How to pass an array of struct using pointer in c/c++?

Question

in C code I'm stuck to pass an array of struct to a function, here's the code that resembles my problem:

typedef struct
{
   int x;
   int y;
   char *str1;
   char *str2;
}Struct1;

void processFromStruct1(Struct1 *content[]);
int main()
{
    Struct1 mydata[]=
    { {1,1,"black","cat"},
      {4,5,"red","bird"},
      {6,7,"brown","fox"},
    };

    processFromStruct1(mydata);//how?!?? can't find correct syntax

    return 0;
}

void processFromStruct1(Struct1 *content[])
{
    printf("%s", content[1]->str1);// if I want to print 'red', is this right?
        ...
}

Compile error in msvc is something like this:

error C2664: 'processFromStruct1' : cannot convert parameter 1 from 'Struct1 [3]' to 'Struct1 *[]'
1>       Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

How to solve this? tnx.

@mhd: though your question has been asnwered, i suggest you to read some book before jumping to code. reading a book will greatly supplement your understanding. — N 1.1, Mar 02 '10 at 04:02

score 16 · Accepted Answer · answered Mar 02 '10 at 03:55

16

You almost had it, either this

void processFromStruct1(Struct1 *content);

or this

void processFromStruct1(Struct1 content[]);

and, as Alok points out in comments, change this

content[1]->str1

to this

content[1].str1

Your array is an array of structures, not an array of pointers, so once you select a particular structure with [1] there is no need to further dereference it.

answered Mar 02 '10 at 03:55

John Knoeller

33,512
4
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6

and `content[1]->str1` becomes `content[1].str1`. – Alok Singhal Mar 02 '10 at 03:56
5

He may also want to pass a length. – Matthew Flaschen Mar 02 '10 at 03:58
1

...which can be obtained by passing `sizeof(mydata)/sizeof(mydata[0])`, but only _before_ the function call – Adam Rosenfield Mar 02 '10 at 04:13
1

@Adam: ...or *after* the call, but not *in* the call :-). – Alok Singhal Mar 02 '10 at 04:24

score 2 · Answer 2 · answered Mar 02 '10 at 03:58

2

Try

processFromStruct1( & mydata[ i ] ); // pass the address of i-th element of mydata array

and the method to

void processFromStruct1(Struct1 *content )
{
    printf("%s", content->str1);
        ...
}

(2nd part already noted by John Knoeller and Alok).

answered Mar 02 '10 at 03:58

Arun

19,750
10
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Then one can't index that pointer with 1, one can only use 0. Not sure if this is what the OP wants. Also the type is wrong. – Alok Singhal Mar 02 '10 at 04:00
OK, now I am happy with the answer. +1. – Alok Singhal Mar 02 '10 at 04:04

score 1 · Answer 3 · answered Mar 02 '10 at 04:28

John Knoeller gave the perfect syntax , I am trying to explain some basic things, I hope that it willsolve your confusions in future. This is very similar to passing pointer to a function in C. Of course struct is also a pointer,

so we can pass the value in 2 ways 0. Via pointer 0. Via array ( since we are using array of struct )

so the problem is simple now , You have to give the data type of a variable as we do in normal pointers , here the data type is user-defined ( that means struct ) Struct1 then variable name, that variable name can be pointer or array name ( choose a compatible way ).

score 0 · Answer 4 · answered Nov 24 '17 at 07:14

0

This works for me. Changed structs to C++ style.

struct Struct1
{
    int x;
    int y;
    char *str1;
    char *str2;
};

Struct1 mydata[]=
{   {1,1,"black","cat"},
    {4,5,"red","bird"},
    {6,7,"brown","fox"},
};

void processFromStruct1(Struct1 content[]);

int main()
{
    processFromStruct1(&mydata[1]);    
    return 0;
}

void processFromStruct1(Struct1 content[])
{
    printf("%s",content->str1);
}

output: red

answered Nov 24 '17 at 07:14

Farnham

19
2

That should not compile and is not "C++ style". Above the keyword, `struct` must precede the `Struct1` name. Example: `void processFromStruct1(struct Struct1 content[]);` – Chef Gladiator Jun 23 '21 at 15:24

score 0 · Answer 5 · answered Jun 23 '21 at 15:18

Perhaps a proper re-factoring from the future:

#include <stdio.h>

typedef struct
{
   int x;
   int y;
   char *str1;
   char *str2;
} struct_1;

static void proc_the_struct_1_arr (
    const int count_ ,
    // array arg declared with min number of arguments
    // also can not be null
    struct_1 content[ static count_ ]
    )
{
    for (unsigned j = 0; j < count_; ++j)
    printf("x:%-4dy:%-4d%-12s%-12s\n", content[j].x,content[j].y,content[j].str1,content[j].str2);
}
int main( void )
{
    struct_1 mydata[3]= { 
      {.str1 = "black", .str2 = "cat" },
      {.str1 = "red",   .str2 = "bird"},
      {.str1 = "brown", .str2 = "fox" },
    };

    proc_the_struct_1_arr (3,mydata);

    return 0;
}

Godbolt

Obviously proc_the_struct_1_arr declaration is interesting. That actually uses Variably Modified Types (VMT). That is a legal syntax only for array arguments.

That is not slower vs the pointer to array solution:

static void proc_the_struct_1_arr_pointer (
    const int count_ ,
    // array arg not declared with min 
    // required number of arguments
    struct_1 (*arr_ptr)[ /* static no allowed here */ count_ ]
    );

I assume the compiler "rewrites" the above to the array pointer, anyway. On the second option arr_ptr can be a null argument.

score -1 · Answer 6 · edited Sep 29 '15 at 07:17

-1

You can try the prototype as void processFromStruct1(Struct1 content[]); and then the declaration should be like void processFromStruct1(Struct1 content[]).

edited Sep 29 '15 at 07:17

IKavanagh

6,089
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answered Sep 29 '15 at 06:40

sunilyadav0201

45
5

How to pass an array of struct using pointer in c/c++?

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