Your function binsearch
takes a function pointer as argument. A lambda and a function pointer are different types: a lambda may be considered as an instance of a struct implementing operator()
.
Note that stateless lambdas (lambdas that don’t capture any variable) are implicitly convertible to function pointer. Here the implicit conversion doesn’t work because of template substitution:
#include <iostream>
template <typename T>
void call_predicate(const T& v, void (*predicate)(T)) {
std::cout << "template" << std::endl;
predicate(v);
}
void call_predicate(const int& v, void (*predicate)(int)) {
std::cout << "overload" << std::endl;
predicate(v);
}
void foo(double v) {
std::cout << v << std::endl;
}
int main() {
// compiles and calls template function
call_predicate(42.0, foo);
// compiles and calls overload with implicit conversion
call_predicate(42, [](int v){std::cout << v << std::endl;});
// doesn't compile because template substitution fails
//call_predicate(42.0, [](double v){std::cout << v << std::endl;});
// compiles and calls template function through explicit instantiation
call_predicate<double>(42.0, [](double v){std::cout << v << std::endl;});
}
You should make your function binsearch
more generic, something like:
template <typename T, typename Predicate>
T binsearch(const std::vector<T> &ts, Predicate p) {
// usage
for(auto& t : ts)
{
if(p(t)) return t;
}
// default value if p always returned false
return T{};
}
Take inspiration from standard algorithms library.