CODEWITHSUNDEEP
c++c++11templatesauto
Martin Zink
Martin Zink

Reputation: 123

Auto return type in template

I have got multiple data classes, in various modules in my code, which i do no intend to modify whatsoever. I have a container classes for each of them, which have a pointer to this data class, and also got a pointer to a class which is intended to hold these classes without knowing about them.

So in the container classes I implemented:

DataClass GetMyData (void){
   return myData;
}

And in the collection of these cointainer classes i created

template<class ContainerClass>
ContainerClass  GetContainer (void);

template<class ContainerClass>
auto    GetData (void){
    GetContainer<ContainerClass> ().GetMyData();
}

But whenever i try to use this GetData function i get the error

a function that returns 'auto' cannot be used before it is defined

So my question is, is it possible to use auto in this case (c++11), or i have to make my template with 2 classes?

Upvotes: 2

Views: 6884

Answers (1)

max66
max66

Reputation: 66200

If you forget return in GetData(), auto can't work is unuseful to return the same type returned by the getData() of the template type (thanks Jarod42!).

Anyway... given a couple of struct as follows

struct A
 { int getData () const { return 1; } };

struct B
 { std::string getData () const { return "abc"; } };

in C++14 GetData() can be written simply as

template <class CC>
auto GetData ()
 { return CC{}.getData(); }

but this doesn't work in C++11; in C++11 you can write

template <class CC>
auto GetData () -> decltype( CC{}.getData() )
 { return CC{}.getData(); }

Before C++11 there isn't (this use of) auto.

--- EDIT ---

As pointed by StoryTeller, the use of decltype( CC{}.getData() ) assumes that the type CC is default constructible.

In the toy example I've written, this isn't a problem because I've used CC{} in the body of the function.

In simple cases, you can mirror in the decltype() expression what do return in the body of the function; by example, if you pass an object of type CC, you can write

template <class CC>
auto GetData (CC const & cc) -> decltype( cc.getData() )
 { return cc.getData(); }

But isn't ever so simple: there are cases when you know that the value returned is a given expression but the function is to complicated to mirror it in the decltype() expression.

So, to avoid the contructible/not constructible problem, it's better use (as suggested by StoryTeller; thanks) std::declval; as follows, by example

template <class CC>
auto GetData () -> decltype( std::declval<CC>().getData() )
 { /* something complicated ... */ return something.getData(); }

Upvotes: 8

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