Passing struct pointer through a function then accessing struct data

Question

I'm working on a function that works as part of a larger program. My C pointer skills are a bit rusty and so I need some help here.

Notice: I've omitted a lot of the program because it is irrelevant and works correctly, all you need to know is that I import a header file that declares the struct ImportedStruct, also, I cannot change the function header, so I need to pass the ImportedStruct as a pointer and the function as a pointer.

I get the following error message in VS code: pointer to incomplete class type is not allowed.

Any ideas?

#include "name.h"
long int *functionName(struct ImportedStruct *structName, int *irrelevant){
    int width = structName->width;
    int height = structName->height;

    // Remaining function ...
}

Header file:

struct ImportedStruct 
{ 
  int width, height;
};

Are you sure you included the header? Also, ImportedStruct -> structName. — Neil
– Neil, Commented Feb 23, 2020 at 19:02
My apologies, @Neil I had a typo. I just edited the code, thanks for the heads up! — Iyad
– Iyad, Commented Feb 23, 2020 at 19:05
Also, I'm sure I imported the header correctly, as otherwise I would have had other errors indicating that "ImportedStruct" is not a valid type. But it is in this case, the only problem is that it's not working when I use pointers. — Iyad
– Iyad, Commented Feb 23, 2020 at 19:09
@Iyad: You actually wouldn't get an error that it's not a valid type. You'd get the error that you got, that the type is not defined. The compiler knows that struct ImportedStruct is a type, it has the keyword struct in front of it. — Bill Lynch
– Bill Lynch, Commented Feb 23, 2020 at 19:16

Tarek Dakhran · Accepted Answer · 2020-02-23 19:12:46Z

3

I just compiled the code you provide

struct ImportedStruct 
{ 
  int width, height;
};

long int *functionName(struct ImportedStruct *structName, int *irrelevant){
    int width = structName->width;
    int height = structName->height;

    // Remaining function ...
    return (long int *)(width + height);
}

and it compiled successfully.

Make sure struct definition is included. Also, you can put

struct ImportedStruct 
{ 
  int width, height;
};

right before your function definition. If compiler does not give you multiple definition error, then struct definition is not included by headers.

edited Feb 23, 2020 at 19:12

answered Feb 23, 2020 at 19:03

Tarek Dakhran

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7 Comments

Iyad Over a year ago

Hi Tarek, thank you for your answer. I've just edited my code segment. Can you take a look again?

Tarek Dakhran Over a year ago

Hi @lyad. Edited the answer. Please check if helps.

Iyad Over a year ago

Hi Tarek, thank you so much for your help. You're right, when I compile it with your edits it does work properly. The problem is, I'm importing that struct from another header file. Which I believe I've imported properly (include "name.h") at the top. Do you have any idea what else it could be? Because I need to import the struct and not re-declare it.

Tarek Dakhran Over a year ago

Just track the headers you include that they actually include the header with ImportedStruct definition. It also could be that definition is protected by preprocessor defines.

William Pursell Over a year ago

The existence of the word 'class' in the original error messages suggests to me that you tried to compile it as C++

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