初始化程序列表与 2013 中的双重删除 [英] Double delete in initializer_list vs 2013

问题描述

今天在我的项目中遇到内存问题，一个类使用 c++ 11 initializer_list.系统发出内存问题信号:dbgdel.cpp 中的表达式 _BLOCK_TYPE_IS_VALID(pHead->nBlockUse).我将代码简化为一个简单示例，它不再抛出表达式，但问题从调试输出中变得明显.在我看来，这代码是正确的，它似乎也适用于 g++.

#include #include <内存>#include <字符串>#include #include <向量>#include <地图>#include #include <initializer_list>使用命名空间标准；B类{上市:字符数据[256]；B(const string& x) {cout<<初始化"<<这<<结束;}B(const B& b) {cout<<复制"<<这<<结束;}~B() {cout<<删除 b" <<这<<结束;}};C类{上市:向量 Bbs;C(initializer_list<B> bb) {for(auto& b : bb) {bs.push_back(b);}}};int main(int argc, char** argv) {C bb { B("foo"), B("bar") };返回0；}

输出为:

<前>初始化 00B7FAE8初始化 00B7FBE8复制 00E108A0复制 00E10AE8 (?????)删除 b 00E108A0复制 00E10BE8删除 b 00B7FBE8删除 b 00B7FAE8删除 b 00B7FAE8(删除两次！)

我在这里犯了什么错误或者这不应该工作?

解决方案

initializer_list 的行为有问题.在其析构函数中，它调用整个范围的向量删除(delete[])，然后再次删除数组中的第一个条目.此行为不是 initializer_list 类的一部分，看起来像是编译器错误.initializer_list 没有析构函数，也不分配用于列表的数组.它看起来就像一个 C 数组的包装器.

至于使用您看到的额外副本，这是由向量在初始化期间调整大小引起的.这是您的流程:

Init 00B7FAE8//构造foo"Init 00B7FBE8//构造bar"Copy 00E108A0//将foo"复制到向量(容量=1)Copy 00E10AE8 (?????)//将上述对象复制到调整大小的向量(容量 = 2)Deleting b 00E108A0//删除较小的向量缓冲区Copy 00E10BE8//将bar"从初始化列表复制到向量删除 b 00B7FBE8//以相反的顺序删除初始化列表.这是酒吧"删除 b 00B7FAE8//最后删除.这是foo"删除 b 00B7FAE8(错误)//后来 C::bs 被销毁

您在这里可以看到，由于复制，通过 push_back 初始化向量的速度非常慢.即使您使用了更优雅的方式，也会发生这种情况:

C(initializer_list bb) : bs(bb) {}

一种更快(没有额外副本)的方法是:

C(initializer_list bb) {bs.reserve(bb.size());bs.insert(bs.end(), bb.begin(), bb.end());}

Today in run into a memory problem in my project, with a class using c++ 11 initializer_list. The system signals a memory problem: "Expression _BLOCK_TYPE_IS_VALID(pHead->nBlockUse) in dbgdel.cpp. I simplified the code to a simple example, it no longer throws an expression but the problem becomes apparent from the debug output. In my eyes this code is correct, also it seems to work with g++.

#include <functional>
#include <memory>
#include <string>
#include <iostream>
#include <vector>
#include <map>
#include <sstream>

#include <initializer_list>

using namespace std;

class B {
public:
    char data[256];
    B(const string& x) {
        cout << "Init " << this << endl;
    }
    B(const B& b) {
        cout << "Copy " << this << endl;
    }
    ~B() {
        cout << "Deleting b " << this << endl;
    }
};

class C {
public:
    vector<B> bs;

    C(initializer_list<B> bb) {
        for(auto& b : bb) {
            bs.push_back(b);
        }
    }
};

int main(int argc, char** argv) {
    C bb { B("foo"), B("bar") };
    return 0;
}

The output is:

Init 00B7FAE8
Init 00B7FBE8
Copy 00E108A0
Copy 00E10AE8 (?????)
Deleting b 00E108A0
Copy 00E10BE8
Deleting b 00B7FBE8
Deleting b 00B7FAE8
Deleting b 00B7FAE8  (Deleted twice!)

What mistake I make here or is this not supposed to work?

解决方案

The initializer_list behavior is buggy. In its destructor it calls a vector delete (a delete[]) of the entire range and then deletes the first entry in the array again. This behavior is not part of the initializer_list class and looks like a compiler bug. initializer_list doesn't have a destructor and doesn't allocate the array used for the list. It just looks like a wrapper for a C array.

As for using the extra copy you see, it's caused by the vector resizing from during its initialization. Here's your flow:

Init 00B7FAE8       // construct "foo"
Init 00B7FBE8       // construct "bar"
Copy 00E108A0       // copy "foo" to vector (capacity=1)
Copy 00E10AE8 (?????) // copy the above object to the resized vector (capacity = 2)
Deleting b 00E108A0   // delete the smaller vector buffer
Copy 00E10BE8         // copy "bar" from initialization_list to vector

Deleting b 00B7FBE8   // delete initialization_list in reverse order. this is "bar"
Deleting b 00B7FAE8   // last to delete. this is "foo"

Deleting b 00B7FAE8  (bug)

// later C::bs is destroyed

What you can see here is the initializing a vector via push_back is quite slow due to copying. This would be happen even if you've used the more elegant way:

C(initializer_list<B> bb) : bs(bb) {}

A faster (no extra copies) method is:

C(initializer_list<B> bb) {
    bs.reserve(bb.size());
    bs.insert(bs.end(), bb.begin(), bb.end());
}

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