struct compar {
bool operator()(const vector<int>& a,
const vector<int>& b) const {
return a[1] < b[1];
}
};
...
auto it = lower_bound(events.begin(), events.end(), {0, events[i][0]}, compar());
This code gives me an error with the {0, events[i][0]}:
/bits/stl_algo.h:2022:5: note: candidate template ignored: couldn't infer template argument '_Tp'
lower_bound(_ForwardIterator __first, _ForwardIterator __last,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/algorithmfwd.h:353:5: note: candidate function template not viable: requires 3 arguments, but 4 were provided
lower_bound(_FIter, _FIter, const _Tp&);
^
1 error generated.
But when I define it as a vector explicitly, it works as desired.
vector<int> point = {0, events[i][0]};
auto it = lower_bound(events.begin(), events.end(), point, compar());
Can someone explain why?
Braced-init-list has no type itself, it can't be used for deduction of template parameter. This is non-deduced context:
In the following cases, the types, templates, and non-type values that are used to compose P do not participate in template argument deduction, but instead use the template arguments that were either deduced elsewhere or explicitly specified. If a template parameter is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.
- The parameter P, whose A is a braced-init-list, but P is not std::initializer_list, a reference to one (possibly cv-qualified), or a reference to an array:
As you're showed, you have to specify the type explicitly, e.g.
auto it = lower_bound(events.begin(), events.end(), vector<int>{0, events[i][0]}, compar());
// ^^^^^^^^^^^
Or specify template arguments as
auto it = lower_bound<decltype(events.begin()), std::vector<int>>(events.begin(), events.end(), {0, events[i][0]}, compar());
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^