ptr<C> obj(new C);    std::shared_ptr<int> p9(obj, obj->data);    //-----output------    std::cout << "use_count:\n";    std::cout << "p1: " << p1.use_count() << '\n';  //0    std::cout << "p2: " << p2.use_count() << '\n';  //0    std::cout << "p3: " << p3.use_count() << '\n';  //1    std::cout << "p4: " << p4.use_count() << '\n';  //1    std::cout << "p5: " << p5.use_count() << '\n';  //2    std::cout << "p6: " << p6.use_count() << '\n';  //0    std::cout << "p7: " << p7.use_count() << '\n';  //2    std::cout << "p8: " << p8.use_count() << '\n';  //1    std::cout << "p9: " << p9.use_count() << '\n';  //2}static void testCtorWithMyDeleter(){    auto deleter = [](int*p) {        std::cout << "[deleter called]\n"; delete p;    };    std::shared_ptr<int> foo(new int, deleter);    std::cout << "use_count: " << foo.use_count() << '\n';}static void testAssignment(){    std::shared_ptr<int> foo;    std::shared_ptr<int> bar(new int(10));    foo = bar;                          // copy    bar = std::make_shared<int>(20);   // move    std::unique_ptr<int> unique(new int(30));    foo = std::move(unique);            // move from unique_ptr    std::cout << "*foo: " << *foo << '\n';    std::cout << "*bar: " << *bar << '\n';}//element_type* get() const noexcept;static void testGet(){    int* p = new int(10);    std::shared_ptr<int> a(p);    if(a.get() == p)        std::cout << "a and p point to the same location\n";    // three ways of accessing the same address:     std::cout << *a.get() << "\n";  //10    std::cout << *a << "\n";  //10    std::cout << *p << "\n";  //10}/* * (1) void reset() noexcept; * (2) template <class U> void reset (U* p); * (3) template <class U, class D> void reset (U* p, D del); * (4) template <class U, class D, class Alloc> void reset (U* p, D del, Alloc alloc); */static void testReset(){    auto deleter = [](int*p) {        std::cout << "[deleter called]\n"; delete p;    };    std::shared_ptr<int> sp(nullptr, deleter);  // empty    //同时将sp的deleter设置为默认的删除器    sp.reset(new int);       // takes ownership of pointer    *sp = 10;    std::cout << *sp << '\n';  //10    sp.reset(new int);       // deletes managed object, acquires new pointer    *sp = 20;    std::cout << *sp << '\n';  //20    sp.reset();               // deletes managed object}//void swap (shared_ptr& x) noexcept;static void testSwap(){    std::shared_ptr<int> foo(new int(10));    std::shared_ptr<int> bar(new int(20));    foo.swap(bar);    std::cout << "*foo: " << *foo << '\n';  //20    std::cout << "*bar: " << *bar << '\n';  //10}//long int use_count() const noexcept;/* * Returns the number of shared_ptr objects that share ownership over the same pointer as this object (including it). * If this is an empty shared_ptr, the function returns zero. */static void testUse_count(){    std::shared_ptr<int> p1;    std::shared_ptr<int> p2(new int);    std::cout << "p1: " << p1.use_count() << '\n';  //0    std::cout << "p2: " << p2.use_count() << '\n';  //1}//bool unique() const noexcept;static void testUnique(){    std::shared_ptr<int> foo;    std::shared_ptr<int> bar(new int);    std::cout << "foo unique?\n" << std::boolalpha;    std::cout << "1: " << foo.unique() << '\n';  // false (empty)    foo = bar;    std::cout << "2: " << foo.unique() << '\n';  // false (shared with bar)    bar = nullptr;  //释放bar的拥有权    std::cout << "3: " << foo.unique() << '\n';  // true}/* template <class U> bool owner_before (const shared_ptr<U>& x) const; * template <class U> bool owner_before(const weak_ptr<U>& x) const; * Returns whether the object is considered to go before x following a strict weak owner-based order. *///NOTE:对于这个函数不理解！！！static void testOwner_before(){    int * p = new int(10);    std::shared_ptr<int> a(new int(20));    std::shared_ptr<int> b(a, p);  // alias constructor    std::cout << "comparing a and b...