Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Negators

The header functional.hpp provides enhanced versions of both the negator adapters from the C++ Standard Library (§20.3.5):

As well as the corresponding helper functions

However, the negators in this library improve on the standard versions in two ways:

Usage

Usage is identical to the standard negators. For example,

bool bad(const Foo &foo) { ... }
...
std::vector<Foo> c;
...
std::find_if(c.begin(), c.end(), boost::not1(bad));

Argument Types

The C++ Standard (§20.3.5) defines unary negate like this (binary negate is similar):

template <class Predicate>
  class unary_negate
    : public unary_function<typename Predicate::argument_type,bool> {
public:
  explicit unary_negate(const Predicate& pred);
  bool operator()(const typename Predicate::argument_type& x) const;
};

Note that if the Predicate's argument_type is a reference, the type of operator()'s argument would be a reference to a reference. Currently this is illegal in C++ (but see the C++ standard core language active issues list).

However, if we instead defined operator() to accept Predicate's argument_type unmodified, this would be needlessly inefficient if it were a value type; the argument would be copied twice - once when calling unary_negate's operator(), and again when operator() called the adapted function.

So how we want to declare the argument for operator() depends on whether or not the Predicate's argument_type is a reference. If it is a reference, we want to declare it simply as argument_type; if it is a value we want to declare it as const argument_type&.

The Boost call_traits class template contains a param_type typedef, which uses partial specialisation to make precisely this decision. If we were to declare operator() as

bool operator()(typename call_traits<typename Predicate::argument_type>::param_type x) const

the desired result would be achieved - we would eliminate references to references without loss of efficiency. In fact, the actual declaration is slightly more complicated because of the use of function object traits, but the effect remains the same.

Limitations

Both the function object traits and call traits used to realise these improvements rely on partial specialisation, these improvements are only available on compilers that support that feature. With other compilers, the negators in this library behave very much like those in the Standard - ptr_fun will be required to adapt functions, and references to references will not be avoided.


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Revised 28 June 2000