共享指针是否会中断尾部调用优化? 前言 我正在练习C++,并试图实现不可变列表。 在我的一个测试中,我试图递归地创建一个包含大量值(100万个节点)的列表。所有的值都是常量,所以我不能执行常规循环,这也不够功能,你知道。 测试失败,出现分段故障
我的系统是64位Xubuntu 16.04 LTS和Linux 4.4。 我使用g++5.4和clang++3.8编译我的代码,使用共享指针是否会中断尾部调用优化? 前言 我正在练习C++,并试图实现不可变列表。 在我的一个测试中,我试图递归地创建一个包含大量值(100万个节点)的列表。所有的值都是常量,所以我不能执行常规循环,这也不够功能,你知道。 测试失败,出现分段故障,c++,recursion,shared-ptr,compiler-optimization,tail-recursion,C++,Recursion,Shared Ptr,Compiler Optimization,Tail Recursion,我的系统是64位Xubuntu 16.04 LTS和Linux 4.4。 我使用g++5.4和clang++3.8编译我的代码,使用--std=c++14-O3标志 源代码 我已经写了一个简单的例子,它展示了这样一种情况,当尾部调用应该很容易优化时,出现了一些错误,出现了分段错误。函数f只需等待amount迭代,然后创建指向singleint的指针并返回它 #include <memory> using std::shared_ptr; shared_ptr<int>
--std=c++14-O3
标志
源代码
我已经写了一个简单的例子,它展示了这样一种情况,当尾部调用应该很容易优化时,出现了一些错误,出现了分段错误
。函数f
只需等待amount
迭代,然后创建指向singleint
的指针并返回它
#include <memory>
using std::shared_ptr;
shared_ptr<int> f(unsigned amount) {
return amount? f(amount - 1) : shared_ptr<int>{new int};
}
int main() {
return f(1E6) != nullptr;
}
注意
两个编译器都很好地优化了使用常规指针的实现
问题:
在我的例子中,shared_ptr
真的打破了尾部调用优化吗?
这是编译器的问题还是共享\u ptr
实现的问题?答案
简单的回答是:是和否
C++中的共享指针不会中断尾调用优化, 但它使这种递归函数的创建变得复杂,编译器可以将其转换为循环
细节 在递归构造长列表期间避免堆栈溢出 <>我记得,<代码> SyddpPTR>代码>有析构函数,C++有RAII。 正如前面讨论的那样,这使得构造可优化的尾部调用变得更加困难 @KennyOstrom建议使用普通指针来解决这个问题static const List* insertBulk_(unsigned amount, const List* tail=nullptr) {
return amount? insertBulk_(amount - 1, new List{tail})
: tail;
}
使用以下构造函数
List(const List* tail): tail{tail} {}
当List
的tail
是shared\u ptr
的实例时,tail调用被成功优化
在销毁期间避免堆栈溢出
需要定制销毁策略。
幸运的是,shared\u ptr
允许我们设置它,
所以我隐藏了List
的析构函数,使其成为私有的,
并使用此列表进行销毁
static void destroy(const List* list) {
if (!list) return;
shared_ptr<const List> tail = list->tail;
delete list;
for (; tail && tail.use_count() == 1; tail = tail->tail);
}
避免内存泄漏
在异常情况下,我不会进行适当的清理,因此问题尚未解决。
我想使用shared\u ptr
,因为它是安全的,但现在我在构建结束之前不会将其用于当前列表头
它需要监视“裸”指针,直到它被包装成共享指针,并在紧急情况下释放它。
让我们将对尾部指针的引用而不是指针本身传递给insertBulk
。
这将允许最后一个良好的指针在函数外部可见
static const List* insertBulk_(unsigned amount, const List*& tail) {
if (!amount) {
const List* result = tail;
tail = nullptr;
return result;
}
return insertBulk_(amount - 1, tail = new List{tail});
}
然后需要模拟最后
,以便自动销毁指针,在异常情况下会泄漏指针
static const shared_ptr<const List> insertBulk(unsigned amount) {
struct TailGuard {
const List* ptr;
~TailGuard() {
List::destroy(this->ptr);
}
} guard{};
const List* result = insertBulk_(amount, guard.ptr);
return amount? shared_ptr<const List>{result, List::destroy}
: nullptr;
}
源代码的较短版本
列表
没有注释的类,并检入main
函数
#include <memory>
using std::shared_ptr;
class List {
private:
const shared_ptr<const List> tail;
static const List* insertBulk_(unsigned amount, const List*& tail) {
if (!amount) {
const List* result = tail;
tail = nullptr;
return result;
}
return insertBulk_(amount - 1, tail = new List{tail});
}
~List() {}
public:
List(const List* tail): tail{tail, List::destroy} {}
List(const shared_ptr<const List>& tail): tail{tail} {}
List(const List&) = delete;
List() = delete;
static const shared_ptr<const List> insertBulk(unsigned amount) {
struct TailGuard {
const List* ptr;
~TailGuard() {
List::destroy(this->ptr);
}
} guard{};
const List* result = insertBulk_(amount, guard.ptr);
return amount? shared_ptr<const List>{result, List::destroy}
: nullptr;
}
static void destroy(const List* list) {
if (!list) return;
shared_ptr<const List> tail = list->tail;
delete list;
for (; tail && tail.use_count() == 1; tail = tail->tail);
}
};
#包括
使用std::shared_ptr;
班级名单{
私人:
const共享_ptr tail;
静态常量列表*插入批量(未签名金额、常量列表*&尾部){
如果(!金额){
常量列表*结果=尾部;
tail=nullptr;
返回结果;
}
返回insertBulk(amount-1,tail=newlist{tail});
}
~List(){}
公众:
List(const List*tail):tail{tail,List::destroy}{}
列表(const shared_ptr&tail):tail{tail}{}
列表(const List&)=删除;
List()=删除;
静态常量共享\u ptr insertBulk(未签名金额){
结构尾护板{
施工清单*ptr;
~TailGuard(){
列表::销毁(本->ptr);
}
}警卫{};
const List*result=insertBulk(金额,guard.ptr);
返回金额?共享{结果,列表::销毁}
:nullptr;
}
静态无效销毁(常量列表*列表){
如果(!list)返回;
共享\u ptr tail=列表->尾部;
删除名单;
对于(;tail&&tail.use_count()==1;tail=tail->tail);
}
};
“但是我想这个例子在使用共享的\u PTR宏时更清楚。”-不是。@NeilButterworth谢谢,我已经写了更简单的例子,看起来你为每个级别构造了一个新的共享\u PTR包装实例。尝试引用?@KennyOstrom是的,引用发挥了作用,因此我更改了示例以更真实地显示问题。为什么使用共享的ptr
而不是唯一的ptr
?您是否计划让多个列表共享同一个尾部,因为它们是不可变的?
