c++ - 使用递归方法遇到段错误

Question

class Apple
{
 public:
   Apple func(int depth) const;
 private:
   int size = 0; // size of data array  
   double *data = nullptr;
}

Apple::func(int depth) const
{
  Apple new_apple;

  '''
  some calculations
  '''

  if(depth>1)
  {
    return new_apple.func(depth-1);
  }else
  {
    return new_apple;
  }
}

在方法 func() 中，我创建了一个新的 Apple 对象，并根据存储在原始 Apple 对象中的数据进行了一些计算。终端给出

Segmentation fault (core dumped)

当我从 valgrind 收到以下消息时，我认为内存有问题。

==4787== HEAP SUMMARY:
==4787==     in use at exit: 5,994 bytes in 63 blocks
==4787==   total heap usage: 1,059 allocs, 996 frees, 146,564 bytes allocated
==4787== 
==4787== LEAK SUMMARY:
==4787==    definitely lost: 0 bytes in 0 blocks
==4787==    indirectly lost: 0 bytes in 0 blocks
==4787==      possibly lost: 0 bytes in 0 blocks
==4787==    still reachable: 5,994 bytes in 63 blocks
==4787==                       of which reachable via heuristic:
==4787==                         newarray           : 1,080 bytes in 3 blocks
==4787==         suppressed: 0 bytes in 0 blocks

Valgrind 跟踪到 Apple 类的重载 operator=。只有一个成员(我这里命名为data)涉及到动态内存，所以这里只展示和它相关的部分。以下是 operator= 的实现方式:

Apple& Apple::operator=(const Apple &obj)
{
  if(this!=&obj)
  {
    if(this->data)
    {
      delete[] this->data;
    }
    this->size = obj.size;
    this->data = new double[this->size];
    for(int i=0; i<this->size; i++)
    {
      this->data[i] = obj.data[i];
    }
  }
  return *this
}

============================================= ==========================

如果我不添加递归组件，方法 func() 工作正常。而且我也用valgrind查过，计算部分的内存没有问题。

Apple::func(int depth) const
{
  Apple new_apple;

  '''
  some calculations
  '''
  return new_apple; // works fine
}

即使我调用 depth = 1 的 func(int depth) 也会发生相同的段错误。

Apple::func(int depth) const
{
  Apple new_apple;

  '''
  some calculations
  '''

  if(depth>1)
  {
    std::cout << 1 << std::endl;
    return new_apple.func(depth-1);
  }else
  {
    std::cout << 2 << std::endl;
    return new_apple;
  }
}

当我调用 func(1) 时，既没有打印出“1”也没有打印出“2”。

============================================= ==========================来自 gdb 的调试信息:

Program received signal SIGSEGV, Segmentation fault.
Apple::operator= (this=0x0, T=...) at src/xxxx.cpp:1011
1011          if(this->data)

============================================= ==========================更新:(添加了导致错误的示例代码)

我添加了一个示例代码，它更接近我的真实代码的结构。

示例代码:

http://coliru.stacked-crooked.com/a/3f7e50e4ebabf717

#include <iostream>
using namespace std;
class Apple
{
public:
  Apple();
  ~Apple();
  Apple(const Apple &obj);
  void alloc(int size);
private:
  double *data = nullptr;
  int size = 0;
};
/////////////////////////////////////////////
class Basket
{
public:
  Basket();
  ~Basket();
  Basket(const Basket &obj);
  void alloc(int size1, int size2);
  Basket func(int depth);
private:
  Apple *data = nullptr;
  int size;
};
//////////////////////////////////////////////
Apple::Apple(){}
Apple::~Apple()
{
  if(this->data)
  {
    delete[] this->data;
  }
}
Apple::Apple(const Apple &obj)
{
  if(this->data)
  {
    delete[] this->data;
  }
  this->size = obj.size;
  this->data = new double[this->size];
  for(int i=0; i<this->size; i++)
  {
    this->data[i] = obj.data[i];
  }
}
void Apple::alloc(int size)
{
  this->size = size;
  if(this->data)
  {
    delete[] this->data;
  }
  this->data = new double[size];
}
///////////////////////////////////////////////////////////////////////
Basket::Basket(){}
Basket::~Basket()
{
  if(this->data)
  {
     delete[] this->data;
  }
}
Basket::Basket(const Basket &obj)
{
  this->size = obj.size;
  // This is the missing piece
  /*
  if(this->data)
  {
      delete[] this->data;
  }
  this->data = new Apple[this->size];
  */
  for(int i=0; i<this->size; i++)
  {
    this->data[i] = obj.data[i];
  }
}
void Basket::alloc(int size1, int size2)
{
  if(this->data)
  {
    delete[] this->data;
  }
  this->size = size1;
  this->data = new Apple[this->size];
  for(int i=0; i<size1; i++)
  {
    this->data[i].alloc(size2);
  }
}
Basket Basket::func(int depth)
{
  Basket new_basket;
  new_basket.alloc(5,5);
  if(depth>1)
  {
    cout << 1 << endl;
    return new_basket.func(depth-1);
  }else
  {
    cout << 2 << endl;
    return new_basket;
  }
}
int main()
{
  Basket basket;
  Basket new_basket = basket.func(1);
  return 0;
}

Answer 1

问题已解决。

错误是由于类Basket的拷贝构造函数实现不当造成的。

我原来的复制构造函数:

Basket::Basket(const Basket &obj)
{
  this->size = obj.size;
  for(int i=0; i<this->size; i++)
  {
    this->data[i] = obj.data[i];
  }
}

正确的拷贝构造函数:

Basket::Basket(const Basket &obj)
{
  this->size = obj.size;
  if(this->data)
  {
      delete[] this->data;
  }
  this->data = new Apple[this->size];
  for(int i=0; i<this->size; i++)
  {
    this->data[i] = obj.data[i];
  }
}