Templated function not called

Question

I have overloaded a function in my string class, however, it never gets called. Why?

template <class T>
class StringT {
public:
    void assign(const T* ptr);
    template <size_t N> void assign(const T(&ptr)[N]);
};

int main() {
    StringT<char> str;
    str.assign("Hello World"); //calls "void assign(const T* ptr)" although type is (const char[12])
}

Answer 1

For more reference, some specific references to the Standard are:

13.3.3 Best viable function

Given these definitions, a viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then...

• F1 is not a function template specialization and F2 is a function template specialization...

In this case, the non-templated function is (obviously) not a function template specialization and the conversion of "Hello World" to char const* is not worse than to const char[N], per the ranking rules defined in the Table in the "Standard conversion sequences" section. According to that Table, both No conversions required and Array-to-pointer conversion are considered an exact match in the context of overload resolution. Likewise, if the templated overloads are changed to a non-template overload (i.e., as void assign(const T(&ptr)[12]);), then compilation of str.assign("Hello World"); will fail due to an ambiguous call.

To make sure the non-template function isn't considered for overload, there is the following note under the "Explicit template argument specification" section:

Note: An empty template argument list can be used to indicate that a given use refers to a specialization of a function template even when a non-template function (8.3.5) is visible that would otherwise be used.

So, you can use str.assign<>("Hello World"); for that.

Answer 2

When there is a choice compiler chooses most specialized function. When there is a non-template function than it is treated as more specialized than any template function

Details here

If you want to keep non-template function but forcibly call template one try

str.template assign("Hello World");