static const shared_ptr<const List> insertBulk(unsigned amount) {
struct TailGuard {
const List* ptr;
~TailGuard() {
List::destroy(this->ptr);
}
} guard{};
const List* result = insertBulk_(amount, guard.ptr);
return amount? shared_ptr<const List>{result, List::destroy}
: nullptr;
}
#include <memory>
#include <cassert>
using std::shared_ptr;
class List {
private:
const shared_ptr<const List> tail;
/**
* I need a `tail` to be an instance of `shared_ptr`.
* Separate `List` constructor was created for this purpose.
* It gets a regular pointer to `tail` and wraps it
* into shared pointer.
*
* The `tail` is a reference to pointer,
* because `insertBulk`, which called `insertBulk_`,
* should have an ability to free memory
* in the case of `insertBulk_` fail
* to avoid memory leak.
*/
static const List* insertBulk_(unsigned amount, const List*& tail) {
if (!amount) {
const List* result = tail;
tail = nullptr;
return result;
}
return insertBulk_(amount - 1, tail = new List{tail});
}
unsigned size_(unsigned acc=1) const {
return this->tail? this->tail->size_(acc + 1) : acc;
}
/**
* Destructor needs to be hidden,
* because it causes stack overflow for long lists.
* Custom destruction method `destroy` should be invoked first.
*/
~List() {}
public:
/**
* List needs custom destruction strategy,
* because default destructor causes stack overflow
* in the case of long lists:
* it will recursively remove its items.
*/
List(const List* tail): tail{tail, List::destroy} {}
List(const shared_ptr<const List>& tail): tail{tail} {}
List(const List&) = delete;
List() = delete;
unsigned size() const {
return this->size_();
}
/**
* Public iterface for private `insertBulk_` method.
* It wraps `insertBulk_` result into `shared_ptr`
* with custom destruction function.
*
* Also it creates a guard for tail,
* which will destroy it if something will go wrong.
* `insertBulk_` should store `tail`,
* which is not yet wrapped into `shared_ptr`,
* in the guard, and set it to `nullptr` in the end
* in order to avoid destruction of successfully created list.
*/
static const shared_ptr<const List> insertBulk(unsigned amount) {
struct TailGuard {
const List* ptr;
~TailGuard() {
List::destroy(this->ptr);
}
} guard{};
const List* result = insertBulk_(amount, guard.ptr);
return amount? shared_ptr<const List>{result, List::destroy}
: nullptr;
}
/**
* Custom destruction strategy,
* which should be called in order to delete a list.
*/
static void destroy(const List* list) {
if (!list) return;
shared_ptr<const List> tail = list->tail;
delete list;
/**
* Watching references count allows us to stop,
* when we reached the node,
* which is used by another list.
*
* Also this prevents long loop of construction and destruction,
* because destruction calls this function `destroy` again
* and it will create a lot of redundant entities
* without `tail.use_count() == 1` condition.
*/
for (; tail && tail.use_count() == 1; tail = tail->tail);
}
};
int main() {
/**
* Check whether we can create multiple lists.
*/
const shared_ptr<const List> list{List::insertBulk(1E6)};
const shared_ptr<const List> longList{List::insertBulk(1E7)};
/**
* Check whether we can use a list as a tail for another list.
*/
const shared_ptr<const List> composedList{new List{list}, List::destroy};
/**
* Checking whether creation works well.
*/
assert(list->size() == 1E6);
assert(longList->size() == 1E7);
assert(composedList->size() == 1E6 + 1);
return 0;
}
#include <memory>
using std::shared_ptr;
class List {
private:
const shared_ptr<const List> tail;
static const List* insertBulk_(unsigned amount, const List*& tail) {
if (!amount) {
const List* result = tail;
tail = nullptr;
return result;
}
return insertBulk_(amount - 1, tail = new List{tail});
}
~List() {}
public:
List(const List* tail): tail{tail, List::destroy} {}
List(const shared_ptr<const List>& tail): tail{tail} {}
List(const List&) = delete;
List() = delete;
static const shared_ptr<const List> insertBulk(unsigned amount) {
struct TailGuard {
const List* ptr;
~TailGuard() {
List::destroy(this->ptr);
}
} guard{};
const List* result = insertBulk_(amount, guard.ptr);
return amount? shared_ptr<const List>{result, List::destroy}
: nullptr;
}
static void destroy(const List* list) {
if (!list) return;
shared_ptr<const List> tail = list->tail;
delete list;
for (; tail && tail.use_count() == 1; tail = tail->tail);
}
};