std::enable_shared_from_this
STD：：启用[医]共享[医]从[医]这
| Defined in header | | |
|---|---|---|
| template< class T > class enable_shared_from_this; | | (since C++11) |
std::enable_shared_from_this允许对象t当前由std::shared_ptr命名pt安全地生成额外的std::shared_ptr实例pt1, pt2, ...所有股份的所有权t带着pt...
公开继承std::enable_shared_from_this<T>提供类型T具有成员函数shared_from_this.如果一个物体t类型T由std::shared_ptr<T>命名pt，然后打电话T::shared_from_this将返回一个新的std::shared_ptr<T>拥有t带着pt...
成员函数
| (constructor) | constructs an enabled_shared_from_this object (protected member function) |
|---|---|
| (destructor) | destroys an enable_shared_from_this object (protected member function) |
| operator= | returns a reference to this (protected member function) |
| shared_from_this | returns a shared_ptr which shares ownership of *this (public member function) |
| weak_from_this (C++17) | returns the weak_ptr which shares ownership of *this (public member function) |
成员对象
| Member name | Definition |
|---|---|
| weak_this (private)(C++17) | std::weak_ptr object tracking the control block of the first shared owner of *this. Exposition only |
注记
的共同实现enable_shared_from_this保存弱引用%28，如std::weak_ptr%29至this...的建设者std::shared_ptr检测到enable_shared_from_this基并分配新创建的std::shared_ptr到内部存储的弱引用。构造一个std::shared_ptr对于已经由另一个对象管理的对象。std::shared_ptr不会查询内部存储的弱引用，从而导致未定义的行为。
它被允许打电话shared_from_this仅在以前共享的对象上，即在std::shared_ptr<T>。否则，在C++17%29之前，该行为是未定义的%28。std::bad_weak_ptr由共享引发%28。[医]来自默认构造的PTR构造函数weak_this%29%28自C++17%29。
enable_shared_from_this提供以下表达式的安全替代方案：std::shared_ptr<T>(this)，这很可能导致this被多个不知道对方的所有者销毁不止一次，参见%29下面的示例。
例
二次
#include <memory>
#include <iostream>
struct Good: std::enable_shared_from_this<Good> // note: public inheritance
{
std::shared_ptr<Good> getptr() {
return shared_from_this(
}
};
struct Bad
{
std::shared_ptr<Bad> getptr() {
return std::shared_ptr<Bad>(this
}
~Bad() { std::cout << "Bad::~Bad() called\n"; }
};
int main()
{
// Good: the two shared_ptr's share the same object
std::shared_ptr<Good> gp1 = std::make_shared<Good>(
std::shared_ptr<Good> gp2 = gp1->getptr(
std::cout << "gp2.use_count() = " << gp2.use_count() << '\n';
// Bad: shared_from_this is called without having std::shared_ptr owning the caller
try {
Good not_so_good;
std::shared_ptr<Good> gp1 = not_so_good.getptr(
} catch(std::bad_weak_ptr& e) {
// undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
std::cout << e.what() << '\n';
}
// Bad, each shared_ptr thinks it's the only owner of the object
std::shared_ptr<Bad> bp1 = std::make_shared<Bad>(
std::shared_ptr<Bad> bp2 = bp1->getptr(
std::cout << "bp2.use_count() = " << bp2.use_count() << '\n';
} // UB: double-delete of Bad
二次
产出：
二次
gp2.use_count() = 2
bad_weak_ptr
bp2.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption
二次
另见
| shared_ptr (C++11) | smart pointer with shared object ownership semantics (class template) |
|---|
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