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c++functionlambdastldecltype

define a custom Compare for multiset

function<bool(const pair<int, int>&, const pair<int, int>&)> cmp = [](const pair<int, int> & left, const pair<int, int> & right){if(left > right) return true;};

//multiset<pair<int, int>, function<bool(const pair<int, int>&, const pair<int, int>&)> mt(cmp);
multiset<pair<int, int>, decltype(cmp)> mt1(cmp);

cout<<typeid(decltype(cmp)).name()<<endl<<typeid(cmp).name()<<endl;

the comment code(mt) will make an error whereas the line below(mt1) won't.

I tried to cout the name of two type and they are the same.

Besides, I also tried like this:

multiset<pair<int, int>, const function<bool(const pair<int, int>&, const pair<int, int>&) &> mt(cmp);

or

multiset<pair<int, int>, function<bool(const pair<int, int>&, const pair<int, int>&) &> mt(cmp);

All of them don't work.

So what is the type of the second argument exactly? If I don't use decltype, how can I declare it?

Solution

  • Look carefully at your code, you lost a >.

    multiset<pair<int, int>, function<bool(const pair<int, int>&, const pair<int, int>&)>> mt(cmp);
//                                                                                   ~

    Using decltype should be a good idea.

    BTW: You should return something when if returns false in lambda. Or just return left > right;.