c++ - How to write a template function where its parameter type is required to be inheriting certain class

Question

Is it possible to declare a template function where a certain type is derived from let's say B?

My goal is to achieve something like this:

template<class T : std::ostream> void write(T os) {
    os << "...";
} 

template<class T : std::string> void write(T s) {
   // ...
}

Edit: I know it is not a solid example since it is not usual to derive from a string, but note that it's just a example.

So any solution like a workaround is welcome, however I would like to be able to explicit instantiate the template functions.

Answer 1

是的，使用 C++11<type_traits>可以实现。
如果你只有 C++03，你可以用 Boost<type_traits>代替。

template <typename T>
typename std::enable_if<std::is_base_of<std::ostream, T>::value>::type
write(T& os) {
}

Answer 2

任何派生自 std::ostream 的对象都可以用作参数

void write(std::ostream os) {
    os << "...";
}