# Re: Function f(lb,x,ub); equivalent to min(ub,max(lb,x)); ?

From:
"Doug Harrison [MVP]" <dsh@mvps.org>
Newsgroups:
microsoft.public.vc.mfc
Date:
Tue, 27 Feb 2007 14:02:13 -0600
Message-ID:
<p729u2hfhu7hdrk140gpoctof7v0cqglhi@4ax.com>
On Tue, 27 Feb 2007 05:15:48 -0500, David Wilkinson <no-reply@effisols.com>
wrote:

Not to beat a dead horse, but I would still follow what the standard
library does for min() and max(): use < only and const T&.

The reason to use < is because that is what many standard library
algorithms (like std::sort()) require to be defined. If you use > you
are placing a requirement on your class that is not required by the
standard library.

That's a good idea.

The reason to use const T& is that it will be much more efficient for a
complex type, particularly one that requires memory allocation in the
copy constructor. I would need detailed evidence that using T is more
efficient for simple types, and even if it was it would be extremely
unusual for it to really matter.

To my surprise, it does make a difference, though I agree it would be
really rare for it be significant. I compiled the following with /O2:

template <typename T>
inline
const T& Clamp1( const T& lower, const T& x, const T& upper )
{
if ( x < lower )
return lower;
if ( x > upper )
return upper;
return x;
}

template <typename T>
inline
T Clamp2( T lower, T x, T upper )
{
if ( x < lower )
return lower;
if ( x > upper )
return upper;
return x;
}

int f(int x, int y, int upper)
{
return Clamp1(x, y, upper);
}

int g(int x, int y, int upper)
{
return Clamp2(x, y, upper);
}

*********************************************************************
The VC8 output was for f was:

PUBLIC ?f@@YAHHHH@Z ; f
; Function compile flags: /Ogtpy
; COMDAT ?f@@YAHHHH@Z
_TEXT SEGMENT
_x\$ = 8 ; size = 4
_y\$ = 12 ; size = 4
_upper\$ = 16 ; size = 4
?f@@YAHHHH@Z PROC ; f, COMDAT

; 25 : return Clamp1(x, y, upper);

mov eax, DWORD PTR _y\$[esp-4]
cmp eax, DWORD PTR _x\$[esp-4]
jge SHORT \$LN4@f
lea eax, DWORD PTR _x\$[esp-4]
mov eax, DWORD PTR [eax]

; 26 : }

ret 0

; 25 : return Clamp1(x, y, upper);

\$LN4@f:
cmp eax, DWORD PTR _upper\$[esp-4]
lea eax, DWORD PTR _upper\$[esp-4]
jg SHORT \$LN7@f
lea eax, DWORD PTR _y\$[esp-4]
\$LN7@f:
mov eax, DWORD PTR [eax]

; 26 : }

ret 0
?f@@YAHHHH@Z ENDP ; f
_TEXT ENDS

*********************************************************************
The VC8 output was for g was:

PUBLIC ?g@@YAHHHH@Z ; g
; Function compile flags: /Ogtpy
; COMDAT ?g@@YAHHHH@Z
_TEXT SEGMENT
_x\$ = 8 ; size = 4
_y\$ = 12 ; size = 4
_upper\$ = 16 ; size = 4
?g@@YAHHHH@Z PROC ; g, COMDAT

; 30 : return Clamp2(x, y, upper);

mov ecx, DWORD PTR _y\$[esp-4]
mov eax, DWORD PTR _x\$[esp-4]
cmp ecx, eax
jl SHORT \$LN5@g
mov eax, DWORD PTR _upper\$[esp-4]
cmp ecx, eax
jg SHORT \$LN5@g
mov eax, ecx
\$LN5@g:

; 31 : }

ret 0
?g@@YAHHHH@Z ENDP ; g
_TEXT ENDS

The first one has three more instructions than the second and is presumably
slower, but in this day and age, I wouldn't bet on it. :) I'm just
surprised there's a difference when the Clamp functions are both inlined...
Ah, it is making the return type a reference that explains it. Make Clamp1
return T, and the assembly output is identical for f and g. That makes
sense.

If there is a higher-level downside to returning a const reference argument
by const reference, it is this:

const int& z = Clamp1(x, y, 100);

Now this requires creation of a temporary to hold the value 100, and if
Clamp1 returns this temporary (NB: by reference), z will be bound to it.
The problem is, this temporary will be destroyed at the end of the
full-expression, so z will end up a dangling reference. This can't happen
if you use Clamp2, because C++ requires its return value of type T to exist
as long as z exists. This is a relatively unlikely bug, and it shows why
it's important to watch what you're binding to const references. Another
example would be a ctor taking a const reference and initializing a member
const reference with it. Basically, you should avoid binding something to a
const reference if the reference lives beyond the full-expression that
performs the binding.

--
Doug Harrison
Visual C++ MVP

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