c++ - move 构造函数可以是隐式的吗？

Question

考虑以下类:

class A
{
public:
   std::string field_a;
   std::string field_b;
}

现在考虑以下复制结构:

A a1(a2);

尽管没有显式的复制构造函数，复制构造仍将充分复制 A，因为 std::string 的复制构造函数将由编译器生成的隐式调用复制构造函数。

我想知道的是， move 构造是否也是如此？

编辑:测试here表明:

A a2(std::move(a1));

实际上会导致复制构造，除非特定的 move 构造函数:

A( A && other ) : a(std::move(other.a)) {}

已定义。

编辑编辑我联系了 Stephan T Lavavej，问他为什么 VC 2012 似乎没有遵循 12.8 草案中关于隐式 move 构造函数生成的规定。他好心解释:

It's more of a "feature not yet implemented" than a bug. VC currently implements what I refer to as rvalue references v2.0, where move ctors/assigns are never implicitly generated and never affect the implicit generation of copy ctors/assigns. C++11 specifies rvalue references v3.0, which are the rules you're looking at.

Answer 1

是的，来自 C++11 草案，12.8:

If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declaredas defaulted if and only if

• X does not have a user-declared copy constructor,
• X does not have a user-declared copy assignment operator,
• X does not have a user-declared move assignment operator,
• X does not have a user-declared destructor, and
• the move constructor would not be implicitly defined as deleted.

后面会详细说明最后一个条件:

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted (8.4.3) if X has:

• a variant member with a non-trivial corresponding constructor and X is a union-like class,
• a non-static data member of class type M (or array thereof) that cannot be copied/moved becauseoverload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or afunction that is deleted or inaccessible from the defaulted constructor,
• a direct or virtual base class B that cannot be copied/moved because overload resolution (13.3), asapplied to B’s corresponding constructor, results in an ambiguity or a function that is deleted orinaccessible from the defaulted constructor,
• any direct or virtual base class or non-static data member of a type with a destructor that is deletedor inaccessible from the defaulted constructor,
• for the copy constructor, a non-static data member of rvalue reference type, or
• for the move constructor, a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable.

简单地说， move 构造函数将在以下情况下被隐式声明:

1. 该类没有用户声明的任何其他特殊成员函数。
2. move 构造函数可以通过 move 其所有成员和基数来实现。

你的类(class)显然符合这些条件。