CM and centre of gravity coincides in the case of 

For any given body centre of mass is the average position of all the mass that makes up the object.

Centre of gravity is simply the average position of weight distribution.

Since mass and weight are proportional C.M and C.G refer to the same point of an object on and near earth’s surface.

Hence C.G and C.M both are coincident in the case of regular or irregular shaped objects and

uniformly or non uniformly distributed mass systems.