Does default argument of template class, instantiate the template?

Question

Does the following default argument for the template instantiates a template with type EmptyClass?

class EmptyClass{};

template <typename TYPE=EmptyClass>
class Sample
{
public:
    static void test()
    {
        TYPE::Serialize();
    }
};

This didn't give me any compilation error. That means, it doesn't actually instantiates the templates. But I couldn't find any line in C++11 standards that talks about instantiation. — Sabhareesh Ravichandran, Mar 21 '16 at 12:35
With templates, nothing is instantiated until you actually *use* the template. Having `static` member function is different, since they don't work on instances. — Some programmer dude, Mar 21 '16 at 12:37
What is my doubt is. Does this aforementioned code, instantiates a template with type `EmptyClass`? — Sabhareesh Ravichandran, Mar 21 '16 at 12:38
actyaully I am not sure, but afaik adding e.g. a line `typedef Sample<> DoesNotWork;` should create a compiler error. — 463035818_is_not_an_ai, Mar 21 '16 at 12:39
Do you declare an instance of `Sample<>` anywhere? If not then no there is no instantiation. — Some programmer dude, Mar 21 '16 at 12:39

score 0 · Answer 1 · answered Nov 27 '16 at 15:38

No. Template are instantiated when used, and are instantiated on a per-function base.

Default parameter values are just the types to use when the parameter is not specified. But does not themselves imply usage.

When you call Sample<>::test() then Sample<Emptyclass>::test() is instantiated and the EmptyClass::serialize() call attempted, resulting in a compile-time error (Since Emptyclass is declared as not having such function)

Try to make up more function, containing different compile-time errors referring to different parameter, and you'll see how, until the function is not used, no error is produced.

score -1 · Accepted Answer · answered Mar 21 '16 at 12:49

-1

No, in that code any instance of EmptyClass is created. Serialize is a static function. And EmptyClass's constructor is never called (in code showed)

answered Mar 21 '16 at 12:49

Ing. Gerardo Sánchez

1,607
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The question was not about constructors and objects. It is about template instantiation – Sabhareesh Ravichandran Mar 21 '16 at 13:49
1

Instance of a template is not an instance of a class which is an instance of a class template. So I do not underszand what your answer has to do with the question. And I can't underszand why this "answer" was excepted... – Klaus Nov 27 '16 at 15:17

Does default argument of template class, instantiate the template?

2 Answers2