Implementation of strafe - Opengl - Camera

I created a camera class. You can rotate 360 ​​degrees with the mouse and move up and down. As I expected, the same as in all games. It is also possible to move back and forth, as in all games. But I do not know how to implement moving left and right.

I do the following:

This calls each frame:

gluLookAt(_posX , _posY , _posZ,
          _viewX, _viewY, _viewZ,
          _upX,   _upY,   _upZ );

My move function
Does not work:

void Camera::moveLeft() 
{

    float rot= (_viewY / 180 * PI);
    _moveX -= float(cos(rot)) * 0.5;
    _moveZ -= float(sin(rot)) * 0.5;
}

Does the move forward in the scene:

void Camera::moveForward() 
{

    float viewX = _viewX - _posX;
    float viewY = _viewY - _posY;
    float viewZ = _viewZ - _posZ;

    _posX += viewX * speed
    _posY += viewY * speed;
    _posZ += viewZ * speed;

    _viewX += viewX * speed;
    _viewY += viewY * speed;
    _viewZ += viewZ * speed;

}

When I only move with the mouse, there is no problem. But if I use this function and rotate with the mouse, I get some strange camera movements.

Any ideas on how to solve this?

thank

@edit

So, I deleted the glTranslated statement, and I changed the moveLeft function to the following:

void Camera::moveLeft(){

float x = ((_viewY * _upZ) - (_viewZ * _upY));
float y = ((_viewZ * _upX) - (_viewX * _upZ));
float z = ((_viewX * _upY) - (_viewY * _upX));

float magnitude = sqrt( (x * x) + (y * y) + (z * z) );

x /= magnitude;
y /= magnitude;
z /= magnitude;

_posX -= x;
_posY -= y;
_posZ -= z;

} Code>

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