Template deduction reversion with function pointer

What have I done wrong and how can I fix it?

The way to fix that is to #include <cmath> instead of using #include <math.h> as mentioned in the reference documentation:

#include <cmath> // <<< official header to use.
#include <iostream>

template<typename T,typename T1>
T apply(T (*func)(T1), const T1 &val)
{
  return func(val);
}

int main(void)
{
  double val1 = 0.5, val2 = apply(ceil,val1);

  std::cout << val1 << ' ' << val2<< std::endl;
}