Passing a lambda into a function template
c++, c++11, function-pointers, lambda, templates
Solution
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.
Problem
I'm learning C++, and I'm trying to implement a binary search function that finds the first element for which a predicate holds. The function's first argument is a vector and the second argument is a function that evaluates the predicate for a given element. The binary search function looks like this: ``` template <typename T> int binsearch(const std::vector<T> &ts, bool (*predicate)(T)) { ... } ``` This works as expected if used like this: ``` bool gte(int x) { return x >= 5; } int main(int argc, char** argv) { std::vector<int> a = {1, 2, 3}; binsearch(a, gte); return 0; } ``` But if I use a lambda function as a predicate, I get a compiler error: ``` search-for-a-range.cpp:20:5: error: no matching function for call to 'binsearch' binsearch(a, [](int e) -> bool { return e >= 5; }); ^~~~~~~~~ search-for-a-range.cpp:6:27: note: candidate template ignored: could not match 'bool (*)(T)' against '(lambda at search-for-a-range.cpp:20:18)' template <typename T> int binsearch(const std::vector<T> &ts, ^ 1 error generated. ``` The above error is generated by ``` binsearch(a, [](int e) -> bool { return e >= 5; }); ``` What's wrong? Why is the compiler not convinced that my lambda has the right type?