延长对象生命周期
Lambda函数可以捕获指向this
的共享指针,因此只要存在至少一个Lambda函数,对象就不会被销毁。
class Foo : public std::enable_shared_from_this<Foo> {
public:
Foo(const std::string& i_name) : name(i_name) {}
std::function<void()> GetPrinter() {
std::shared_ptr<Foo> that = shared_from_this();
return [that]() {
std::cout << that->name << std::endl;
};
}
std::string name;
};
http://ideone.com/Ucm2p8
通常情况下,这不是一个好的解决方案,因为对象的生命周期在这里以非常隐式的方式被延长。这是产生对象之间循环引用的非常简单的方法。
跟踪对象的生命周期
Lambda表达式可以跟踪捕获对象的生命周期,并仅在该对象仍然存在时使用它。
class Foo : public std::enable_shared_from_this<Foo> {
public:
Foo(const std::string& i_name) : name(i_name) {}
std::function<void()> GetPrinter() {
std::weak_ptr<Foo> weak_this = shared_from_this();
return [weak_this]() {
auto that = weak_this.lock();
if (!that) {
std::cout << "The object is already dead" << std::endl;
return;
}
std::cout << that->name << std::endl;
};
}
std::string name;
};
http://ideone.com/Wi6O11
不使用shared_ptr跟踪对象生命周期
正如hvd 所指出的,我们并不能总是确定一个对象是否由shared_ptr
管理。在这种情况下,我建议使用以下的lifetime_tracker
。它是自包含的,不会影响您管理对象生命周期的方式。
struct lifetime_tracker
{
private:
struct shared_state
{
std::uint32_t count : 31;
std::uint32_t dead : 1;
};
public:
struct monitor
{
monitor() : state(nullptr) {}
monitor(shared_state *i_state) : state(i_state) {
if (state)
++state->count;
}
monitor(const monitor& t) : state(t.state) {
if (state)
++state->count;
}
monitor& operator=(monitor t) {
std::swap(state, t.state);
return *this;
}
~monitor() {
if (state) {
--state->count;
if (state->count == 0 && state->dead)
delete state;
}
}
bool alive() const {
return state && !state->dead;
}
private:
shared_state *state;
};
public:
lifetime_tracker() : state(new shared_state()) {}
lifetime_tracker(const lifetime_tracker&) : state(new shared_state()) {}
lifetime_tracker& operator=(const lifetime_tracker& t) { return *this; }
~lifetime_tracker() {
if (state->count == 0)
delete state;
else
state->dead = 1;
}
monitor get_monitor() const {
return monitor(state);
}
private:
shared_state *state;
};
使用示例
class Foo {
public:
Foo(const std::string& i_name) : name(i_name) {}
std::function<void()> GetPrinter() {
auto monitor = tracker.get_monitor();
return [this, monitor]() {
if (!monitor.alive()) {
std::cout << "The object is already dead" << std::endl;
return;
}
std::cout << this->name << std::endl;
};
}
private:
lifetime_tracker tracker;
std::string name;
};