CODEWITHSUNDEEP
c++arrayspointers
Arpit Jain
Arpit Jain

Reputation: 9

c++ pointers & arrays

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
using namespace std;

int width = 100;
int height = 100;

float cam[] = {-1.1,-1.0,1.2};
float D[] = {0.2,0.2,-0.2};
float U[] = {0.0,1.0,0.0};

float* cross(float* v1,float* v2)
{
    float el1[] = {(v1[1]*v2[2]-v1[2]*v2[1]),(v1[2]*v2[0]-v1[0]*v2[2]),(v1[0]*v2[1]-v1[1]*v2[0])};
    return el1;
}

float* neg3(float* v)
{
    float arr[3];
    for(int i=0;i<3;i++)
    arr[i] = 0.0-(v[i]);

    return arr;
}

/*

float* cam_space(float* p)
{
    float* e1 = cross(D,U);
    float* e2 = cross(D,cross(U,D));
    float* e3_n = D;

    float ar[4];
    ar[0] = e1[0]*p[0]+e1[1]*p[1]+e1[2]*p[2];
    ar[1] = e2[0]*p[0]+e2[1]*p[1]+e2[2]*p[2];
    ar[2] = -(e3_n[0]*p[0]+e3_n[1]*p[1]+e3_n[2]*p[2]);
    ar[3] = p[3];
    return ar;
}

*/
float* translate(float* p,float* v)
{

    float arr1[3];
    for(int i=0;i<=2;i++)
    arr1[i] = p[i] + v[i];

    return arr1;
}


int main()
{
    float* poi;
    poi = cam;   //undo later

    float* nc;
    nc = neg3(cam);
    cout<<" "<<nc[0]<<" "<<nc[1]<<" "<<nc[2]<<endl;


    float arbit[3] = {0.1,0.1,0.1};

    float* temp1;
    temp1 = translate(poi,arbit);
    //float* temp2;
    //temp2 = cam_space(temp);

    cout<<" "<<nc[0]<<" "<<nc[1]<<" "<<nc[2]<<endl;
    cout<<" "<<poi[0]<<" "<<poi[1]<<" "<<poi[2]<<endl;


    cout<<" "<<temp1[0]<<" "<<temp1[1]<<" "<<temp1[2]<<endl;
    return 0;
}

As you can see, I am outputting nc twice. But both the values differ. The second time nc is displayed, it actually shows the temp1 value, and temp1 actually shows garbage values. Any help?

Upvotes: 1

Views: 172

Answers (4)

Mahesh
Mahesh

Reputation: 34625

float* translate(float* p,float* v)
{

    float arr1[3];
    for(int i=0;i<=2;i++)
    arr1[i] = p[i] + v[i];

    return arr1;
}// arr1 ceases to exist from this point.

You are returning the reference of a local variable, arr1. It resides on stack and gets de-allocated on the return of function call. But you are holding a reference to it which is yielding you garbage values. Instead new new[] arr1 and return it. Remember to delete[] it when you are done.

Upvotes: 5

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361412

translate() returns a local pointer (converted from array type). So what temp1 is referring to, doesn't exist after the function translate() returns.

Same is the case with neg3() also.

If you're using C++, then std::vector<float> will solve all such problem.

You could write this:

std::vector<float> translate(const std::vector<float> & p, const std::vector<float> & v)
{
   std::vector<float> vf(3);
   for(int i=0;i <3;i++)
      vf[i] = p[i] + v[i];
   return vf; //semantically, it returns a copy of the local object.
}

Similarly, use std::vector whereever you're using float[3]. And don't use global variables.

Upvotes: 1

c-smile
c-smile

Reputation: 27460

Instead of returning pointers to local variables you should return values.

Consider this:

struct V3 { float data[3]; }

V3 neg3(V3 v)
{
    for(int i=0;i<3;i++)
      v.data[i] = -v.data[i];
    return v;
}

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477040

You are returning pointers to local variables left right and centre. Those variables go out of scope at the end of the function body, and the result is undefined behaviour.

A nice way of handling array-modifying functions is to pass the array as a parameter:

void modify_me(float arr[])  // or `modify_me(float * arr)`, same thing
{
  arr[0] = 0.5;
  arr[1] = -2.25;
}

int main()
{
  float myarray[2];
  modify_me(myarray);  // now myarray[0] == 0.5, etc.

  // ...
}

Since you're in C++, you could even use template magic:

template <unsigned int N>
void modify_me(float (&arr)[N])
{
  static_assert(N == 3, "You're doing it wrong!");  // C++11 feature
  arr[0] = /* ... */
}

Now you get a compile-time error if you try to call this with anything that's not an automatic array of size 3.

Upvotes: 1

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