Default constructor won't compile inside template class when brackets are included (g++4.6.1)

Question

I couldn't find any information on Google about this, In the following example:

    #include <iostream>

    class Default
    {
    public:
      void Print()
      {
        std::cout << "This is a message\n";
      }
    };

    template <class C = Default>
    class Template
    {
    public:
      static void Test()
      {
        Default oDefault();
      }
    };

    int main()
    {
      return 0;
    }

the code fails to compile with the error:

In static member function ‘static void Template::Test()’: 19:22: error: default template arguments may not be used in function templates without -std=c++0x or -std=gnu++0x

The trouble is that it doesn't like the brackets appearing on that line and I don't understand why. If I remove the brackets the code compiles just fine. Also if I remove the template declaration (line 13) it also compiles just fine. Is this a bug or is there some rule somewhere about exactly this situation?

I'm using g++4.6.1 (gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3))

FWIW, VC10 has no problems with it. (Of course, it takes `oDefault` as a function declaration, which I agree is legal). — MSalters, Mar 19 '12 at 09:35

score 2 · Accepted Answer · edited May 23 '17 at 12:20

Change it to:

 Default oDefault = Default();  // Version 1

Though you can use:

 Default oDefault;              // Version 2

This has a slightly different meaning.

Version 1: Causes the POD members to be zero initialized (in some situations).
Version 2: Causes the POD members to be un-initialized (in some situations).

Through version one looks like it invokes an extra copy construction this is not the case (in most compilers) as the extra copy will be elided by the compiler and a simple normal (zero-initialized) construction will happen.

You should prefer zero-initialization to default (in general) as if the class (or any members type) does not have a user defined constructor (Like Default) then the difference is that default-initialization leaves POD members undefined while zero-initialized leaves POD members initialized to zero. Now you may think now that my class has no members so it does not matter. But what happens if you modify the class are you going to go and find all instances and update them; Best to use version one and let the compiler do the correct thing.

For all the mind blowing details see:

The reason your initial version did not work is that it is actually a forward declaration of a function. This is caused by the complex syntax of C++ and just one of the rules you need to remember. You can look this up as "The Most Vexing Parse".

Version 1 causes _value_ initialization (which may in turn be defined in terms of zero initialization for certain types). — ildjarn, Mar 19 '12 at 04:59
@ildjarn: Yes absolutely correct. But I did not want to go into all the details here. The final result is zero-initialization of POD members (making them well defined). — Martin York, Mar 19 '12 at 05:05
That makes sense (parsing it as a function declaration), but why does having the template <...> in front of the class cause an error? — SirGuy, Mar 19 '12 at 13:34
It's not the template per say. Its the default arguments. `= Default` that is causing the problem. — Martin York, Mar 19 '12 at 18:08
I don't know why the default template parameter is causing trouble, but this is far enough away from the original problem that I'm not going to worry about it further — SirGuy, Mar 20 '12 at 01:02

score 1 · Answer 2 · answered Mar 19 '12 at 04:07

1

Default oDefault();

The compiler sees this as an function declaration and not as creating an object.
It declares a function oDefault() which takes no parameters and returns a Default object.

Change it to:

Default oDefault;

answered Mar 19 '12 at 04:07

Alok Save

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While this is certainly what's _intended_, it doesn't explain why the function declaration is illegal. – MSalters Mar 19 '12 at 09:33

Default constructor won't compile inside template class when brackets are included (g++4.6.1)

2 Answers2