initialize a set with a comparison structure that depends on "this" c++ - c++

Question

I want to define a set that does its comparison based on the value of other members of the current class:
std::set<ContentType> mySet(doComparison(*this));
where doCompare is a struct:
struct doCompare{
doCompare( MyClass& mc ) : _mc(mc) { }
MyClass& _mc;
bool operator()( const ContentType & i1, const ContentType & i2 ){
return _mc.otherMember[i1] < _mc.otherMemeber[i2];
}
};
Here the mySet is a member of MyClass and when I try to initialize the set with the comparison function in the initialization list: mySet(doCompare(*this)) the code does not compile.
What am I doing wrong here?
The error is:
no matching function for call to ``std::set<ContentType>::set(MyClass::doCompare)`
Here is the full message (with the change of names for better readability):
./myclass.h:74:165: error: no matching function for call to ‘std::set<ContentType>::set(MyClass::doCompare)’
: mySet(doCompare(*this)) {
^
./myclass.h:74:165: note: candidates are:
In file included from /usr/include/c++/4.8/set:61:0,
from ./myclass.h:12,
from [blah blah]
/usr/include/c++/4.8/bits/stl_set.h:193:7: note: std::set<_Key, _Compare, _Alloc>::set(const std::set<_Key, _Compare, _Alloc>&) [with _Key = ContentType; _Compare = std::less<ContentType >; _Alloc = std::allocator<ContentType >]
set(const set& __x)
^
/usr/include/c++/4.8/bits/stl_set.h:193:7: note: no known conversion for argument 1 from ‘MyClass::doCompare’ to ‘const std::set<ContentType >&’
/usr/include/c++/4.8/bits/stl_set.h:180:2: note: template<class _InputIterator> std::set<_Key, _Compare, _Alloc>::set(_InputIterator, _InputIterator, const _Compare&, const allocator_type&)
set(_InputIterator __first, _InputIterator __last,
^
/usr/include/c++/4.8/bits/stl_set.h:180:2: note: template argument deduction/substitution failed:
In file included from [blahblah]:
./myclass.h:74:165: note: candidate expects 4 arguments, 1 provided
: mySet(doCompare(*this)) {
^
In file included from /usr/include/c++/4.8/set:61:0,
from ./myclass.h:12,
/usr/include/c++/4.8/bits/stl_set.h:163:2: note: template<class _InputIterator> std::set<_Key, _Compare, _Alloc>::set(_InputIterator, _InputIterator)
set(_InputIterator __first, _InputIterator __last)
^
/usr/include/c++/4.8/bits/stl_set.h:163:2: note: template argument deduction/substitution failed:
In file included from [blahblah]:
./myclass.h:74:165: note: candidate expects 2 arguments, 1 provided
: mySet(doCompare(*this)) {
^
In file included from /usr/include/c++/4.8/set:61:0,
from ./myclass.h:12,
from blahblah:
/usr/include/c++/4.8/bits/stl_set.h:148:7: note: std::set<_Key, _Compare, _Alloc>::set(const _Compare&, const allocator_type&) [with _Key = ContentType; _Compare = std::less<ContentType >; _Alloc = std::allocator<ContentType >; std::set<_Key, _Compare, _Alloc>::allocator_type = std::allocator<ContentType >]
set(const _Compare& __comp,
^
/usr/include/c++/4.8/bits/stl_set.h:148:7: note: no known conversion for argument 1 from ‘MyClass::doCompare’ to ‘const std::less<ContentType >&’
/usr/include/c++/4.8/bits/stl_set.h:139:7: note: std::set<_Key, _Compare, _Alloc>::set() [with _Key = ContentType; _Compare = std::less<ContentType>; _Alloc = std::allocator<ContentType >]
set()
^
To summarize the issue:
It seems I can't initialize the set with the comparison in its declaration, because it depends on this.
The initialization with comparison function after the set is declared is failing, but I don't know why.
SOLUTION thanks to #WhozCraig
declare mySet as:
std::set<ContentType, doCompare> mySet;
and initialize it in the initialization list as:
mySet(doCompare(*this))

Answer 1

You're not declaring your set type with the proper template comparator parameter. This:
std::set<ContentType> mySet;
means this when expanded:
std::set<ContentType, std::less<ContentType>> mySet
leaving out the allocator for brevity. This means when constructing mySet and specifying an alternate comparator functor, it must be of type std::less<ContentType>, but yours is not. It is of type doCompare. The compiler tries to match against every other constructor argument list, failing to find any match, finally resulting in your error.
Change your declaration of mySet to:
std::set<ContentType,doCompare> mySet;
Now the types should wire up correctly.
And as I said in comments, i see no reason the reference to your MyClass being held in your comparator object should be non-const. unless you can think of a good reason, I suggest changing the reference to const instead, i.e. const MyClass&