C++ Pointer Issues when passing to a function

Question

Solution Below For almost a year, I thought I completely understood pointers, and now it's failing. I will post the entire file if it is even needed.

// Test Structure and Function
struct You {
    int x;
    int y;
    string str;
};

bool Show(You* showValue);


// Should (delete) in whatever way possible and update its address to the     "You* update" you sent
void Update(You* update, int n) {

    // Create a new "You"
    You* youTwo = new You();
    youTwo->x = 55;
    youTwo->y = 43;
    youTwo->str = "Twin";

    // Update?
    update = youTwo;

    return; 
};




bool Show(You* showValue) {
    cout << "Show:" << endl;
    cout << showValue->x << '\t';
    cout << showValue->y << '\t';
    cout << showValue->str << '\t'; 
    cout << endl << endl;
};



int main(int argc, char** argv) {

    // Original You
    You* currentYou = new You();
    currentYou->x = 1;
    currentYou->y = 2;
    currentYou->str = "You";

    // Update the current you to a new you
    Show(currentYou);   // works
    Update(currentYou, 5);  // no compile errors
    Show(currentYou); // shows initial values instead of the updated

    return 0;
};

The Update function is where the issue is. My intentions is to delete (or get rid) of the original. Replacing it with a new You() and be done with it.

Well, congrats to solving the problem. Meanwhile, if you keep the original problematic code there and provide the solution separately, it will be more useful to other readers. — Earth Engine
– Earth Engine, Commented Jul 1, 2015 at 1:25

songyuanyao · Accepted Answer · 2015-07-01 01:21:35Z

3

You pass the pointer to You by value to void Update(You*,int). Thus update = youTwo; this has no effect on currentYou.

Change Update to void Update(You*& update, in n) { //... and read up on references.

Btw, you have a memory leak. You update the pointer, but you never deallocate the old currentYou nor the new currentYou. You should use "smart pointers" (explicitly shared_ptr<You>) to clean up everything behind you, without having to call delete every time yourself.

edited Jul 1, 2015 at 1:21

songyuanyao

174k16 gold badges327 silver badges432 bronze badges

answered Jul 1, 2015 at 0:58

WorldSEnder

5,1063 gold badges33 silver badges68 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Evan Carslake Over a year ago

thanks, the memory leak I actually left, I was trying to rule that out of the issue. I will try your advise.

WorldSEnder Over a year ago

@EvanCarslake, you have to delete update; before you assign youTwo so that you delete the previous You. Other than that it should be fine (after formatting)

Evan Carslake · Accepted Answer · 2015-07-02 00:30:35Z

0

#include <iostream>
#include <string>
using namespace std;



// Test Structure and Function
    struct You {
    int x;
    int y;
    string str;
};


bool Show(You* showValue);



// Should (delete) in whatever way possible and update its address to the "You* update" you sent
void Update(You*& update, int n) {

// Clean Up
delete update;

// Create a new "You"
You* youTwo = new You();
youTwo->x = 55;
youTwo->y = 43;
youTwo->str = "Twin";

// Update address
update = youTwo;

return; 
};




bool Show(You* showValue) {
cout << "Show:" << endl;
cout << showValue->x << '\t';
cout << showValue->y << '\t';
cout << showValue->str << '\t'; 
cout << endl << endl;
};



int main(int argc, char** argv) {

// Original You
You* currentYou = new You();
currentYou->x = 1;
currentYou->y = 2;
currentYou->str = "You";

// Update the current you to a new you
Show(currentYou);
Update(currentYou, 5); 
Show(currentYou); 

delete currentYou;
return 0;
};

The memory leak is fixed (the delete's were added,) and the pointer is a pointer-ref now. Works perfect.

answered Jul 2, 2015 at 0:30

Evan Carslake

2,36915 gold badges39 silver badges57 bronze badges

1 Comment

WorldSEnder Over a year ago

What if there's another reference to the old update when calling Update? You invalidate it completely, without anybody knowing

Collectives™ on Stack Overflow

C++ Pointer Issues when passing to a function

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related