Question

Given the following base class:

struct ValueType {
    String
    ToString(String const& format) const;
};

I want this overload to be called for types deriving from ValueType:

String FormatValue(const ValueType& value, const String& format)
{
    return value.ToString(format);
}

Otherwise, I want this overload to be called:

template <typename T>
String FormatValue(const T& value, const String& format);

How can I ensure that derived types do not instead call the second overload?

The original question is located here.

Answer 1

I am not too fond of what you are trying to do for different reasons (including the ValueType interface, why not use AnyToString always?), but at any rate you should be able to solve your problem with SFINAE

template <typename T>
typename enable_if< !is_base_of<ValueType, T>::value, String>::type
FormatValue( T const & value, const String& format ) { ... }

What that code does (once you make it compile 🙂 is inhibiting the template function when the condition is met. When the compiler considers the template as an overload it will try to substitute the type, and the enable_if instantiation will fail to have a nested type if the condition is met, so the substitution fails and the template is discarded. After that, the best overload will be the version that takes a ValueType object.