Thread: Passing Typedef Struct to a function

  1. #1
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    Passing Typedef Struct to a function

    Hello,

    I am a bit confused as to why this code compiles, but is not working, and how to fix it. Can you help? Thx

    Code:
    #include <stdlib.h>
    
    typedef struct {
    	int x;
    } Stype;
    
    void Fun(Stype a) {
    	a.x = a.x + 1;
    }
    
    int main() {
    	Stype sexample;
    	sexample.x = 0;
    	
    	Stype arr[10];
    	arr[0] = sexample;
    	
    	Fun(arr[0]);
    	
    	printf("%d", arr[0].x); // why not 1?
    	
    	return 0;
    }

  2. #2
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    When you pass arr[0] to Fun, you pass it by copy. So, the change you make to the copy is not reflected back in main. If you want it the change to be reflected back in main, pass your value by reference (or maybe pointer).

    If you want to see 1 as the output, you should do something like this:
    Code:
    void Fun(Stype& a) {
          a.x++;
    }
    
    int main() {
          .....
          Fun(arr[0]);
          .....
    }
    
    // or maybe,
    
    void Fun(Stype* a) {
          *a.x++;
    }
    
    int main() {
          .....
          Fun(&arr[0]);
          .....
    }
    Last edited by Zeus_; 03-27-2020 at 04:03 PM.
    "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning." - Rick Cook, The Wizardry Compiled

  3. #3
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    But I thought I was passing an element of the array, not the whole array?

    Are you sure your examples compile? This should work

    Code:
    #include <stdlib.h>
    
    typedef struct {
    	int x;
    } Stype;
    
    void Fun(Stype *a) {
    	a->x = a->x + 1;
    }
    
    int main() {
    	Stype sexample;
    	sexample.x = 0;
    	
    	Stype arr[10];
    	arr[0] = sexample;
    	
    	Fun(&arr[0]);
    	
    	printf("%d", arr[0].x); // why not 1?
    	
    	return 0;
    }

  4. #4
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    Quote Originally Posted by serge View Post
    Are you sure your examples compile? This should work
    No, it doesn't compile. Zeus is confused between C++ and C.
    Your code is correct, although using ++ is more usual.

    But I thought I was passing an element of the array, not the whole array
    Passing a pointer to the first element is essentially passing the whole array since in your function you could increment a to access the next element.
    If you want the truth to stand clear before you, never be for or against. - Sent-ts'an

  5. #5
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    My bad. I should have written (*a).x++
    ++ has higher precedence than dereferencing a pointer. I was busy with something else at the time you posted and I happened to write wrong code. Sorry Nonetheless, the reference version works right.
    "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning." - Rick Cook, The Wizardry Compiled

  6. #6
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    Thx a lot. I am doing a pure c project atm, and it happens to me all the time.

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