I have learned to overload operator< in order to make custom classes compatible with STL algorithms, just like this:
struct A
{ int a; };
bool operator< (const A& x, const A& y)
{ return x.a < y.a; }
std::vector<A> aOne, aTwo, aResult;
std::set_difference(aOne.begin(), aOne.end(),
aTwo.begin(), aTwo.end(),
std::inserter(aResult, aResult.begin()));
However, when I try to do the same thing with a ValueTree object from the JUCE library, it fails:
bool operator< (const juce::ValueTree& x, const juce::ValueTree& y)
{
// let's now worry about the implementation of this function here...
return true;
}
std::vector<juce::ValueTree> vOne, vTwo, vResult;
std::set_difference(vOne.begin(), vOne.end(),
vTwo.begin(), vTwo.end(),
std::inserter(vResult, vResult.begin()));
// COMPILER ERROR: Failed to specialize function template 'unknown-type std::less<void>::operator ()(_Ty1 &&,_Ty2 &&) const'
Can anyone see what is wrong with my operator< function?
I am aware that the answer probably has to do with the inner workings of ValueTree, and that this is therefore an imperfect question. But I have no idea of the type of thing that could be going wrong here. To my eyes, the two cases seem entirely symmetrical, so I don't know why one would fail while the other succeeds.
Please note: I am aware that I have left my arrays unsorted, and that set_difference would therefore throw an exception. Right now I'm just trying to get the code to compile, and keeping the example brief.
