[英]Comparator operator in weak_ptr C++
I am still a novice in the new stl members.Can anyone point out why this code is giving segmentation fault? 我仍然是新stl成员的新手。任何人都可以指出为什么这段代码会给出分段错误?
#include<memory>
#include<stdio.h>
#include<map>
#include<set>
#include<string>
using namespace std;
struct StubClass
{
weak_ptr<string> b;
int c;
friend bool operator==(StubClass x,StubClass y);
friend bool operator<(StubClass x,StubClass y);
StubClass(weak_ptr<string> x):b(x){c=5;}
};
bool operator==(StubClass d,StubClass c) { return d.b==c.b;}
bool operator<(StubClass d,StubClass c) { return d.b<c.b; }
int main()
{
shared_ptr<string> spPtr(new string("Hello"));
weak_ptr<string> wpPtr(spPtr);
StubClass hello(wpPtr);
set<StubClass> helloSet;
helloSet.insert(hello);
if(helloSet.find(StubClass(wpPtr))!=helloSet.end()) printf("YAYA");
else puts("Bye");
}
The error is in line 错误符合
if(helloSet.find(StubClass(wpPtr))!=helloSet.end()) printf("YAYA"); if(helloSet.find(StubClass(wpPtr))!= helloSet.end())printf(“YAYA”);
More research reveals there is a problem when the StubClass's comparator function is called. 更多的研究表明,调用StubClass的比较器函数时会出现问题。 I am compiling the program here 我在这里编译程序
EDIT: 编辑:
bool operator==(StubClass d,StubClass c) { return d.b.lock()==c.b.lock();}
bool operator<(StubClass d,StubClass c) { return d.b.lock()<c.b.lock(); }
This resolved the issue.I should be reading more.:( Anyways can anyone from the community explain the reason why the first code gives SIGSEGV.I figured it out eventually,but still a nice explanation won't hurt. :) 这解决了这个问题。我应该阅读更多。:(无论如何,社区中的任何人都可以解释第一个代码给出SIGSEGV的原因。我最终想出来了,但仍然有一个很好的解释不会受到伤害。:)
Your original code segfaults because you've accidentally set up an infinite recursion: 您的原始代码段错误,因为您不小心设置了无限递归:
bool operator<(StubClass d,StubClass c) { return d.b<c.b; }
There is no operator<
for weak_ptr
. 没有operator<
for weak_ptr
。 However you do have an implicit conversion from weak_ptr
to StubClass
. 但是,您确实有从weak_ptr
到StubClass
的隐式转换。 And StubClass
has an operator<
. StubClass
有一个operator<
。 So this function calls itself indefinitely: thus the segfault. 所以这个函数无限地调用自己:因此是段错误。
The currently accepted answer from inkooboo will also lead to undefined behavior, probably resulting in a crash. 来自inkooboo的当前接受的答案也将导致未定义的行为,可能导致崩溃。 As weak_ptrs
become expired during the execution of your program (something more involved than your test case), then the ordering of them will change. 由于weak_ptrs
在程序执行期间过期(比测试用例更复杂),因此它们的顺序将发生变化。 When this happens between two weak_ptrs
in the set
, the set
will become corrupted, likely leading to a crash. 当在set
两个weak_ptrs
之间发生这种情况时,该set
将被破坏,可能导致崩溃。 However there is a way around this using owner_less
which was designed specifically for this use case: 然而,有一种方法可以使用owner_less
专门为此用例设计:
bool operator==(const StubClass& d, const StubClass& c)
{
return !owner_less<weak_ptr<string>>()(d.b, c.b) &&
!owner_less<weak_ptr<string>>()(c.b, d.b);
}
bool operator<(const StubClass& d, const StubClass& c)
{
return owner_less<weak_ptr<string>>()(d.b, c.b);
}
Or, if you prefer, this can also be coded using the member function owner_before
. 或者,如果您愿意,也可以使用成员函数owner_before
进行编码。 Both are equivalent: 两者都是等价的:
bool operator==(const StubClass& d, const StubClass& c)
{
return !d.b.owner_before(c.b) && !c.b.owner_before(d.b);
}
bool operator<(const StubClass& d, const StubClass& c)
{
return d.b.owner_before(c.b);
}
Using these functions, even when one weak_ptr
expires and the other doesn't, their ordering remains stable. 使用这些函数,即使一个weak_ptr
到期而另一个没有,它们的顺序仍然稳定。 And thus you'll have a well-defined set
. 因此,你将有一个明确定义的set
。
If you want to compare strings stored in weak_ptr do this: 如果你想比较存储在weak_ptr中的字符串,请执行以下操作:
bool operator<(StubClass d, StubClass c)
{
std::shared_ptr<std::string> a = d.b.lock();
std::shared_ptr<std::string> b = c.b.lock();
if (!a && !b)
return false;
if (!a)
return true;
if (!b)
return false;
return *a < *b;
}
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