c++ - How to hide an implementation helper template?

Question

Suppose that I have two template functions declared in a header file:

template <typename T> void func1(const T& value);
template <typename T> void func2(const T& value);

And suppose that the implementation of these functions (also in a header file and not in a source file, because they are templates) uses some implementation helper function, which is also a template:

template <typename T> void helper(const T& value) {
    // ...
}

template <typename T> void func1(const T& value) {
    // ...
    helper(value);
}

template <typename T> void func2(const T& value) {
    // ...
    helper(value);
}

In any source file that I include the header file, the helper function will be visible. I don't want that, because the helper function is just an implementation detail. Is there a way to hide the helper function?

Answer 1

A common approach (as used in many Boost libraries, for example) is to put the helper in a namespace called details, possibly in a separate header (included from the "public" header).

There's no way to prevent it from being visible, and callable, but this quite clearly indicates that it is part of the implementation, not the interface.