[Boost-users] Re: enable_if and constructors

From: David Abrahams (dave_at_[hidden])
Date: 2005-04-05 15:31:12

Angus Leeming <angus.leeming_at_[hidden]> writes:

> What are the advantages of the Boost.MPL 
> approach over the functionally identical:
>
> #include <iostream>
>
> class foo {
> public:
>     enum state {
>         state1,
>         state2,
>         state3
>     };
>
>     // Only the specializations (below) of this template will compile.
>     template <int N>
>     static foo
>     set() { return invalid_value; }

Without a definition for invalid_value, this is invalid code, and on a
conforming compiler, compilation fails at the point it is parsed.

> private:
>     foo(state)
>     {
>         std::cout << "foo" << std::endl;
>     }
> };
>
> template <>
> foo foo::set<foo::state2>() { return foo::state2; }
>
> template <>
> foo foo::set<foo::state3>() { return foo::state3; }
>
> int main()
> {
>     // Compiles, as expected.
>     foo f1 = foo::set<foo::state2>();
>     foo f2(foo::set<foo::state2>());
>     // Fail to compile, as expected.
> //  foo f3 = foo::set<foo::state1>();
> //  foo f4(foo::set<foo::state1>());
>
>     return 0;
> }

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com