警告：带有值的“返回”，在函数中返回void return next; ex19.c：95：10：错误：无效值不应该被忽略，因为它应该被忽略

Question

I am going through the Learn Code the hard way book and stuck on exercise 19 http://c.learncodethehardway.org/book/ex19.html . I got desperate, copied and pasted the code, yet still I am getting errors:

cc -Wall    -g    ex19.c object.o   -o ex19

ex19.c: In function ‘Room_move’:

ex19.c:65:5: warning: ‘return’ with a value, in function returning void
     return next;

     ^
ex19.c: In function ‘Map_move’:

ex19.c:95:10: error: void value not ignored as it ought to be
     next = location->_(move)(location, direction);

          ^
ex19.c: In function ‘Room_attack’:

ex19.c:145:5: warning: initialization from incompatible pointer type
     .move = Map_move,

     ^
ex19.c:145:5: warning: (near initialization for ‘MapProto.move’)

ex19.c:199:5: warning: ‘main’ is normally a non-static function [-Wmain]
 int main(int argc, char *argv[])

     ^
ex19.c:214:1: error: expected declaration or statement at end of input
 }
 ^
ex19.c:214:1: error: expected declaration or statement at end of input

ex19.c:214:1: warning: control reaches end of non-void function [-Wreturn-type]
 }

This is what I got:

void Room_move(void *self, Direction direction)
{
    Room *room = self;
    Room *next = NULL;

    if(direction == NORTH && room->north) {
        printf("You go north, into:\n");
        next = room->north;
    } else if(direction == SOUTH && room->south) {
        printf("You go south, into:\n");
        next = room->south;
    } else if(direction == EAST && room->east) {
        printf("You go east, into:\n");
        next = room->east;
    } else if(direction == WEST && room->west) {
        printf("You go west, into:\n");
        next = room->west;
    } else {
        printf("You can't go that direction.");
        next = NULL;
    }

    if(next) {
        next->_(describe)(next);
    }

    return next;
}

void *Map_move(void *self, Direction direction)
{
    Map *map = self;
    Room *location = map->location;
    Room *next = NULL;

    next = location->_(move)(location, direction);

    if(next) {
        map->location = next;
    }

    return next;
}

int main(int argc, char *argv[])
{
    // simple way to setup the randomness
    srand(time(NULL));

    // make our map to work with
    Map *game = NEW(Map, "The Hall of the Minotaur.");

    printf("You enter the ");
    game->location->_(describe)(game->location);

    while(process_input(game)) {
    }

    return 0;
}

Answer 1

This line:

void Room_move(void *self, Direction direction)

should be:

void *Room_move(void *self, Direction direction)

This is the case on the page you linked, so you messed up while copy/pasting the code.

Answer 2

You're missing up a pointer variable while specifying the return type for Room_move() function. You need to use

void* Room_move(void *self, Direction direction)

instead of void Room_move(void *self, Direction direction) .

void* Room_move() makes the return type as void * , but void Room_move() essentially means a void return. Side effect:

1. warning: 'return' with a value, in function returning void

2. error: void value not ignored as it ought to be

So, as you can see, a void and void* are not the same.