我在看下面的例子就移动构造函数/赋值: https://msdn.microsoft.com/en-us/library/dd293665.aspx为什么拷贝构造函数choosen搬过来contrstructor
我已经加入了交换功能,以简化移动构造函数/分配和修改了一些拷贝赋值:
#include <iostream>
#include <vector>
#include <algorithm>
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=(MemoryBlock& other)
{
std::cout << "In operator=(const MemoryBlock&). length = "
<< other._length << ". Copying resource." << std::endl;
swap(*this, other);
return *this;
}
// Retrieves the length of the data resource.
size_t Length() const
{
return _length;
}
// Move constructor.
MemoryBlock(MemoryBlock&& other)
: _data(nullptr)
, _length(0)
{
std::cout << "In MemoryBlock(MemoryBlock&&). length = "
<< other._length << ". Moving resource." << std::endl;
*this = std::move(other);
}
// Move assignment operator.
MemoryBlock& operator=(MemoryBlock&& other)
{
std::cout << "In operator=(MemoryBlock&&). length = "
<< other._length << "." << std::endl;
swap(*this, other);
return *this;
}
void swap(MemoryBlock& first, MemoryBlock& second)
{
using std::swap;
swap(first._length, second._length);
swap(first._data, second._data);
}
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));
}
现在,当我跑,我有以下输出的代码:
In MemoryBlock(size_t). length = 25.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In operator=(MemoryBlock&&). length = 25.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 75.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In operator=(MemoryBlock&&). length = 75.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(MemoryBlock&&). length = 50. Moving resource.
In operator=(MemoryBlock&&). length = 50.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In MemoryBlock(const MemoryBlock&). length = 75. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.
我不明白为什么有时通过移动构造函数调用复制构造函数?
如果我删除了移动构造函数的定义,并宣布它作为默认:
// Move constructor.
MemoryBlock(MemoryBlock&& other) = default;
然后我得到正确的输出:从输出
In MemoryBlock(size_t). length = 25.
In ~MemoryBlock(). length = 25. Deleting resource.
In MemoryBlock(size_t). length = 75.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.
In MemoryBlock(size_t). length = 50.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 75. 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.
(构造函数调用缺少这意味着移动构造函数被使用)
你大胆的错了句子吗?我认为在加粗之前的句子与你答案的其余部分更相关。虽然很好的答案。 – hvd