I want to be able to supply functions which take an existing object of a given type by const reference.
i.e. I want by-const-ref-but-no-temporaries-created. [edited this phrase to clarify it!]
I can't think of any way to express that in C++ directly:
1 void fn(const T & t)
will create a temporary T from an S if a (non-explicit) T(S) exists.
2 void fn(T & t)
will only allow a non-const t as an argument (I need this to work for const t's as well).
3 void fn(T && t)
requires a non-const rvalue.
4 void fn(const T && t)
requires a const rvalue
Is there any way to do this - accept any argument by reference, const, without allowing a temp to be created?
Failed thoughts:
Supplying #2 and #4 without supply the other two, and now we have an unambiguous fn that takes a reference to any existing T (but won't silently generate a temporary)?
Nope, that isn't adequate either, because I need to also be able to bind to a const T & - Where the target object is itself been passed in by const reference to our caller...
I guess I see no way to achieve this except in the case where I can control T to ensure it doesn't supply an implicit T(S).
Am I right, or am I overlooking some solution?
Judging from the comments, it seems like your actual question has nothing to do with rvalues and lvalues, but simply preventing implicit conversions. You can do this by changing the function to a function template, and then constraining the template with sfinae:
template <class T, std::enable_if_t<std::is_same<std::decay_t<T>, CString>::value, int> = 0>
void fn(const T&) {
}
This will only work if called with some kind of CString, a type that implicit converts to a CString will not work.
Live example: http://coliru.stacked-crooked.com/a/80602c39cdc4d35e
That said, a better solution would simply be to change those functions to accept a string_view, which can be cheaply implicitly constructed from various kinds of strings. I don't know if it handles CString or not, but you can always right your own. It's a fairly simple class to write. http://en.cppreference.com/w/cpp/string/basic_string_view