value-category.rst

C++ Value Category

History

• 196x - CPL - Christopher Strachey

  lvalue, rvalue

• 1966 - BCPL

• 1978 - (K&R) C - Dennis Ritchie

  lvalue, not lvalue

• 1986 - early C++ - Bjarne Stroustrup

  lvalue, rvalue

C++

C++ 的 expression 會用兩種性質來做區分， 一個是 type， 另一個是 value category。

其中 C++ 的 Value 又會依照兩種性質來區分， 一個是 identity， 另一個是可不可以被 move。

• has identity: i.e. an address, a pointer, the user can determine whether two copies are identical, etc.
• can be moved from: i.e. we are allowed to leave to source of a "copy" in some indeterminate, but valid state

	Identity	Move
lvalue	O	X	iM
prvalue	X	O	Im
xvalue	O	O	im
not used	X	X	IM
glvalue	O	X/O	i	lvalue or xvalue
rvalue	X/O	O	m	prvalue or xvalue

lvalue   xvalue    prvalue

   \        /\        /
iM  \      /  \      / Im
     \    / im \    /
      \  /      \  /
       \/ i      \/ m

     glvalue   rvalue

   std::move
   +---------+
   |         |
   |         v
 lvalue   xvalue    prvalue

   \        /\        /
iM  \      /  \      / Im
     \    / im \    /
      \  /      \  /
       \/ i      \/ m

     glvalue   rvalue

      std::move
      +---------+
      |         |
      |         v
+-> lvalue   xvalue    prvalue
|
|     \        /\        /
|  iM  \      /  \      / Im
|       \    / im \    /
|        \  /      \  /
|         \/ i      \/ m
|
|       glvalue   rvalue
|
|                   |
+-------------------+
  rvalue reference

• prvalues are the implicit moves
• xvalues are the explicit moves

Examples

std::move : lvalue -> xvalue

#include <utility>

int main() {
    int x = 42;
    std::move(x) = 5566;
    return 0;
}

$ g++ test.cpp
test.cpp: In function ‘int main()’:
test.cpp:5:18: error: using xvalue (rvalue reference) as lvalue
     std::move(x) = 5566;

rvalue reference : prvalue -> lvalue

#include <utility>

int main() {
    int&& x = 42;
    int&& y = x;
    return 0;
}

$ g++ test.cpp
test.cpp: In function ‘int main()’:
test.cpp:5:15: error: cannot bind ‘int’ lvalue to ‘int&&’
     int&& y = x;
               ^
test.cpp:5:11: warning: unused variable ‘y’ [-Wunused-variable]
     int&& y = x;
           ^

rvalue reference : xvalue -> lvalue

#include <utility>

int main() {
    int x = 42;
    int&& y = std::move(x);
    int&& z = y;
    return 0;
}

$ g++ test.cpp
test.cpp: In function ‘int main()’:
test.cpp:6:15: error: cannot bind ‘int’ lvalue to ‘int&&’
     int&& z = y;
               ^
test.cpp:6:11: warning: unused variable ‘z’ [-Wunused-variable]
     int&& z = y;
           ^

Reference

• cppreference - Value categories
• cppreference - C - Value categories
• Bjarne Stroustrup - "New" Value Terminology
• Understanding lvalues and rvalues in C and C++