CODEWITHSUNDEEP
c++c++11type-inference
varnie
varnie

Reputation: 2595

Auto-deduce method's return type

I have the following functions:

//method takes 2 or more template parameters    
template <class A1, class A2, class ...Ax>
Value<FooMany> getValue() {

    //note, FooAll's ctor takes std::string and std::initializer_list<std::size_t>

    FooAll<Item> hairyStructure("abc", { Foo<A1>::getIndex(), Foo<A2>::getIndex(), Foo<Ax>::getIndex() ... } );
    return Value<FooMany>(someData, hairyStructure);
}

//method takes only 1 template parameter
template <class A>
Value<FooSingle> getValue() {

    //note, FooOne's ctor takes std::string and std::size_t

    FooOne<Item> hairyStructure("abc", Foo<A>::getIndex() );
    return Value<FooSingle>(someData, hairyStructure);
}

.

Obviously, the types of these functions are different.

I wonder, is it possible to squash these two into the single method, which, utilizing the C++11 features (decltype, I suppose), would auto-deduce the return type?

So, basically, it should return Value<FooSingle> if getValue is invoked as

GetValue<A>();

and it should return Value<FooMany> if it is invoked for example as

GetValue<A, B>();

or

GetValue<A, B, C>();

I am not sure if my terminology is correct regarding "method takes 2 or more template parameters". Please correct me if it is wrong.

If it helps, my question continues the previous topic: C++11 parameters pack overload

Thank you.

Upvotes: 2

Views: 196

Answers (1)

Piotr Skotnicki
Piotr Skotnicki

Reputation: 48447

#include <type_traits>

template <class A1, class... Ax>
auto getValue()
    -> Value<typename std::conditional<sizeof...(Ax) == 0
                                     , FooSingle, FooMany>::type>
{
    typename std::conditional<sizeof...(Ax) == 0
                            , FooOne<Item> 
                            , FooAll<Item>>::type
              hairyStructure("abc", { Foo<A1>::getIndex(), Foo<Ax>::getIndex()... } );
    return Value<typename std::conditional<sizeof...(Ax) == 0
                                         , FooSingle, FooMany>::type>(hairyStructure);
}

DEMO

Upvotes: 1

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