How is the return type here being deduced? In other words, how based on the return type of the function pointer does it instantiate the correct function.
template<typename From, typename To>
To Convert(const From& f)
{
std::cout << sizeof(From) << " " << sizeof(To) << std::endl;
return To(f);
}
int main()
{
float (*p)(const double &e) = Convert;
p(20.2);
}
This prints
8 4
It's true that return types cannot be deduced in a function call even if there is context that could be used:
template<class T> T get();
struct X {} x=get(); // error: cannot deduce T
No type except (reference to) cv X could work here, but you just don't consider that it is exactly the initializer of a variable.
However, return types can be deduced when a function is named in a context where a particular type is "obviously" expected, as in the question, as well as when conversion function templates are used (since there's nothing else to deduce).