\n" << std::boolalpha;    //std::cout << "value-based: " << (!(a < b) && !(b < a)) << '\n';    cout << a.owner_before(b) << endl;    cout << b.owner_before(a) << endl;    std::cout << "owner-based: " << (!a.owner_before(b) && !b.owner_before(a)) << '\n';    std::shared_ptr<int> c(new int(20));    std::shared_ptr<int> d(c);  // copy constructor    cout << c.owner_before(d) << endl;    cout << d.owner_before(c) << endl;    delete p;}//测试成员函数static void testMemberFunction(){    ////测试成员函数get()   注意此时源地址未失效    //testGet();    //测试成员函数get_deleter()    //testGet_deleter();    ////测试reset(),        --注意没有release()，因为可能不只是一个占用指针，因此单个shared_ptr没有权利释放    //testReset();    ////测试swap    //testSwap();    ////测试use_count()    //testUse_count();    ////测试unique()    //testUnique();    //测试owner_before()    testOwner_before();    //其他符号重载函数operator*, ->, bool }static void testNonSwap(){    std::shared_ptr<int> foo(new int(10));    std::shared_ptr<int> bar(new int(20));    swap(foo, bar);    std::cout << "foo: " << *foo << '\n';    std::cout << "bar: " << *bar << '\n';}static void testNonRelationalOperators(){    std::shared_ptr<int> a, b, c, d;    a = std::make_shared<int>(10);    b = std::make_shared<int>(10);    c = b;    std::cout << "comparisons:\n" << std::boolalpha;    std::cout << "a == b: " << (a == b) << '\n';  //false    std::cout << "b == c: " << (b == c) << '\n';  //true    std::cout << "c == d: " << (c == d) << '\n';  //false    std::cout << "a != nullptr: " << (a != nullptr) << '\n';  //true    std::cout << "b != nullptr: " << (b != nullptr) << '\n';  //true    std::cout << "c != nullptr: " << (c != nullptr) << '\n';  //true    std::cout << "d != nullptr: " << (d != nullptr) << '\n';  //false}//测试make_shared<>()static void testNonMake_shared(){    std::shared_ptr<int> foo = std::make_shared<int>(10);    // same as:    std::shared_ptr<int> foo2(new int(10));    auto bar = std::make_shared<int>(20);    auto baz = std::make_shared<std::pair<int, int>>(30, 40);    std::cout << "*foo: " << *foo << '\n';    std::cout << "*bar: " << *bar << '\n';    std::cout << "*baz: " << baz->first << ' ' << baz->second << '\n';}//测试static_pointer_cast<>()struct A{    static const char* static_type;    const char* dynamic_type;    A()    {        dynamic_type = static_type;    }};struct B : A{    static const char* static_type;    B()    {        dynamic_type = static_type;    }};const char* A::static_type = "class A";const char* B::static_type = "class B";/* * template <class T, class U> * shared_ptr<T> static_pointer_cast (const shared_ptr<U>& sp) noexcept; */static void testNonStatic_pointer_cast(){    std::shared_ptr<A> foo;    std::shared_ptr<B> bar;    foo = std::make_shared<A>();    // statically from A* to B*.    // cast of potentially incomplete object, but ok as a static cast:    bar = std::static_pointer_cast<B>(foo);    std::cout << "foo's static  type: " << foo->static_type << '\n';  //class A    std::cout << "foo's dynamic type: " << foo->dynamic_type << '\n';  //class A    std::cout << "bar's static  type: " << bar->static_type << '\n';  //class B    std::cout << "bar's dynamic type: " << bar->dynamic_type << '\n';  //class A}/* * template <class T, class U> * shared_ptr<T> dynamic_pointer_cast (const shared_ptr<U>& sp) noexcept; */static void testNonDynamic_pointer_cast(){    std::shared_ptr<A> foo;    std::shared_ptr<B> bar;    bar = std::make_shared<B>();    //dynamically from B* to A*.    