Compound literals (since C99)

#include <assert.h>
 
int main(void)
{
 struct S
 {
 int i;
 }
 *p = 0, *q;
 int j = 0;
again:
 q = p,
 p = &((struct S){ j++ }); // creates an unnamed object of type S,
 // initializes it to the value formerly
 // held in j, then assigns the address
 // of this unnamed object to the pointer p
 if (j < 2)
 goto again; // note: if a loop were used, it would end scope here,
 // which would terminate the lifetime of the compound
 // literal leaving p as a dangling pointer
 assert (p == q && q->i == 1);
}

#include <stdio.h>
 
int *p = (int[]){2, 4}; // creates an unnamed static array of type int[2]
 // initializes the array to the values {2, 4}
 // creates pointer p to point at the first element of
 // the array
const float *pc = (const float []){1e0, 1e1, 1e2}; // read-only compound literal
 
struct point {double x,y;};
 
int main(void)
{
 int n = 2, *p = &n;
 p = (int [2]){*p}; // creates an unnamed automatic array of type int[2]
 // initializes the first element to the value formerly
 // held in *p
 // initializes the second element to zero
 // stores the address of the first element in p
 
 void drawline1(struct point from, struct point to);
 void drawline2(struct point *from, struct point *to);
 drawline1(
 (struct point){.x=1, .y=1}, // creates two structs with block scope and
 (struct point){.x=3, .y=4}); // calls drawline1, passing them by value
 drawline2(
 &(struct point){.x=1, .y=1}, // creates two structs with block scope and
 &(struct point){.x=3, .y=4}); // calls drawline2, passing their addresses
}
 
void drawline1(struct point from, struct point to)
{
 printf ("drawline1: `from` @ %p {%.2f, %.2f}, `to` @ %p {%.2f, %.2f}\n",
 (void*)&from, from.x, from.y, (void*)&to, to.x, to.y);
}
 
void drawline2(struct point *from, struct point *to)
{
 printf ("drawline2: `from` @ %p {%.2f, %.2f}, `to` @ %p {%.2f, %.2f}\n",
 (void*)from, from->x, from->y, (void*)to, to->x, to->y);
}

drawline1: `from` @ 0x7ffd24facea0 {1.00, 1.00}, `to` @ 0x7ffd24face90 {3.00, 4.00}
drawline2: `from` @ 0x7ffd24facec0 {1.00, 1.00}, `to` @ 0x7ffd24faced0 {3.00, 4.00}