Re: [boost] [proto] design of expr<>

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-03-22 13:39:23

On 03/22/08 10:24, Steven Watanabe wrote:
> AMDG
> 
> Larry Evans wrote:
>> Could you post a test case with the before (only 1 expr with 3 args)
[snip]
>> arity and show how the 2 expr template method wouldn't work?
> 
> template<class Tag, class Args, int arity = Args::arity>
> struct expr3;
> 
> template<class Tag, class Args>
> struct expr3<Tag, Args, 2> {
>     typename Args::arg0 arg0;
>     typename Args::arg1 arg1;
> };
> 
> template<class Tag, class Args>
> struct expr2 {
>     expr3<Tag, Args> args;
> };
> 
> struct test {
>     int i;
> };
> 
> struct my_args {
>     enum { arity = 2 };
>     typedef test arg0;
>     typedef test arg1;
> };
> 
> struct my_tag {};
> 
> int main() {
>     expr3<my_tag, my_args> x3 = { {1}, {1} };
>     expr2<my_tag, my_args> x2 = { {1}, {1} };  // error C2078: too many 
> initializers
> }

To make x2 compile, add an extra {}, IOW

   x2 = { { {1}, {1} } }

where the extran {} is for expr2::args.

So, the cost of this extra indirection is
1 more curly braces in
every expr initializer list?