问题
You can pass a function pointer, function object (or boost lambda) to std::sort to define a strict weak ordering of the elements of the container you want sorted.
However, sometimes (enough that I've hit this several times), you want to be able to chain "primitive" comparisons.
A trivial example would be if you were sorting a collection of objects that represent contact data. Sometimes you will want to sort by
last name, first name, area code. Other times
first name, last name- yet other times
age, first name, area code... etc
Now, you can certainly write an additional function object for each case, but that violates the DRY principle - especially if each comparison is less trivial.
It seems like you should be able to write a hierarchy of comparison functions - the low level ones do the single, primitive, comparisons (e.g. first name < first name), then higher level ones call the lower level ones in succession (probably chaining with && to make use of short circuit evaluation) to generate the composite functions.
The trouble with this approach is that std::sort takes a binary predicate - the predicate can only return a bool. So if you're composing them you can't tell if a "false" indicates equality or greater than. You can make your lower level predicates return an int, with three states - but then you would have to wrap those in higher level predicates before they could be used with std::sort on their own.
In all, these are not insurmountable problems. It just seems harder than it should be - and certainly invites a helper library implementation.
Therefore, does anyone know of any pre-existing library (esp. if it's a std or boost library) that can help here - of have any other thoughts on the matter?
[Update]
As mentioned in some of the comments - I've gone ahead and written my own implementation of a class to manage this. It's fairly minimal, and probably has some issues with it in general. but on that basis, for anyone interested, the class is here:
http://pastebin.com/f52a85e4f
And some helper functions (to avoid the need to specify template args) is here:
http://pastebin.com/fa03d66e
回答1:
You could build a little chaining system like so:
struct Type {
string first, last;
int age;
};
struct CmpFirst {
bool operator () (const Type& lhs, const Type& rhs) { return lhs.first < rhs.first; }
};
struct CmpLast {
bool operator () (const Type& lhs, const Type& rhs) { return lhs.last < rhs.last; }
};
struct CmpAge {
bool operator () (const Type& lhs, const Type& rhs) { return lhs.age < rhs.age; }
};
template <typename First, typename Second>
struct Chain {
Chain(const First& f_, const Second& s_): f(f_), s(s_) {}
bool operator () (const Type& lhs, const Type& rhs) {
if(f(lhs, rhs))
return true;
if(f(rhs, lhs))
return false;
return s(lhs, rhs);
}
template <typename Next>
Chain <Chain, Next> chain(const Next& next) const {
return Chain <Chain, Next> (*this, next);
}
First f;
Second s;
};
struct False { bool operator() (const Type& lhs, const Type& rhs) { return false; } };
template <typename Op>
Chain <False, Op> make_chain(const Op& op) { return Chain <False, Op> (False(), op); }
Then to use it:
vector <Type> v; // fill this baby up
sort(v.begin(), v.end(), make_chain(CmpLast()).chain(CmpFirst()).chain(CmpAge()));
The last line is a little verbose, but I think it's clear what's intended.
回答2:
One conventional way to handle this is to sort in multiple passes and use a stable sort. Notice that std::sort is generally not stable. However, there’s std::stable_sort.
That said, I would write a wrapper around functors that return a tristate (representing less, equals, greater).
回答3:
You can try this:
Usage:
struct Citizen {
std::wstring iFirstName;
std::wstring iLastName;
};
ChainComparer<Citizen> cmp;
cmp.Chain<std::less>( boost::bind( &Citizen::iLastName, _1 ) );
cmp.Chain<std::less>( boost::bind( &Citizen::iFirstName, _1 ) );
std::vector<Citizen> vec;
std::sort( vec.begin(), vec.end(), cmp );
Implementation:
template <typename T>
class ChainComparer {
public:
typedef boost::function<bool(const T&, const T&)> TComparator;
typedef TComparator EqualComparator;
typedef TComparator CustomComparator;
template <template <typename> class TComparer, typename TValueGetter>
void Chain( const TValueGetter& getter ) {
iComparers.push_back( std::make_pair(
boost::bind( getter, _1 ) == boost::bind( getter, _2 ),
boost::bind( TComparer<TValueGetter::result_type>(), boost::bind( getter, _1 ), boost::bind( getter, _2 ) )
) );
}
bool operator()( const T& lhs, const T& rhs ) {
BOOST_FOREACH( const auto& comparer, iComparers ) {
if( !comparer.first( lhs, rhs ) ) {
return comparer.second( lhs, rhs );
}
}
return false;
}
private:
std::vector<std::pair<EqualComparator, CustomComparator>> iComparers;
};
回答4:
std::sort is not guaranteed to be stable because stable sorts are usually slower than non-stable ones ... so using a stable sort multiple times looks like a recipe for performance trouble...
And yes it's really a shame that sort ask for a predicate: I see no other way than create a functor accepting a vector of tristate functions ...
回答5:
The chaining solution is verbose. You could also use boost::bind in conjunction with std::logical_and to build your sorting predicate. See the linked article for more information: How the boost bind library can improve your C++ programs
回答6:
Variadic templates in C++ 11 give a shorter option:
#include <iostream>
using namespace std;
struct vec { int x,y,z; };
struct CmpX {
bool operator() (const vec& lhs, const vec& rhs) const
{ return lhs.x < rhs.x; }
};
struct CmpY {
bool operator() (const vec& lhs, const vec& rhs) const
{ return lhs.y < rhs.y; }
};
struct CmpZ {
bool operator() (const vec& lhs, const vec& rhs) const
{ return lhs.z < rhs.z; }
};
template <typename T>
bool chained(const T &, const T &) {
return false;
}
template <typename CMP, typename T, typename ...P>
bool chained(const T &t1, const T &t2, const CMP &c, P...p) {
if (c(t1,t2)) { return true; }
if (c(t2,t1)) { return false; }
else { return chained(t1, t2, p...); }
}
int main(int argc, char **argv) {
vec x = { 1,2,3 }, y = { 2,2,3 }, z = { 1,3,3 };
cout << chained(x,x,CmpX(),CmpY(),CmpZ()) << endl;
return 0;
}
来源：https://stackoverflow.com/questions/274951/chaining-of-ordering-predicates-e-g-for-stdsort