CODEWITHSUNDEEP
c++c++11stdmovevalue-categories
user9396941
user9396941

Reputation:

Why does the std::move() work in c++?

Following is the code snippet:

int i=0;
int&&k=std::move(i);

In c++ primer the move is 

template <typename T>
typename remove_reference<T>::type &&move(T&& t)
{return static_cast<typename remove_reference<T>::type&&>(t);}

As far as i know,this std::move template will deduct a function like

int&& move(int& t){return static_cast<int&&>(t);}

As a comparison and to elaborate my question,consider an example like this:

 int test(int k){k=66;return k;}
 int k;
 int a=test(k);

The code above will be compiled as: 

int temp;//the temporary object
temp=k;
int a=temp;

Similarly,i think the first code snippet will be compiled as:

int&& temp=0;
int&& k=temp;//oop!temp is an lvalue!

which seems wrong because temp is an lvalue,did i get something wrong?

Upvotes: 6

Views: 414

Answers (1)

songyuanyao
songyuanyao

Reputation: 172934

Similarly,i think the first code snippet will be compiled as:

int&& temp=0;
int&& k=temp;//oop!temp is an lvalue!

You might be confusing type with value category.

Each C++ expression (an operator with its operands, a literal, a variable name, etc.) is characterized by two independent properties: a type and a value category.

int&&k=std::move(i); and int&& k=temp; are not identical. The return type of std::move is rvalue-reference and then what std::move returns is an rvalue (more precisely it's an xvalue), which could be bound to rvalue-reference. On the other hand temp is a named variable then it's always an lvalue, regardless of whether its type is int, int&, int&& etc. Lvalue can't be bound to rvalue-reference.

More infomations about lvalue:

The following expressions are lvalue expressions:

  • the name of a variable, ...

and xvalue(rvalue):

The following expressions are xvalue expressions:

  • a function call or an overloaded operator expression, whose return type is rvalue reference to object, such as std::move(x);
  • ...
  • a cast expression to rvalue reference to object type, such as static_cast<char&&>(x);

Upvotes: 4

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