Problems with structs and pointers

[Mattz333]

For a project I need to have a vector array of pointers to a struct, I have no problem with the vector array, but when I try to set the values of member in the struct the program crashes. Here is some basic code that I am having the problem with:

struct point  // grid point
{
int x;
int y;
int z;
};

 int main()
{

point* new_point;
new_point->x = 5;  // Crashes on this line, Segmentation Fault

return 0;
}

I tried looking this up but I found examples that were also using the -> operator to set the values (i have not tried copy-pasting the examples yet).

Anyone have any ideas of what could be going wrong/ any alternate solutions?

[jA_cOp]

You need to create the struct, not just the pointer to it.
[cpp]
point* new_point;
[/cpp]
This only creates an [I]uninitialized[/I] pointer.
[cpp]
point new_point;
[/cpp]

This creates a new [I]uninitialized[/I] struct on the stack. As this is uninitialized, you need to set all its members:
[cpp]
point new_point;
new_point.x = 1;
new_point.y = 2;
new_point.z = 3;
/*now fully initialized*/
[/cpp]

If you want to create an uninitialized struct on the heap so you can use it outside the current scope, you can do:
[cpp]
point* new_point = malloc(sizeof(*new_point));

//or, preferably, since you're using C++,
point* new_point = new point();
[/cpp]

In C++, you can do the initialization in a constructor.

[gparent]

You haven't allocated any memory to the struct, you're dereferencing invalid memory.

C: point* new_point = malloc(sizeof(point));
C++: point* new_point = new point;

[Jallen]

Short answer.
[cpp]point* new_point = new point();
new_point->x = 5;[/cpp]
Your pointer wasn't pointing to anything. (well, anything valid..)

[Mattz333]

Ah I see what's going on there. Thanks :)

[PvtCupcakes]

I thought you had to declare struct objects like:

struct point *new_point;

Is that only a C thing?

[jA_cOp]

[QUOTE=PvtCupcakes;16624495]I thought you had to declare struct objects like:

struct point *new_point;

Is that only a C thing?[/QUOTE]

You can do that in both C and C++, but if you want to use it without the "struct" keyword in C you need to typedef it:
[cpp]
struct A_t
{
...
};
typedef struct A_t A;

/*or simply;*/
typedef struct
{
...
} A;
[/cpp]

[QUOTE=Jallen;16623238]Short answer.
[cpp]point* new_point = new point();
new_point->x = 5;[/cpp]
Your pointer wasn't pointing to anything. (well, anything valid..)[/QUOTE]

Poor timing for a "short answer" really, as he should be using a constructor in that case...

[Mattz333]

It's all working now, but after closing the SFML window and closing the console output, I'm getting a memory read error from somewhere. Even though the broken line appears to be AFTER the 'return EXIT_SUCCESS'. Very confusing indeed.

[jA_cOp]

[QUOTE=Mattz333;16627428]Even though the broken line appears to be AFTER the 'return EXIT_SUCCESS'. Very confusing indeed.[/QUOTE]

You should really confirm that.

[nos217]

[QUOTE=Mattz333;16627428]It's all working now, but after closing the SFML window and closing the console output, I'm getting a memory read error from somewhere. Even though the broken line appears to be AFTER the 'return EXIT_SUCCESS'. Very confusing indeed.[/QUOTE]

Why did you give up SDL Matt. Why oh why.

[jA_cOp]

[QUOTE=nos217;16627680]Why did you give up SDL Matt. Why oh why.[/QUOTE]

SDL is in C, he's using C++.

Choosing SFML is perfectly natural, and should improve his productivity quite a bit :)

[nos217]

Yeah you're right. I just don't want to change from SDL now though.

Hey Matt, making anything interesting?

[Mattz333]

After seeing all those Isometric projects in the 'What are you working on' thread, I decided to try and make an isometric terrain editor that draws everything with polygons.

Also just tried running it again and the error isn't coming up anymore.

[Jallen]

[QUOTE=Mattz333;16642331]Also just tried running it again and the error isn't coming up anymore.[/QUOTE]

That doesn't mean it won't come back.
You were probably just lucky this time around running it. Runtime errors are never consistent, especially ones to do with memory management.

[Shammah]

Nobody even bothered about delete?
Jeez

Anyway, at the end of the program don't forget to

delete new_point;

Or you'll be having lots of memory leaks in the future.

[Agent766]

I'm not very good with pointers, but don't you have to set it to null or something before deleting it?

[gparent]

[QUOTE=Agent766;16646957]I'm not very good with pointers, but don't you have to set it to null or something before deleting it?[/QUOTE]
No, otherwise how could delete know where to delete?
You can set it to NULL after you use delete, but that's not necessary. It can help avoid errors.

[Jallen]

[QUOTE=Agent766;16646957]I'm not very good with pointers, but don't you have to set it to null or something before deleting it?[/QUOTE]

Lol no.
I am pointing at a random space.
I now point at this new item I have created.
I'm finished with it, I stop pointing at it.
I delete it... wait, I'm not pointing to it WHERE IS IT!??

Point the pointer to 0 [I]after[/I] deleting it.

:argh: gparent! :ninja: