Unhandled exception in debug mode but works fine in release

Question

I don't know why but this code below gives this error in debug mode when compiling in 64-bit:

Unhandled exception at 0x000000013f488f55 in OpenGL.exe: 0xC0000005: Access violation reading location 0x000000053f4d9778.

However it works perfectly fine in release mode and both debug and release when compiled in 32-bit! Help is very much appreciated.

I'm using Visual Studio 2010.

float g_History[20] = { 0.0f };
const float g_WeightModifier = 0.25f;

void CInput::SmoothMouseMovement()
{
    if(!m_SmoothMouse) return;

    for(UINT i = 0; i < 10; i++)
    {
        g_History[i * 2] = g_History[(i - 1) * 2]; // This line gives the error
        g_History[i * 2 + 1] = g_History[(i - 1) * 2 + 1];
    }

    g_History[0] = m_MouseState.X;
    g_History[1] = m_MouseState.Y;

    float AverageX = 0.0f;
    float AverageY = 0.0f;
    float AverageTotal = 0.0f;
    float WeightModifier = 1.0f;

    for(UINT i = 0; i < 10; i++)
    {
        AverageX += g_History[i * 2] * WeightModifier;
        AverageY += g_History[i * 2 + 1] * WeightModifier;
        AverageTotal += 1.0f * WeightModifier;
        WeightModifier *= g_WeightModifier;
    }

    m_MouseState.X = AverageX / AverageTotal;
    m_MouseState.Y = AverageY / AverageTotal;
}

Answer 1

The first time through your loop, g_History[(i - 1) * 2] is going to be the equivalent of g_History[-2], which is obviously a bad access. It's just coincidence on how memory is arranged with 32v64 and debug v release. That line is an error regardless of whether your app is crashing or not.