On Tuesday 12 May 2009, Andreas Buykx wrote:
> Hi all,
>
> Yesterday I ran into a g++ (3.4.6) compiler problem for code that I have
> been compiling without a problem using the Intel (9.0) compiler. Here's a
> code snippet that shows what happened:
>
> template<typename A, typename B>
> class Foo { };
>
> struct Bar {
> void method ( Foo<int,int> const& stuff = Foo<int,int>() );
> };
>
The following compiles with g++-4.2.3, see the extra parentheses
--------------------------------------------------
template< typename A, typename B>
class Foo {};
struct Bar {
void method( Foo<int,int> const& stuff = (Foo<int,int>()) );
};
--------------------------------------------------
Though I don't know the exact differences
> The g++ compiler error is:
>
> foo.cpp:5: error: expected `,' or `...' before '>' token
> foo.cpp:5: error: wrong number of template arguments (1, should be 2)
> foo.cpp:2: error: provided for `template<class A, class B> struct Foo'
> foo.cpp:5: error: default argument missing for parameter 2 of `void
> Bar::method(const Foo<int, int>&, int)'
>
> Apparently, the default argument is not accepted when written this way,
> and the compiler assumes that instead of the second template argument a
> new function argument is specified, for which it then expects a default
> value because the `stuff` argument has one. I can help the compiler by
> creating a typedef, and then everything compiles fine:
>
> template<typename A, typename B>
> class Foo { };
>
> struct Bar {
> typedef Foo<int,int> FooType;
> void method ( FooType const& stuff = FooType() );
> };
>
> So I can solve my problem, but I don't understand what is going on. Do I
> miss a C++ (template?) language feature here and am I doing something
> wrong, or is the g++ compiler wrong in not accepting the first piece of
> code?
>
> Note BTW that this also compiles ...
>
> template<typename A, typename B>
> class Foo { };
>
> void method ( Foo<int,int> const& stuff = Foo<int,int>() );
>
> >From stackoverflow, where I posted the exact same question, two other
>
> people confirmed that they could compile the code with two other
> compilers: IBM's xlC V7.0 and Comeau.
