C++ static classes & shared_ptr memory leaks

问题:

I can't understand why does the following code produce memory leaks (I am using boost::shared_ptr with static class instance). Could someone help me?

#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;
}

回答1:

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++

#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:

ctor
dtor

回答2:

This is a memory leak. You are initializing a static instance of myclass called myclass_instance. You are also initializing the "shared_ptr myclass::ptr".

According to Stroustrup[3], statics are initialized in the order that they are defined. Therefore you have the static definition of myclass_instance, which initializes the internal ptr on construction. However, you then have the definition of the static myclass::ptr, which invokes the default constructor for shared_ptr.

This is an example of the classic statics ordering problem. The compiler thinks that myclass::ptr wasn't actually initialized, so there's no destruction of the original shared_ptr. Instead, it is just leaked.

You'll need a bare pointer of some kind. 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. It's pretty rough, but it works.

Here's how I'd do it in C++11:

#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:

1. Lakos, J, 1996, Large-Scale C++ Software Design. Section 7.8.1.3, Addison Wesley, Reading, Massachusetts.
2. Meyers, S, 2005, Effective C++, Third Edition. Item 4: Make sure that objects are initialized before they're used. Addison Wesley, Reading, Massachusetts.
3. Stroustrup, B, 2000, The C++ Programming Language Special Edition. Section 10.4.9, Addison Wesley, Reading, Massachusetts.

回答3:

Most likely the leak is detected before the global objects get destroyed and shared_ptr has a chance to release the object, so it's likely a false leak.