本文学习了微软的官方实例，用于理解 std::move 语义。
#pragma once
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
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 != nullptr)
{
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;
}
// Move constructor.
MemoryBlock(MemoryBlock&& other) noexcept
: _data(nullptr)
, _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 = nullptr;
//other._length = 0;
*this = std::move(other);
}
// Move assignment operator.
MemoryBlock& operator=(MemoryBlock&& other) noexcept
{
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 = nullptr;
other._length = 0;
}
return *this;
}
private:
size_t _length; // The length of the resource.
int* _data; // The resource.
};
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));
}
运行结果：
In MemoryBlock(size_t). length = 25.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In operator=(MemoryBlock&&). length = 25.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 25. Deleting resource.
从输出可以看出，MemoryBlock仅仅初始化了一次，没有调用 Copy constructor 和 Copy assignment operator，只是调用了 Move constructor。而且，在最终释放资源时也没有出现内存的二次释放问题。