[英]C++ static classes & shared_ptr memory leaks
I can't understand why does the following code produce memory leaks (I am using boost::shared_ptr
with static class instance). 我不明白为什么以下代码会产生内存泄漏(我在静态类实例中使用
boost::shared_ptr
)。 Could someone help me? 有人可以帮我吗?
#include <crtdbg.h>
#include <boost/shared_ptr.hpp>
using boost::shared_ptr;
#define _CRTDBG_MAP_ALLOC
#define NEW new(_NORMAL_BLOCK, __FILE__, __LINE__)
static struct myclass {
static shared_ptr<int> ptr;
myclass() {
ptr = shared_ptr<int>(NEW int);
}
} myclass_instance;
shared_ptr<int> myclass::ptr;
int main() {
_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF |
_CRTDBG_CHECK_ALWAYS_DF | _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG));
return 0;
}
This is a memory leak. 这是内存泄漏。 You are initializing a static instance of myclass called myclass_instance.
您正在初始化myclass的静态实例myclass_instance。 You are also initializing the "shared_ptr myclass::ptr".
您还正在初始化“ shared_ptr myclass :: ptr”。
According to Stroustrup[3], statics are initialized in the order that they are defined. 根据Stroustrup [3],按定义的顺序初始化静态变量。 Therefore you have the static definition of myclass_instance, which initializes the internal ptr on construction.
因此,您具有myclass_instance的静态定义,该定义初始化构造时的内部ptr。 However, you then have the definition of the static myclass::ptr, which invokes the default constructor for shared_ptr.
但是,您然后有了静态myclass :: ptr的定义,该定义调用了shared_ptr的默认构造函数。
This is an example of the classic statics ordering problem. 这是经典静态排序问题的一个示例。 The compiler thinks that myclass::ptr wasn't actually initialized, so there's no destruction of the original shared_ptr.
编译器认为myclass :: ptr实际上并未初始化,因此不会破坏原始的shared_ptr。 Instead, it is just leaked.
相反,它只是泄漏。
You'll need a bare pointer of some kind. 您将需要某种裸露的指针。 If you're using C++11, you can do the Nifty Counter Technique with a ternary assignment statement which does a move to itself if you determine that the object has already been initialized.
如果您使用的是C ++ 11,则可以使用三元赋值语句执行Nifty Counter Technique,如果您确定该对象已经初始化,则该语句会自动执行。 It's pretty rough, but it works.
这很粗糙,但是可以。
Here's how I'd do it in C++11: 这是我在C ++ 11中的处理方式:
#include <crtdbg.h>
#include <memory>
using std;
#define _CRTDBG_MAP_ALLOC
#define NEW new(_NORMAL_BLOCK, __FILE__, __LINE__)
// Note that the count could also be a field in an initializer static used in the Nifty Counter
// Technique covered in many texts.
static int count = 0; // This gets implicitly initialized to 0 by the executable load into memory.
static struct myclass {
static shared_ptr<int> ptr;
myclass() {
if (count++ == 0) {
ptr = make_shared<int>(0); //initialization
}
}
} myclass_instance;
shared_ptr<int> myclass::ptr = count == 0 ? make_shared<int>(0) : move(myclass::ptr);
int main() {
_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF |
_CRTDBG_CHECK_ALWAYS_DF | _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG));
return 0;
}
See the following for more information: 有关更多信息,请参见以下内容:
最有可能在全局对象被销毁之前检测到泄漏,并且shared_ptr
有机会释放该对象,因此很可能是错误的泄漏。
At a guess the CRT is reporting a false positive - the following code illustrates that the shared pointer is working correctly, at least with g++ 猜测CRT报告的是误报-以下代码说明了共享指针至少在g ++上正常工作
#include <iostream>
#include "boost/shared_ptr.hpp"
using namespace std;
using namespace boost;
struct R {
R() {
cerr << "ctor" << endl;
}
~R() {
cerr << "dtor" << endl;
}
};
struct A {
static shared_ptr<R> ptr;
A() {
ptr = shared_ptr<R>(new R);
}
};
shared_ptr<R> A::ptr;
static A a;
int main() {
}
It prints: 它打印:
ctor
dtor
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