如何：撰寫移動建構函式

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06/09/2015

本主題說明如何撰寫 C++ 類別的「移動建構函式」(move constructor) 和移動指派運算子。移動建構函式可讓您實作移動語意，大幅改善應用程式的效能。如需有關移動語意的詳細資訊，請參閱右值參考宣告子：&&。

這個主題是以下列管理記憶體緩衝區的 C++ 類別 MemoryBlock 為基礎。

// MemoryBlock.h
#pragma once
#include <iostream>
#include <algorithm>

class MemoryBlock
{
public:

   // Simple constructor that initializes the resource.
   explicit MemoryBlock(size_t length)
      : _length(length)
      , _data(new int[length])
   {
      std::cout << "In MemoryBlock(size_t). length = "
                << _length << "." << std::endl;
   }

   // Destructor.
   ~MemoryBlock()
   {
      std::cout << "In ~MemoryBlock(). length = "
                << _length << ".";
      
      if (_data != NULL)
      {
         std::cout << " Deleting resource.";
         // Delete the resource.
         delete[] _data;
      }

      std::cout << std::endl;
   }

   // Copy constructor.
   MemoryBlock(const MemoryBlock& other)
      : _length(other._length)
      , _data(new int[other._length])
   {
      std::cout << "In MemoryBlock(const MemoryBlock&). length = " 
                << other._length << ". Copying resource." << std::endl;

      std::copy(other._data, other._data + _length, _data);
   }

   // Copy assignment operator.
   MemoryBlock& operator=(const MemoryBlock& other)
   {
      std::cout << "In operator=(const MemoryBlock&). length = " 
                << other._length << ". Copying resource." << std::endl;

      if (this != &other)
      {
         // Free the existing resource.
         delete[] _data;

         _length = other._length;
         _data = new int[_length];
         std::copy(other._data, other._data + _length, _data);
      }
      return *this;
   }

   // Retrieves the length of the data resource.
   size_t Length() const
   {
      return _length;
   }

private:
   size_t _length; // The length of the resource.
   int* _data; // The resource.
};

下列程序說明如何撰寫 C++ 類別的移動建構函式和移動指派運算子。

建立 C++ 類別的移動建構函式

定義空的建構函式方法，該方法會接受以類別類型的右值參考做為其參數，如下列範例所示範：
```
MemoryBlock(MemoryBlock&& other)
   : _data(NULL)
   , _length(0)
{
}
```
在移動建構函式中，將類別資料成員從來源物件指派給將建構的物件：
```
_data = other._data;
_length = other._length;
```
將來源物件的資料成員指派為預設值。這樣可防止解構函式多次釋放資源 (例如記憶體)：
```
other._data = NULL;
other._length = 0;
```

建立 C++ 類別的移動指派運算子

定義空的指派運算子，該運算子會接受以類別類型的右值參考做為其參數，並傳回對類別類型的參考，如下列範例所示範：
```
MemoryBlock& operator=(MemoryBlock&& other)
{
}
```
在移動指派運算子中，如果您嘗試將物件指派給其本身，請新增不執行任何作業的條件陳述式。
```
if (this != &other)
{
}
```

在條件陳述式中，從所指派的物件釋放所有資源 (例如記憶體)。

下列範例會從所指派的物件釋放 _data 成員：

// Free the existing resource.
delete[] _data;

依照第一個程序的步驟 2 和步驟 3，將資料成員從來源物件傳送至將建構的物件：

// Copy the data pointer and its length from the 
// source object.
_data = other._data;
_length = other._length;

// Release the data pointer from the source object so that
// the destructor does not free the memory multiple times.
other._data = NULL;
other._length = 0;

傳回目前物件的參考，如下列範例所示範：
```
return *this;
```

範例

下列範例示範 MemoryBlock 類別的完整移動建構函式和移動指派運算子：

// Move constructor.
MemoryBlock(MemoryBlock&& other)
   : _data(NULL)
   , _length(0)
{
   std::cout << "In MemoryBlock(MemoryBlock&&). length = " 
             << other._length << ". Moving resource." << std::endl;

   // Copy the data pointer and its length from the 
   // source object.
   _data = other._data;
   _length = other._length;

   // Release the data pointer from the source object so that
   // the destructor does not free the memory multiple times.
   other._data = NULL;
   other._length = 0;
}

// Move assignment operator.
MemoryBlock& operator=(MemoryBlock&& other)
{
   std::cout << "In operator=(MemoryBlock&&). length = " 
             << other._length << "." << std::endl;

   if (this != &other)
   {
      // Free the existing resource.
      delete[] _data;

      // Copy the data pointer and its length from the 
      // source object.
      _data = other._data;
      _length = other._length;

      // Release the data pointer from the source object so that
      // the destructor does not free the memory multiple times.
      other._data = NULL;
      other._length = 0;
   }
   return *this;
}

下列範例示範移動語意如何改善應用程式的效能。此範例會在向量物件中加入兩個元素，然後在兩個現有元素之間插入新的元素。在 Visual C++ 2010 中，vector 類別會使用移動語意，透過移動向量元素而非複製向量元素的方式，有效率地執行插入作業。

// rvalue-references-move-semantics.cpp
// compile with: /EHsc
#include "MemoryBlock.h"
#include <vector>

using namespace std;

int main()
{
   // Create a vector object and add a few elements to it.
   vector<MemoryBlock> v;
   v.push_back(MemoryBlock(25));
   v.push_back(MemoryBlock(75));

   // Insert a new element into the second position of the vector.
   v.insert(v.begin() + 1, MemoryBlock(50));
}

這個範例會產生下列輸出：

In MemoryBlock(size_t). length = 25.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 75.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In ~MemoryBlock(). length = 0.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(MemoryBlock&&). length = 50. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 50. Moving resource.
In operator=(MemoryBlock&&). length = 75.
In operator=(MemoryBlock&&). length = 50.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.

在 Visual C++ 2010 之前，這個範例會產生下列輸出：

In MemoryBlock(size_t). length = 25.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In MemoryBlock(size_t). length = 75.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In MemoryBlock(const MemoryBlock&). length = 75. Copying resource.
In ~MemoryBlock(). length = 75. Deleting resource.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(const MemoryBlock&). length = 50. Copying resource.
In MemoryBlock(const MemoryBlock&). length = 50. Copying resource.
In operator=(const MemoryBlock&). length = 75. Copying resource.
In operator=(const MemoryBlock&). length = 50. Copying resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.

這個使用移動語意的範例版本比不使用移動語意的版本更有效率，因為前者執行較少的複製、記憶體配置和記憶體解除配置作業。

穩固程式設計

為避免資源流失，請一律釋放移動指派運算子中的資源 (例如記憶體、檔案控制代碼和通訊端)。

為避免解構資源後無法復原，請適當地處理移動指派運算子中的自我指派。

如果您同時為類別提供移動建構函式和移動指派運算子，可以撰寫移動建構函式來呼叫移動指派運算子，藉此去除冗餘碼。下列範例示範呼叫移動指派運算子的修訂版移動建構函式。

// Move constructor.
MemoryBlock(MemoryBlock&& other)
   : _data(NULL)
   , _length(0)
{
   *this = std::move(other);
}

std::move 函式會保留 other 參數的右值屬性。

請參閱

參考

右值參考宣告子：&&

其他資源

<utility> move

共用方式為

如何：撰寫移動建構函式

建立 C++ 類別的移動建構函式

建立 C++ 類別的移動指派運算子

範例

穩固程式設計

請參閱

參考

其他資源

其他資源