How do I specify a pointer to an overloaded function?

Lambdas to the rescue! (note: C++11 required)

std::for_each(s.begin(), s.end(), [&](char a){ return f(a); });

Or using decltype for the lambda parameter:

std::for_each(s.begin(), s.end(), [&](decltype(*s.begin()) a){ return f(a); });

With polymorphic lambdas (C++14):

std::for_each(s.begin(), s.end(), [&](auto a){ return f(a); });

Or disambiguate by removing overloading (only works for free functions):

void f_c(char i)
{
    return f(i);
}
    
void scan(const std::string& s)
{
    std::for_each(s.begin(), s.end(), f_c);
}

You can use static_cast<>() to specify which f to use according to the function signature implied by the function pointer type:

// Uses the void f(char c); overload
std::for_each(s.begin(), s.end(), static_cast<void (*)(char)>(&f));
// Uses the void f(int i); overload
std::for_each(s.begin(), s.end(), static_cast<void (*)(int)>(&f)); 

Or, you can also do this:

// The compiler will figure out which f to use according to
// the function pointer declaration.
void (*fpc)(char) = &f;
std::for_each(s.begin(), s.end(), fpc); // Uses the void f(char c); overload
void (*fpi)(int) = &f;
std::for_each(s.begin(), s.end(), fpi); // Uses the void f(int i); overload

If f is a member function, then you need to use mem_fun, or for your case, use the solution presented in this Dr. Dobb's article.