Lvalues, Value Categories, and Object Lifetime

Lvalue vs value

An lvalue is an expression that refers to an object — it has an address. A non-lvalue is a value: x is an lvalue; x + 1 is not. Assignment needs an lvalue on the left precisely because storing requires an object to store into.

The subtle corners:

• An array expression decays to a pointer to its first element — the value is an address, not the array object.
• String literals are lvalues (unnamed arrays of char) — you can take their address, but must not modify them.
• A function designator decays to a function pointer.

int x = 1;
int *p = &x;        /* &x: operand must be an lvalue (or function) */
x + 1 = 3;          /* ERROR: not an lvalue */
"hi"[0] = 'H';      /* compiles; UB at runtime: modifying a literal */

Lifetime begins and ends

An object's lifetime — the time during which its representation holds stable values — is fixed by storage duration: automatic objects live their enclosing block; static/allocated objects live until program end or free. Accessing an object outside its lifetime (after the block, after free, before initialization) is UB.

The classic dangling case is not exotic — it is this:

int *bad(void) {
    int local = 7;
    return &local;      /* lifetime ends at the closing brace */
}