Template Return Type with Default Value

Question

So in writing a C++ template class, I have defined a method that returns an object of the templated type, as such:

template 
class Foo
{
public         


        

Answer 1

Boost addresses this problem in one of its utility template classes, namely boost::value_initialized and its relatives.

Answer 2

The line

T t;

Default constructs objects, yet declares uninitialized built-in types. There is no syntax to 'default construct' a local variable.

Therefore it's not trivial to write generic code that initializes a variable, whether built-in or class.

Wrapping the type into a member variable, however, may present a workaround that does not need copy construction (apart from the return statement):

template< typename T > struct Initializer {
   T t;
   Initializer()
   :t() // ====> default construction, works for classes _and_ built-in
   {}
};

Using this wrapper, you can build your code in a generic way:

template<typename T> T foo() {
  Initializer<T> i;
  // fill in i.t to your liking
  return i.t;
}

See a full-blown snippet at codepad.

Answer 3

This seems to work for primitive types (such as int and float) and classes, at least so long as that class has a copy constructor

For classes, not only [public] copy-constructor, you also need public default constructor!

My question is - is this the accepted way of solving this problem?

Usually it's a good idea to provide public default constructor. STL containers use it all the time! If you don't have one, most (or maybe, all) STL containers would not work at many occasions.

See this example with private default constructor: http://ideone.com/EdPLu

Answer 4

Do you really want to return an un-initialized value from this function? Sounds like it could lead to a whole lot of crap later down the line.

Why don't you use an appropriate wrapper - if you have access to boost, consider boost::optional. This way, if you don't initialize it, it can be tested properly.

Answer 5

Sounds OK, if the class has no copy constructor, you will not be able to return it anyway.

Answer 6

Indeed that's the standard way to solve the problem. I don't believe there should be any side effects to worry about in this case.