Compile error when retrieving a value from a struct in C

Question

I am using C++Builder XE4. I am trying to compile some C code into a console application. The C file is large so I have tried to focus on the problem. The code sets up two structures and then tries to call a value which is failing.

struct ephloc
{
  long first_item_ordinal;
  long last_item_ordinal;
  int days_per_record;
  int items_per_record;
  int total_records;
};

struct ephloc objs[15] = {
  {    641, 2210500,  8, 44, 50224},  
  {2210501, 3014088, 16, 32, 25112},  
  {3014089, 4043684, 16, 41, 25112},  
  {4043685, 4483148, 32, 35, 12556},  
  {4483149, 4809608, 32, 26, 12556},  
  {4809609, 5098400, 32, 23, 12556}, 
  {5098401, 5349524, 32, 20, 12556},  
  {5349525, 5600648, 32, 20, 12556},  
  {5600649, 5851772, 32, 20, 12556}, 
  {5851773, 6730696, 16, 35, 25112},  
  {6730697, 10849068, 4, 41, 100448}, 
  {10849069,14967440, 4, 41, 100448}, 
  {14967441,14967452, 401792, 8, 1},  
  {14967453,14967464, 401792, 8, 1},  
};

The code below stops on [2] and shows the two errors below. How can I modify this code to make it work?

E2110: Incompatible type conversion (C++) The cast requested can't be done.

E2062 Invalid indirection (C++)

int LoadData( D, iobj, p, v )
double D;
int iobj;
double p[], v[];
{
int s;
//--Lots of code here--

s = objs[iobj][2];

//--more code here--
}

What is `int LoadData( D, iobj, p, v )` supposed to do? I don't see any types. — Arnav Borborah, Feb 26 '18 at 16:31
`objs[iobj][2]` would work if `objs` was a 2 dimensional array but it isn't, it's an array of structs, what are you expecting to happen? If you want to get data out of a struct you have to use the name of the field in the struct. — john, Feb 26 '18 at 16:32
@ArnavBorborah: they follow between the `)` and the `{`. K&R-style declarations. Smells of very old C. — Mat, Feb 26 '18 at 16:34
Decide on the tags, C **or** C++. They are two separate languages. In C, the common idiom is to use `typedef` when declaring structures. In C++, the `typedef` is not needed. — Thomas Matthews, Feb 26 '18 at 16:45
I recommend getting rid of `[15]` and using `[]` to let the compiler determine the number of elements. — Thomas Matthews, Feb 26 '18 at 20:15

score 5 · Accepted Answer · answered Feb 26 '18 at 16:49

struct ephloc objs[15] = { /* whatever */ };

This defines objs as an array with 15 elements of type ephloc.

objs[iobj]

This accesses an element of that array. Since each element has type ephloc, this expression gives you an ephloc.

objs[iobj][2]

This attempts to access the second element of an array. But objs[iobj] is not an array, so the compiler is telling you that you can't do that.

To access members of that element, use member names:

objs[iobj].first_item_ordinal

This accesses the first member of the object objs[iobj].

I believe s=objs[iobj][2]; should be s=objs[iobj].days_per_record; This seems to resolve the compiler problem and return the correct number. — homebase, Feb 27 '18 at 01:44

StoryTeller - Unslander Monica · Answer 2 · 2018-02-26T16:53:54.063

2

This is a case of "C++ is just a superset of C" being proven wrong, again. Your C function declaration is in an outdated from called K&R syntax. Those predates modern function prototypes, and are now invalid in C. Furthermore, they were never valid in C++ to begin with. The only valid function declaration, as far as your ,C++ compiler is concerned, is this:

int LoadData( double D, int iobj, double p[], double v[] )
{

And you better well make sure the corresponding header file matches it. K&R comes hand in hand with the "function without a prototype" declaration, i.e. int LoadDate();, which in C++ is a function taking no arguments. So that will be a declaration mismatch if you don't adjust that too.

edited Feb 26 '18 at 16:53

answered Feb 26 '18 at 16:48

StoryTeller - Unslander Monica

165,132
21
377
458

I'm sorry, does the code in the OP *not* contain K&R style functions? – StoryTeller - Unslander Monica Feb 26 '18 at 16:54
I believe this function declaration was originally written for Linux. I tried to make as few changes as possible when compiling it in a C file for Windows. Your function declaration is better and correct. – homebase Feb 27 '18 at 01:47

Compile error when retrieving a value from a struct in C

2 Answers2