C++ Template problem adding two data types

Question

I have a template class with an overloaded + operator. This is working fine when I am adding two ints or two doubles. How do I get it to add and int and a double and return th

Answer 1

The add operator should generally be a free function to avoid preferring any operand type as @Stephen nicely explains. This way it's completely symmetric. Assuming you have a function get that returns the stored value, this can be like the following (alternatively, you can declare it as a friend if such a get function does not exist)

template
TemplateTest operator+(const TemplateTest& t1, const TemplateTest& t2)
{
  return TemplateTest(t1.get() + t2.get());
}

The problem now is to find a result type. As other answers show this is possible with decltype in C++0x. You can also simulate this by using the rules of the ?: operator which are mostly quite intuitive. promote<> is a template that uses that technique

template
TemplateTest< typename promote::type > 
operator+(const TemplateTest& t1, const TemplateTest& t2)
{
  return TemplateTest< typename promote::type >(t1.get() + t2.get());
}

Now for example if you add double and int, it will yield double as the result. Alternatively as shown in the promote<> answer, you can also specialize promote so you can apply it directly to TemplateTest types.