汇总类实例时 C++ 崩溃答案

【问题标题】：C++ crash when summarazing class instances汇总类实例时 C++ 崩溃
【发布时间】：2015-07-29 06:24:02
【问题描述】：

这是一个简单的Hello World代码，应该使用复制构造函数来汇总对象

下面是它生成的代码和输出

我猜崩溃是因为析构函数调用了它不应该调用的地方（或者我的 C++ 学习书的作者没有预料到），但也许你可以给我一些建议

我使用带有额外选项的默认 GNU GCC 代码块编译器 -std=c++11 -fno-elide-constructors（无论如何在这种情况下它们都无关紧要）

#include <iostream>
#include <string>
#include <cstring>
#include <sstream>

using namespace std;

class MyString
{
private:
  char* Buffer;

  MyString(): Buffer(NULL)
  {
    cout << "Default constructor called" << endl;
  }

public:
  MyString( const char* InitialInput )
  {
    cout << "Constructor called for: " << InitialInput << endl;
    if(InitialInput != NULL)
    {
      Buffer = new char [strlen(InitialInput)+1];
      strcpy( Buffer, InitialInput );
    }
    else
      Buffer = NULL;
  }

  MyString operator+ (const MyString& AddThis)
  {
    cout << "operator+ called for '" << Buffer << "' to add: " << AddThis.Buffer << endl;
    MyString NewString;
    if (AddThis.Buffer != NULL)
    {
      NewString.Buffer = new char[GetLenght() + strlen( AddThis.Buffer ) + 1];
      strcpy( NewString.Buffer, Buffer );
      strcat( NewString.Buffer, AddThis.Buffer );
    }
  }

  MyString& operator= (const MyString& CopySource)
  {
    cout << "Copy assignment operator for '" << Buffer << "' to copy from: " << CopySource.Buffer << endl;
    if ((this != &CopySource) && (CopySource.Buffer != NULL))
    {
      if (Buffer != NULL)
        delete[ ] Buffer;
      // гарантирует глубокую копию с предварительным
      // резервированием собственного буфера
      Buffer = new char [strlen(CopySource.Buffer) + 1];
      // копирование оригинала в локальный буфер
      strcpy(Buffer, CopySource.Buffer);
    }
    return *this;
  }

  MyString( const MyString& CopySource )
  {
    cout << "Copy constructor for '" << Buffer << "' to copy from: " << CopySource.Buffer << endl;
    if(CopySource.Buffer != NULL)
    {
      Buffer = new char [strlen(CopySource.Buffer)+1];
      strcpy(Buffer,CopySource.Buffer);
    }
    else
      Buffer = NULL;
  }

  ~MyString()
  {
    cout << "Destructor called for: " << Buffer << endl;
    if( Buffer != NULL )
      delete [] Buffer;
  }

  int GetLenght()
  {
    return strlen(Buffer);
  }

  operator const char*()
  {
    return Buffer;
  }
};

int main( )
{
  MyString Hello("Hello ");
  MyString World("World");
  MyString CPP(" of C++");

  MyString sayHelloAgain ("overwrite this");
  sayHelloAgain = Hello + World + CPP;

  return 0;
}

输出是

构造函数调用：你好
构造函数调用：World
构造函数调用：C++
构造函数调用：覆盖 this
operator+ 要求添加“Hello”：World
调用的默认构造函数
调用的析构函数：Hello World
运算符+ 调用 '├РРРРР■ ' 来添加：C++
调用的默认构造函数
调用的析构函数：C++ 的├РРРРР■
将“覆盖此”的赋值运算符移动到：
崩溃
进程返回 -1073741819 (0xC0000005) 执行时间：37.566 s
按任意键继续。

【问题讨论】：

输出看起来与代码不一致：我没有在代码中找到输出的“移动分配..”。
编译时出现警告。这将表明 operator + 应该返回一个值
这个输出是从哪里来的？ Move assignment operator for 'overwrite this' to move from: 请分享此代码。

