Value categories in C++
rvalue, lvalue and the others explained
Historically, when there were only two categories: rvalue and lvalue the names resembled the place in an assignation related to the equal sign, it can be either on the left or on the right.
Furthermore, since C++11 there 3 more categories summing up to five.
lvalue
An lvalue(left value) is an expression which refers to a location in memory and may be assigned a value to. Long story short, any kind of expression that may be found on the right side of an assignment is an lvalue.
int x = 12;
int a[] = {5, 2, 4, 10, 2};
x = 30; // x is a left value
a[4] = 6;
*(a+1) = 7; // *(a+1) is left value
// a[] = {5, 7, 4, 10, 6}rvalue
An rvalue(right value) is any expression which may lay on the right side of the equal sign from an assignment operation.
const int s = 10;
int q = s + 2; // 's + 2' is an rvalue
int w = 3 * 4; // 3*4 is an rvalue
int e = min(q, w); // min(q, w) is an rvalue
int r = max(q, w); // max(q, w) is an rvaluexvalue
The name xvalue(expiring value) comes from expiring because of its temporarily state of being. Case in point, function return value or an object’s method.
int age(){
return 14;
}
char firstLetter() {
return 'c';
}
// main:
a = age(); // xvalue
c = firstLetter(); // xvalueglvalue
A glvalue is a generalised value, which is either an lvalue or an xvalue. How is that possible?
int&& foo() {
int x = 10;
return std::move(x);
}
int&& result = foo(); // this is an xvalue, thus also a glvalue
int a = 5;
int* p = &a;
*p = 6; // this is an lvalue, thus also a glvalueprvalue
A prvalue is a pure value and explicitly is a rvalue which is not an xvalue. For instance, a variable may be must be assigned with a literal like 5, or 54.23 or 3.14 or ‘c’ or This is a string” or with a function call which is temporarily stored and then wiped out. An xvalue is an rvalue which is a function call, but a prvalue is a rvalue which is a literal.
int a = 12; // 12 is a prvalue
string name = "Thomas"; // "Thomas" is a prvalueStory end
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