c++ pointers & arrays - c++

Question

#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?

Answer 1

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.

Answer 2

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.

Answer 3

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;
}

Answer 4

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.