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移动构造函数理解

移动构造函数是C++11标准引入的机制，VS2010后面的版本支持，要对比优势就要安装两个IDE进行对比。

移动构造函数里面的移动很有意思，其实是交给的意思，把参数里的值交给构造对象，参数里的数据清空。

参数的类型一定要是右值引用，不是右值引用不行，右值是一种数值态，这种状态才能做到交给。

初学者不理解，可以有代码运行输出来理解，MSDN上有例子，可以搜索到如下调用例子：

// 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));
}

这类代码涉及到很多临时对象，临时对象又涉及到构造和销毁，所以更体现了移动构造函数的使用好处。

下面是体现优化的日志输出：

In MemoryBlock(size_t). length = 25.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In ~MemoryBlock(). length = 0.//这里优化了
In MemoryBlock(size_t). length = 75.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(MemoryBlock&&). length = 50. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In ~MemoryBlock(). length = 0.//这里优化了
In ~MemoryBlock(). length = 0.//这里优化了
In ~MemoryBlock(). length = 0.//这里优化了
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.

下面是性能差的日志：

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.