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

Answer 1

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.

Answer 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).

Answer 3

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 ).

Answer 4

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

Answer 5

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.

Answer 6

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