A temporary object is an unnamed C object created implicitly while evaluating an intermediate expression or initialization. It has automatic storage duration, but unlike an ordinary local variable, its lifetime follows the temporary-use context rather than a named block variable's definition. section 13.2.2 section 17.2
A function result used as an intermediate object
struct demo { unsigned ory[1]; };\nstruct demo mem(void);\n\nprintf("mem().ory[0] is %u\\n", mem().ory[0]);\n\nThe call produces a temporary struct value, whose member can be selected immediately. The expression uses that unnamed object without storing it in a named local variable. section 13.2.2
Compound literals provide another important case. In ((T){ (X) }), the compound literal first creates a temporary object of type T and then initializes it with X. This deliberately uses initialization rules to check whether X is assignment-compatible with T; an optimizing compiler can usually remove the temporary's runtime cost. section 17.2
TAKEAWAY
- Treat temporary objects as implicit intermediate values rather than named storage. section 13.2.2
- Use a compound literal when initialization compatibility must be checked without retaining a named object. section 17.2
struct demo { unsigned ory[1]; };
struct demo mem(void);
printf("mem().ory[0] is %u\n", mem().ory[0]);
Inspect the returned-struct example to see a temporary object participate directly in member access. section 13.2.2
