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Subject: Re: [boost] [Review:Algorithms] Order of args to clamp
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-09-23 14:05:40

Phil Endecott wrote:
> template<typename V>
> V clamp ( V val, V lo, V hi );
> Like min & max, clamp has a single type template parameter;
> what are the implications of this? For example, if I pass a
> mixture of std::string and const char* arguments, what will
> happen? Ideally, (a) all combinations would compile, and (b)
> conversions from const char* to std::string will only happen
> when necessary, not unconditionally for all of the arguments.

Let's examine the implications:

   std::string value("2");
   clamp(value, "1", "3");

That results in the following code:

   std::string const lo("1");
   std::string const hi("2");
   std::less<std::string> p;
   return p(value, lo) ? lo : p(hi, value) ? value : hi;

Since the return type must match _val_ (or _middle_ ;-), it is not unreasonable to permit _lo_ and _hi_ to have different types:

   template <class V, class L, class H>
   clamp(V _value, L _low, H _high);

(I'll discuss pass-by-value later.)

Assuming the current implementation, that would result in the following code:

   std::less<std::string> p;
   std::string const low("1");
   std::string const high("2");
   return p(value, low)
      ? std::string("1")
      : p(high, value) ? std::string("2") : value;

This code is the result because to std::less<T>::operator () requiring two T const & arguments.

An optimizer may recognize that it needn't create additional temporaries for the return value, resulting in this code:

   std::less<std::string> p;
   std::string const low ("1");
   std::string const high ("2");
   return p(value, low) ? low : p(high, value) ? high: value;

If the non-predicate overload didn't use std::less<V>, then the code would be:

   return value < "1"
      ? std::string("1")
      : "2" < value ? std::string("2") : value;

Since std::string provides < that works with char * directly, the comparisons don't require instantiating a std::string. The result is zero or one temporary versus always two.

Conclusion: The multiple-template-parameter version is better, but only if the non-predicate overload doesn't use std::less<V>.

As for pass-by-value versus references, std::string is an excellent argument against pass-by-value. However, without the extra template parameters, calling clamp(value, "1", "2") would still create temporary std::strings from "1" and "2" to bind to the std::string const & function parameters. With the extra template parameters, however, pass-by-reference-to-const is viable. (Of course call_traits<T>::param_type can be used to select the parameter type to use pass-by-value for appropriately small built-in types.)

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP


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