指针访问冲突问题

Question

i know a ton of these questions appear, but i have tried/searched everything to no avail.

UPDATE START

Test Class

#include "stdafx.h"

#include "testerClasser.h"

Tester::Tester(){

}

void Tester::GetNum(int * num){

    int num2 = 6;

    *num = num2;// error thrown here

}

an example of calling GetNum function

int _tmain(int argc, _TCHAR* argv[])
{

    int* num = NULL;

    Tester* tester = new Tester();

    tester->GetNum(num);

    return 0;
}

error Unhandled exception at 0x77c115de in Tester.exe: 0xC0000005: Access violation writing location 0x00000000. UPDATE END

i have a method

void CCCamera::getEyeXYZ(float *pEyeX, float *pEyeY, float *pEyeZ) {

 *pEyeX = m_fEyeX; *pEyeY = m_fEyeY; *pEyeZ = m_fEyeZ; 

}

i have tried to call the method

float* pEyeX = new float(10);
float* pEyeY= new float(10);
float* pEyeZ= new float(10);
this->m_pCamera->getEyeXYZ (pEyeX,pEyeY,pEyeZ);

and

float* pEyeX;
float* pEyeY;
float* pEyeZ;
this->m_pCamera->getEyeXYZ (pEyeX,pEyeY,pEyeZ);

even

    float pEyeX;
float pEyeY;
float pEyeZ;
this->m_pCamera->getEyeXYZ (&pEyeX,&pEyeY,&pEyeZ);

their is obviously something i am missing can anyone help ?

Answer 1

In the following code you are trying to dereference a NULL pointer inside GetNum function.

int _tmain(int argc, _TCHAR* argv[])
{

    int* num = NULL;

    Tester* tester = new Tester();

    tester->GetNum(num); //will result in NULL pointer dereference inside GetNum!

    return 0;
}

What you need is most likely the following:

int _tmain(int argc, _TCHAR* argv[])
{
    int num = 0;

    Tester* tester = new Tester();

    tester->GetNum(&num); // pass a pointer to local variable

    return 0;
}

Answer 2

So what's the question?

The first block works, the second block does not (pointers pointing 'nowhere', which doesn't actually exist in programming so they are just pointing to more-or-less random locations), the third block is fine again.

Answer 3

The first thing to do would be to figure out which pointer is causing the issue. The symptoms that you reported strongly suggest that there's something wrong with your this or this->m_pCamera pointers. Trying to play with parameter pEye... pointers will make no difference because they probably have nothing to do with the issue at all.