标签： c++ c++11 constructor crash copy

【解决方案1】：

你的错误在你的copy constructor这一行

cout << "Copy constructor for '" << Buffer << "' to copy from: " << CopySource.Buffer << endl;

当对象在缓冲区中有 NULL 时，您正在尝试打印 buffer。

【讨论】：

感谢这是导致崩溃的原因，但之所以调用析构函数是因为我忘记了从 operator+ 返回值，正如“黑暗”所说的那样

【解决方案2】：

谢谢大家如果有人感兴趣，下面是固定代码（添加了复制使用的移动构造函数）

#include <iostream>
#include <string>
#include <cstring>
#include <sstream>

using namespace std;

class MyString
{
private:
  char* Buffer;

  MyString(): Buffer(NULL)
  {
    cout << "Default constructor called" << endl;
  }

public:
  MyString( const char* InitialInput )
  {
    cout << "Constructor called for: " << InitialInput << endl;
    if(InitialInput != NULL)
    {
      Buffer = new char [strlen(InitialInput)+1];
      strcpy( Buffer, InitialInput );
    }
    else
      Buffer = NULL;
  }

  MyString operator+ (const MyString& AddThis)
  {
    cout << "operator+ called: " << AddThis.Buffer << endl;
    MyString NewString;
    if (AddThis.Buffer != NULL)
    {
      NewString.Buffer = new char[GetLenght() + strlen( AddThis.Buffer ) + 1];
      strcpy( NewString.Buffer, Buffer );
      strcat( NewString.Buffer, AddThis.Buffer );
    }
    return NewString;
  }

  MyString& operator= (const MyString& CopySource)
  {
    cout << "Copy assignment operator to copy from: " << CopySource.Buffer << endl;
    if ((this != &CopySource) && (CopySource.Buffer != NULL))
    {
      if (Buffer != NULL)
        delete[ ] Buffer;
      Buffer = new char [strlen(CopySource.Buffer) + 1];
      strcpy(Buffer, CopySource.Buffer);
    }
    return *this;
  }

  MyString( const MyString& CopySource )
  {
    cout << "Copy constructor to copy from: " << CopySource.Buffer << endl;
    if(CopySource.Buffer != NULL)
    {
      Buffer = new char [strlen(CopySource.Buffer)+1];
      strcpy(Buffer,CopySource.Buffer);
    }
    else
      Buffer = NULL;
  }

  MyString( MyString&& MoveSource)
  {
    cout << "Move constructor to move from: " << MoveSource.Buffer << endl;
    if(MoveSource.Buffer != NULL)
    {
      Buffer = MoveSource.Buffer;
      MoveSource.Buffer = NULL;
    }
  }

  MyString& operator= (MyString&& MoveSource)
  {
    cout << "Move assignment operator to move from: " << MoveSource.Buffer << endl;
    if ((this != &MoveSource) && (MoveSource.Buffer != NULL))
    {
      if (Buffer != NULL)
        delete[ ] Buffer;
      Buffer = new char [strlen(MoveSource.Buffer) + 1];
      strcpy(Buffer, MoveSource.Buffer);
    }
    return *this;
  }

  ~MyString()
  {
    if( Buffer != NULL )
      delete [] Buffer;
  }

  int GetLenght()
  {
    return strlen(Buffer);
  }

  operator const char*()
  {
    return Buffer;
  }
};

int main( )
{
  MyString Hello("Hello ");
  MyString World("World");
  MyString CPP(" of C++");

  MyString sayHelloAgain ("overwrite this");
  sayHelloAgain = Hello + World + CPP;

  return 0;
}

出来

构造函数调用：你好
构造函数调用：World
构造函数调用：C++
构造函数调用：覆盖 this
运算符+调用：世界
调用的默认构造函数
移动构造函数以移动：Hello World
运算符+调用：C++
调用的默认构造函数
移动构造函数以从：Hello World of C++
将赋值运算符移至从：Hello World of C++

【讨论】：