#include <string>
#include <iostream>
#include <utility>
struct A {
std::string s;
A() : s("test") {}
A(const A& o) : s(o.s) { std::cout << "move failed!\n"; }
A(A&& o) : s(std::move(o.s)) {}
A& operator=(const A&) { std::cout << "copy assigned\n"; return *this; }
A& operator=(A&& other) {
s = std::move(other.s);
std::cout << "move assigned\n";`enter code here`
return *this;
}
};
A f(A a) { return a; }
struct B : A {
std::string s2;
int n;
// implicit move assignment operator B& B::operator=(B&&)
// calls A's move assignment operator
// calls s2's move assignment operator
// and makes a bitwise copy of n
};
struct C : B {
~C() {}; // destructor prevents implicit move assignment
};
struct D : B {
D() {}
~D() {}; // destructor would prevent implicit move assignment
//D& operator=(D&&) = default; // force a move assignment anyway
};
int main()
{
A a1, a2;
std::cout << "Trying to move-assign A from rvalue temporary\n";
a1 = f(A()); // move-assignment from rvalue temporary
std::cout << "Trying to move-assign A from xvalue\n";
a2 = std::move(a1); // move-assignment from xvalue
std::cout << "Trying to move-assign B\n";
B b1, b2;
std::cout << "Before move, b1.s = \"" << b1.s << "\"\n";
b2 = std::move(b1); // calls implicit move assignment
std::cout << "After move, b1.s = \"" << b1.s << "\"\n";
std::cout << "Trying to move-assign C\n";
C c1, c2;
c2 = std::move(c1); // calls the copy assignment operator
std::cout << "Trying to move-assign D\n";
D d1, d2;
// d2 = std::move(d1);
}
执行
a2 = std::move(a1)
语句时,其行为与执行b2 = std::move(b1)
语句的行为不同。在下面的语句中,移动操作后b1.s
并未变为空,而a1.s
变为空。有人可以解释一下这到底发生了什么吗?
std::string
实现的副作用,它不符合 C++11 标准。无论如何,对于已移动的对象,除了处于有效但未指定状态之外,没有任何保证,因此库可能将移动赋值实现为调用swap
,这会使源包含目标包含的任何内容,或者它可能将其实现为调用assign
(或类似的东西),这会使源字符串为空。 - Praetorian