Re: [boost] Geometry/Vector Lib

From: james.jones_at_[hidden]
Date: 2006-09-22 16:23:18

From: Marcus Lindblom <macke_at_[hidden]>
> Martin Bonner wrote:
> > A more serious problem is if you start mixing usages.  A compiler is
> > quite likely to assume that assignments through the result of the index
> > operator don't affect .y or .z, and optimize on that basis.
> 
> Why should it? The offset to memory will be the same, so it should treat 
> it as access to the same memory location, if it optimizes well in the 
> backend ('assembly' AST).

vector3d v;
v.y = 4.0;
v[1] = 5.0;	// only touches v.x!!
cout << v.y;

Since v.x is assigned to but never used, the compiler might optimize away the assignment.

In practice, compilers typically don't do this optimization when pointers are present, precisely because hackers^W programmers do stuff like this. I certainly wouldn't want to *assume* this for my compiler, though.

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