Exact difference between rvalue and lvalue

Question

While I was reading http://thbecker.net/articles/rvalue_references/section_01.html, I got following snippiest.

// lvalues:
//
int i = 42;
i = 43; // 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

Why int* p2 = &foobar(); is error statement, while int* p1 = &foo(); is not an error. How later one is lvalue while first one is rvalue?

Answer 1

So we have two functions:

int& foo();
int foobar();

• foo is a function returning lvalue-reference to int
• foobar is a function returning int

The function call expressions:

foobar()
foo()

both have type int (references are removed from expressions, so foo() has type int and not lvalue-reference to int). The two expressions have different value categories:

• foobar() is a prvalue (a function call to a function returning a non-reference is a prvalue)
• foo() is an lvalue (a function call to a function returning an lvalue-reference is an lvalue)

You can't take the address of an rvalue (a prvalue is a kind of rvalue), so &foobar() is not allowed.

You can take the address of an lvalue so &foo() is allowed.