How to apply function to all elements in array (in C++ template class)

I still think you should use std::transform:

template <class OutputIter, class UnaryFunction>
void apply_pointwise(OutputIter first, OutputIter last, UnaryFunction f)
{
    std::transform(first, last, first, f);
}

This function works not only for std::vector types but indeed any container that has a begin() and end() member function, and it even works for C-style arrays with the help of the free functions std::begin and std::end. The unary function may be any free function, a functor object, a lambda expression or even member functions of a class.

As for the problem with std::sin, this free function is templated and so the compiler cannot know which template instantiation you need.

If you have access to C++11, then simply use a lambda expression:

std::vector<float> v;
// ...
apply_pointwise(v.begin(), v.end(), [](const float f)
{
    return std::sin(f);
});

This way, the compiler knows that it should substitute T=float as the template parameter.

If you can use C functions, you can also use the function sinf, which is not templated and takes a float as a parameter:

apply_pointwise(v.begin(), v.end(), sinf);