c++ 使用移动语义来提高 vector 性能

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。

参考：https://learn.microsoft.com/en-us/cpp/cpp/move-constructors-and-move-assignment-operators-cpp?view=msvc-170