Can C++ templates check if a function has been overloaded for a given type?

I have a template function that I expect to be templatized for different types at different places.

The problem is that I would like to know at compile time if there is an specialization for the given type to generate in 2 different ways another template.

template<typename T>
bool tobool(const T&){ throw Exception("Can't cast to bool");};
template<> bool tobool<bool>(const bool &value){ return val开发者_如何学Goue;}

I know you can test for function existance like in here.

Any chance on how to test if tobool has been specialized?

Imagine that I want to generate a isbool() that returns true if tobool() has been specialized and returns false if not.

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As a (somewhat ugly and brittle) workaround, you could require specialization of a struct rather than a function and include a class constant to indicate whether the struct has been specialized:

template <typename T> 
struct ToBool {
   static bool tobool(const T&);
   static const bool specialized = false;
};

Another option is to only define tobool in specializations. That way, ToBool<Foo>::tobool(f) won't compile for any classes Foo that ToBool hasn't been specialized for.

As an alternative to tobool, you can use explicit conversion operators if you have control over the classes to be converted.

class Foo {
public:
    operator bool();
    ...
};
...
    Foo f;
    if (f) ...

If the type doesn't have a bool conversion (well, a conversion to a numeric or pointer type, both of which have standard conversions to bool), the program won't compile. Voila, compile time checking for a conversion.

If you don't want implicit conversion to bool, you can define an operator! and use a double-bang for explicit conversion (though this isn't as readable):

class Foo {
public:
    bool operator!();
    ...
};

...
    Foo f;
    if (!!f) ...

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The answer to your specific question is this: No, you cannot check whether T is using the primary or the specialized template. @Martin York's question is a very good one: why on earth would you want to check that? :)