C++Talk.NET

Thomas Mang · Guest

Hi,

Given is this very simple template function:

template <class T, class U>
resultType foo(T v1, U v2) {
return v1 + v2;
}

And the question:
What is the best way to determine what resultType should be?

Using the normal type-conversion rules, if say T is an int and U is a
double, the result of the expression of v1 + v2 is of course double, so
ideally double should be returned. But how can I code this in returnValue?

Of course, I can write something along the lines of:

template <class T, class U>
struct builtinReturnValue;

and specialize that, such as:

template <>
struct builtinResultType<int, double>{
typedef double result_type;
};

and use that.
Does a thing doing this already exist in a library? (boost::result_of
does not seem to do that job)

Furthermore, if I take overloaded operators into account, ideally that
should work too.

My question is probably closely related to N1454:
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1454.html

Once T and U are deduced, the compiler is clearly able to determine the
type of the expression in the return - statement. Is a future version of
the std likely to support access to that information - it would be kind
of the auto-proposal for return-types, something along these lines:

// example pseudo code:

template <class T, class U>
auto foo(T v1, U v2) {
return v1 + v2
}

??

thanks,
Thomas

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