13.57能不能详细解释一下呢 about cpp-primer HOT 6 CLOSED

Can you tell me where did you have problem to understand?
e.g...Do you understand the exercise 13.56?

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Hi~ @dangfeng
To fully understand what's going on here, several syntax need to grasp, which are:
1 Lvalue,Rvalue
2 Lvalue reference, Rvalue reference
3 move semantics
4 reference qualifiers for function members
5 recursion
6 this pointer

Which part do you want to discuss in detail?

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Foo(_this).sorted()为什么会调用右值的版本呢？Foo(_this)和Foo ret(*this)有什么区别吗？

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OK. I got where you were stuck.

The statement Foo(*this).sorted(); is equivalent to (Foo(*this)).sorted();. The expression Foo(*this) is an Rvalue. That's why the Rvalue version sorted will be called.

To tell Rvalue from Lvalue, you can go though other parts of CP5. Also you can use a thurm of rule like this:

Any object that can be accessed again is an Lvalue, otherwise it's an Rvalue.

Let us use this rule to compare the two expressions you listed:
Foo(*this)
This expression construct a temporary object of classFoo, but we have no way to access that object.Therefore it's an Rvalue.
Foo ret(*this)
This expression construct a persistent object of classFoo, we can access that object using it's variable name ret. Therefore it's an Lvalue.

Hope it helps.

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明白了。谢谢你！

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No worries.

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