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
– N 1.1, Commented Mar 2, 2010 at 4:02

John Knoeller · Accepted Answer · 2010-03-02 03:55:50Z

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 2, 2010 at 3:55

John Knoeller

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4 Comments

Alok Singhal Over a year ago

and content[1]->str1 becomes content[1].str1.

Matthew Flaschen Over a year ago

He may also want to pass a length.

Adam Rosenfield Over a year ago

...which can be obtained by passing sizeof(mydata)/sizeof(mydata[0]), but only before the function call

Alok Singhal Over a year ago

@Adam: ...or after the call, but not in the call :-).

Arun · Accepted Answer · 2010-03-02 03:58:42Z

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 2, 2010 at 3:58

Arun

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2 Comments

Alok Singhal Over a year ago

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 Over a year ago

OK, now I am happy with the answer. +1.

Abubacker Nainamohamed · Accepted Answer · 2010-03-02 04:28:23Z

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 ).

Farnham · Accepted Answer · 2017-11-24 07:14:06Z

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

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 · Accepted Answer · 2021-06-23 15:18:53Z

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.

IKavanagh · Accepted Answer · 2015-09-29 07:17:08Z

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

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