foo = std::dynamic_pointer_cast<A>(bar);    std::cout << "foo's static  type: " << foo->static_type << '\n';  //class A    std::cout << "foo's dynamic type: " << foo->dynamic_type << '\n';  //class B    std::cout << "bar's static  type: " << bar->static_type << '\n';  //class B    std::cout << "bar's dynamic type: " << bar->dynamic_type << '\n';  //class B}/* * template <class T, class U> * shared_ptr<T> const_pointer_cast (const shared_ptr<U>& sp) noexcept; */static void testNonConst_pointer_cast(){    std::shared_ptr<int> foo;    std::shared_ptr<const int> bar;    foo = std::make_shared<int>(10);    //const casted from int* to const int*.    bar = std::const_pointer_cast<const int>(foo);    std::shared_ptr<int> foo1;    foo1 = std::const_pointer_cast<int>(bar);    std::cout << "*bar: " << *bar << '\n';  //10    *foo = 20;    std::cout << "*bar: " << *bar << '\n';  //20    *foo1 = 30;    std::cout << "*foo: " << *foo << '\n';  //30    std::cout << "*bar: " << *bar << '\n';  //30    std::cout << "*foo1: " << *foo1 << '\n';  //30    std::cout << "use_count: " << foo.use_count() << ", " << bar.use_count() << ", " << foo1.use_count() << endl;  //3 3 3}struct D{    // a verbose array deleter:    void operator()(int* p)    {        std::cout << "[deleter called]\n";        delete[] p;    }};/* * template <class D, class T> * D* get_deleter (const shared_ptr<T>& sp) noexcept; */static void testNonGet_deleter(){    std::shared_ptr<int> foo(new int[10], D());    int * bar = new int[20];    // use foo's deleter to delete bar (which is unmanaged):    (*std::get_deleter<D, int>(foo))(bar);  //[deleter called]}/* * template <class Ptr> struct owner_less; * template <class T> struct owner_less<shared_ptr<T>>; * template <class T> struct owner_less<weak_ptr<T>>; */static void testNonOwner_less(){    int * p = new int(10);    std::shared_ptr<int> a(new int(20));    std::shared_ptr<int> b(a, p);  // alias constructor: co-owns a, points to p                                   // standard set container: cannot contain duplicates.    std::set < std::shared_ptr<int> > value_based;  // uses std::less    std::set < std::shared_ptr<int>, std::owner_less<std::shared_ptr<int>> > owner_based;    value_based.insert(a);    value_based.insert(b);  // ok, different value    owner_based.insert(a);    owner_based.insert(b);  // overwrites (same owned pointer)    std::cout << "value_based.size() is " << value_based.size() << '\n';    std::cout << "owner_based.size() is " << owner_based.size() << '\n';    delete p;}struct C1 : std::enable_shared_from_this<C1>{};static void testEnable_shared_from_this(){    std::shared_ptr<C1> foo, bar;    foo = std::make_shared<C1>();    bar = foo->shared_from_this();    if(!foo.owner_before(bar) && !bar.owner_before(foo))        std::cout << "foo and bar share ownership";}//测试非成员函数static void testNonMemberFunction(){    /*testNonSwap();    testNonRelationalOperators();*/    ////测试make_shared()    //testNonMake_shared();    ////测试static_pointer_cast()    //testNonStatic_pointer_cast();    ////测试dynamic_pointer_cast()    //testNonDynamic_pointer_cast();    ////测试const_pointer_cast()    //testNonConst_pointer_cast();    ////测试get_deleter()    //testNonGet_deleter();    ////测试类模板owner_less    //testNonOwner_less();    //测试类模板enable_shared_from_this    testEnable_shared_from_this();}//从cplusplus摘取的测试用例。static void testFromCPlusPlus(){    ////测试构造函数    //testCtor();    ////测试带自定义删除器    //testCtorWithMyDeleter();    ////测试赋值函数    //testAssignment();    ////测试成员函数    //testMemberFunction();    //测试非成员函数    testNonMemberFunction();}//总测试static void test(){    //测试shared_ptr的各种成员函数  测试用例来自cplusplus.com    testFromCPlusPlus();    //测试多态，包括指针的upcast，和downcast的转换。    //wait to test}int main(){    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);    test();    return 0;}

总结

• shared_ptr没有成员函数release()，可以直接进行拷贝和赋值。

• 盲点

成员函数：owner_before()

非成员函数：除了make_shared()

参考

1. cplusplus