how to use pointer notation to access a dynamic struct

Question

I'm confused on how to use pointer notation to access a struct used with malloc. I can use array notation but what's the syntax on pointer?

  #include "stdio.h"
#include "stdlib.h"

using namespace std;

typedef struct flightType  {

    int altitude;
    int longitude;
    int latitude;
    int heading;

    double airSpeed;

} Flight;

int main() {

    int airbornePlanes;
    Flight *planes;

    printf("How many planes are int he air?");
    scanf("%d", &airbornePlanes);

    planes =  (Flight *) malloc ( airbornePlanes * sizeof(Flight));
    planes[0].altitude = 7;  // plane 0 works

    // how do i accessw ith pointer notation
    //*(planes + 1)->altitude = 8; // not working
    *(planes + 5) -> altitude = 9;
    free(planes);

}

I'm receving error - main.cpp|34|error: request for member 'altitude' in '(planes + 120u)', which is of non-class type 'Flight*'| ||=== Build finished: 1 errors, 0 warnings ===| — runners3431
– runners3431, Commented Oct 23, 2012 at 16:57
If this is a c code, please use stdio instead of iostream and namespace. Those are of C++. — halfelf
– halfelf, Commented Oct 23, 2012 at 16:57

johv · Accepted Answer · 2012-10-23 17:32:16Z

2

Basically x->y is shorthand for (*x).y.

The following are equivalent:

(planes + 5) -> altitude = 9

and

(*(planes + 5)).altitude = 9;

and

planes[5].altitude = 9;

A more concrete example:

Flight my_flight; // Stack-allocated
my_flight.altitude = 9;

Flight* my_flight = (Flight*) malloc(sizeof(Flight)); // Heap-allocated
my_flight->altitude = 9;

edited Oct 23, 2012 at 17:32

answered Oct 23, 2012 at 16:54

johv

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7 Comments

runners3431 Over a year ago

When do i use -> or periods?

runners3431 Over a year ago

When i use *(planes +5) - i'm getting - |error: invalid type argument of 'unary *'| ||=== Build finished: 1 errors, 0 warnings ===|

johv Over a year ago

I'm sorry, I missed a pair of parentheses. If you think that's ugly, then you know why you got the -> syntax ;)

alk Over a year ago

@StarPilot Ooh, this is not correct! You might like to read here: stackoverflow.com/q/394767/694576

johv Over a year ago

@StarPilot Pointer arithmetics would take the size of a Flight struct into account.

|

DrummerB · Accepted Answer · 2012-10-23 16:54:37Z

1

You don't need the -> notation for this, because you are already dereferencing the pointer using the asterisk. Simply do:

*(places + 5).altitude = 5;

The -> is shorthand for "dereference this struct pointer and access that field", or:

(*myStructPointer).field = value;

is the same as

myStructPointer->field = value;

You can use either notation, but (should) not both at the same time.

answered Oct 23, 2012 at 16:54

DrummerB

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Comments

shinkou · Accepted Answer · 2012-10-23 17:06:40Z

0

You need an extra parenthesis after dereferencing the pointer.

(*(planes + 5)).altitude = 9;

answered Oct 23, 2012 at 17:06

shinkou

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