What do they mean by having identity but not movable for Lvalue in C++ 11?

Question

I am now studying C++ 11 and getting confused by value category of expressions in C++ 11. According to terminology the Lvalue is the top-left point of the W, that is iM (or i-m sometimes) meaning that "has identity but cannot be moved from". This really makes me confused. Please consider the example below:

#include <iostream>

int main()
{
    int a = 0, b = 1, c = 2;
    a = std::move(b = c);
    std::cout << a << '\n';
}

This example compiled well.

We all know that the assignment b = c is an Lvalue then what do they mean by 'cannot be moved from'? Please give examples that can clearly illustrate this!

Thanks!

[why is std move named so](https://stackoverflow.com/questions/21358432/why-is-stdmove-named-stdmove) — Caleth, May 31 '17 at 15:57

score 5 · Accepted Answer · answered May 31 '17 at 15:58

Roughly speaking:

an lvalue has identity because you can take its address
```
int x;
&x; // ok
```
an lvalue cannot be moved from because it cannot be used as an argument to move constructor/assignment
```
struct Foo 
{
    Foo(Foo&);  // 0
    Foo(Foo&&); // 1
};

Foo x;
Foo y{x}; // calls 0, not 1
```
in the example above, x is an lvalue: the copy constructor is invoked. If you want to move from x, you need to make it an rvalue: this is why you must use std::move(x) to cast it to an rvalue reference.
```
Foo y{std::move(x)}; // calls 1
```

In your example std::move(b = c) is an rvalue, as std::move is literally just a static_cast to an rvalue reference.

Can you please provide an example that should show compile errors due to "cannot be moved from"? — duong_dajgja, May 31 '17 at 16:03

What do they mean by having identity but not movable for Lvalue in C++ 11?

1 Answers1