当向量增长时如何实施移动语义？

问题描述

我有一个std ::对象的某个类的对象A.该类是非平凡的，并有复制构造函数和定义的移动构造函数。

I have a std::vector of objects of a certain class A. The class is non-trivial and has copy constructors and move constructors defined.

std::vector<A>  myvec;

如果我用A对象填充向量（使用例如 myvec。 push_back（a）），向量将使用复制构造函数 A（const A&）来实例化元素的新副本在向量中。

If I fill-up the vector with A objects (using e.g. myvec.push_back(a)), the vector will grow in size, using the copy constructor A( const A&) to instantiate new copies of the elements in the vector.

我可以以某种方式强制执行 A 类的move构造函数，而不是使用beging？

Can I somehow enforce that the move constructor of class A is beging used instead?

推荐答案

你需要通知C ++（具体来说 std :: vector ）你的移动构造函数和析构函数不抛出，使用 noexcept 。然后移动构造函数将在向量增长时被调用。

You need to inform C++ (specifically std::vector) that your move constructor and destructor does not throw, using noexcept. Then the move constructor will be called when the vector grows.

这是如何声明和实现std :: vector遵循的移动引擎：

This is how to declare and implement a move constuctor which is respected by std::vector:

A(A && rhs) noexcept {
  std::cout << "i am the move constr" <<std::endl;
  ... some code doing the move ...
  m_value=std::move(rhs.m_value) ; // etc...
}

如果构造函数不是 noexcept ，std :: vector不能使用它，因为它不能确保标准要求的异常保证。

If the constructor is not noexcept, std::vector can't use it, since then it can't ensure the exception guarantees demanded by the standard.

有关标准中所述内容的详情，请参阅
C ++移动语义和异常

For more about what's said in the standard, readC++ Move semantics and Exceptions

对Bo的信任暗示了它可能与例外有关。也请遵循Kerrek SB的建议，并尽可能使用 emplace_back 。

Credit to Bo who hinted that it may have to do with exceptions. Also follow Kerrek SB's advice and use emplace_back when possible.

编辑默认是你想要的：移动可以移动的一切，复制其余。要明确要求，请写入

Edit, often the default is what you want: move everything that can be moved, copy the rest. To explicitly ask for that, write

A(A && rhs) = default;

这样做的话，你可能会得到noexcept：

Doing that, you will get noexcept when possible: Is the default Move constructor defined as noexcept?

请注意，早期版本的Visual Studio 2015和更早版本不支持，即使它支持移动语义。

Note that early versions of Visual Studio 2015 and older did not support that, even though it supports move semantics.

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