Lvalue And Rvalue
Once iam sitting here reading about these two types of value and i don't understand the difference. can someone help me out?
Thomas Becker in http://thbecker.net/articles/rvalue_references/section_01.html#section_01
Kilpeläinen's ACCU article: http://accu.org/index.php/journals/227
| I need to remind you of what lvalues and rvalues are in C++. Giving a rigorous definition is surprisingly difficult, but the explanation below is good enough for the purpose at hand. The original definition of lvalues and rvalues from the earliest days of C is as follows: An lvalue is an expression e that may appear on the left or on the right hand side of an assignment, whereas an rvalue is an expression that can only appear on the right hand side of an assignment. For example, int a = 42; int b = 43; // a and b are both l-values: a = b; // ok b = a; // ok a = a * b; // ok // a * b is an rvalue: int c = a * b; // ok, rvalue on right hand side of assignment a * b = 42; // error, rvalue on left hand side of assignment In C++, this is still useful as a first, intuitive approach to lvalues and rvalues. However, C++ with its user-defined types has introduced some subtleties regarding modifiability and assignability that cause this definition to be incorrect. There is no need for us to go further into this. Here is an alternate definition which, although it can still be argued with, will put you in a position to tackle rvalue references: An lvalue is an expression that refers to a memory location and allows us to take the address of that memory location via the & operator. An rvalue is an expression that is not an lvalue. Examples are: // lvalues: // int i = 42; i = 43; // ok, i is an lvalue int* p = &i; // ok, i is an lvalue int& foo(); foo() = 42; // ok, foo() is an lvalue int* p1 = &foo(); // ok, foo() is an lvalue // rvalues: // int foobar(); int j = 0; j = foobar(); // ok, foobar() is an rvalue int* p2 = &foobar(); // error, cannot take the address of an rvalue j = 42; // ok, 42 is an rvalue If you are interested in a rigorous definition of rvalues and lvalues, a good place to start is Mikael Kilpeläinen's ACCU article on the subject. |
Kilpeläinen's ACCU article: http://accu.org/index.php/journals/227
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