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c++ - C++ std::move() performance wise with lvalues

问题描述

I've just finished learning about lvalues, rvalues, move constructor and move operator. I can see the added value of using move constructor with rvalues performance and usability wise. But i can't see the added value of using move operator with move constructor with lvalues performance wise, sure there is a added value usability wise, but can't we achieve the same functionality for lvalues using some other technologies like pointers for example. so my question is: what is the added value of using move operator with move constructor with lvalues performance wise. Thanks for reading my question.

example:

class string{
public:
char* data;

string(){
    data = nullptr;
}

string(const char* p)
{
    size_t size = std::strlen(p) + 1;
    data = new char[size];
    std::memcpy(data, p, size);
}

 ~string()
{
    delete[] data;
}

string(const string& that)
{
    size_t size = std::strlen(that.data) + 1;
    data = new char[size];
    std::memcpy(data, that.data, size);
}

string(string&& that)
{
    data = that.data;
    that.data = nullptr;
}

string movee(string &that){
    data = that.data;
    that.data = nullptr;
}};

what is the difference performence wise:

string s1("test");
string s2(std::move(s1));

string s1("test");
string s2 = string();
s2.movee(s1);

In the case of rvalues, the compiler spares us the time and memory taken to reform a new object and assign the rvalue to it, and then using that new object to change the values in the moved-to object, thus increasing performance. But in the case of a lvalue object, the move operator is not increasing performance, it is increasing usability and readability of course, but it is not increasing the performance as if it was a rvalue, am I wrong?

标签： c++performancec++11stdmove

解决方案

您可以使用带有指针的间接来实现相同的功能。事实上，我们曾经，甚至现在，这正是您的移动构造函数/赋值运算符内部发生的事情！

是的，变量的大部分好处std::move在于代码的清洁度。但这并非毫无意义。使用移动构造函数/赋值运算符窃取资源使我们能够巧妙、快速地完成此操作，而不会受到黑客攻击或任何性能损失。您的代码将更易于维护且更易于合理化。您将不太想添加一个额外的间接层，动态分配不需要动态分配的东西，只是为了获得任何可以理解的代码，因为现在这样做没有任何好处。

但！不要忘记您的标准容器在幕后为您执行此操作，例如在std::vector调整大小时。这是非常有价值的。如果我们只能从临时人员那里搬走，那将是不可能的，我们将浪费大量机会。

用临时工做这件事并没有什么不同。只是它为您完成而无需键入std::move。

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