L/R Values in C

Every expression in C and C++ is either an l-value or an r-value. An l-value is an expression that designates (refers to) an object. Every l-value is, in turn, either modifiable or non-modifiable. An r-value is any expression that isn't an l-value. Operationally, the difference among these kinds of expressions is this:
The address of each l-value is known at compile time and is where the variable will be kept at runtime, i.e., a modifiable lvalue is addressable (can be the operand of unary &) and assignable (can be the left operand of =). So if the compiler needs to do something with an address (add an offset to it, perhaps), it can do that directly and does not need to plant code to retrieve the address first. In contrast, an r-value is neither addressable nor assignable. For example, the followings are typical r-values and could not be modified.
- function return value
- the result of ? :

A non-modifiable l-value is addressable, but not assignable, like pointer to const value or the array name.