c++ - 返回不能按值复制的对象

Question

我有一个对象，其复制操作太慢，所以我决定删除它并强制用户只能移动。这个对象的拷贝无论如何都没有多大意义。但是后来我有了这个功能:

Object loadFromFile(const std::string& name) {
    Object obj;
    ...
    return obj;
}

即使这里发生了复制省略并且没有调用复制构造函数，这也无法编译，因为复制构造函数需要存在并且可以访问。这是我的第二次尝试:

Object&& loadFromFile(const std::string& name) {
    Object obj;
    ...
    return std::move(obj);
}

这编译。耶!

但是在尝试使用它时出现了一个新问题:

Object x = loadFromFile("test.txt");

这又需要一个复制构造函数。即使明确使用移动，我也无法让它工作:

Object x = std::move(loadFromFile("test.txt"));

我唯一的解决方案是:

const Object& x = loadFromFile("test.txt");

但是 x 必须是非常量，因为它稍后会被更改。

如何处理？

Answer 1

这个是错误的:

Object&& loadFromFile(const std::string& name) {
    Object obj;
    ...
    return std::move(obj);
}

您正在返回对局部变量的引用，这是未定义的行为。对象死了，你返回的是对你 Nose 的引用，所以恶魔可以从中出来。

第二个是对的:

Object loadFromFile(const std::string& name) {
    Object obj;
    ...
    return obj;
}

事实上，在这种情况下，首先执行查找，就好像 obj 是一个右值(标准 12.8.32):

When the criteria for elision of a copy/move operation are met, but not for an exception-declaration, and the object to be copied is designated by an lvalue, or when the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object’s type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [ Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided. —end note ]

并且应该选择移动构造函数Object(Object&&)。