Re: [Boost-users] lexical_cast and default params

Subject: Re: [Boost-users] lexical_cast and default params
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2009-12-01 13:55:45

In article <MPG.257f02b13a41ac01989684_at_[hidden]>, 
roberts.noah_at_[hidden] says...
> 
> Running into something very strange with boost::lexical_cast in 1.39.
> 
> This code works, but notice the f().

Interestingly, when actually applied to the production code I'm having 
trouble with this method did not work.  I got the same error for my 
wrapper and had to set it up to return std::string only.

In my production code I put the wrapper in a detail_ namespace for what 
I was working on.  Turns out that scope resolution at the call site 
causes the problem.  Tested this fact by placing a "using namespace 
boost" above the test code and replacing f() with lexical_cast(); it 
worked fine.

This seems like a compiler bug to me unless someone can tell me why a 
template parameter supplied at the call site would be unresolved only 
when you're using the scope operator to find it.  AFAIK a supplied 
parameter should never be "unresolved" and I can't see why this would be 
a special case.

Leaving the original code pasted below for reference.

> #include <boost/lexical_cast.hpp>
> #include <string>
> #include <iostream>
> 
> 

// if f() is in a nampace and we use scope resolution in test
// test constructor, this will fail to resolve the R supplied at
// the call site.  MSVC++ 8.0

> template < typename R, typename T >
> R f(T const& x)
> {
>   return boost::lexical_cast<R>(x);
> }
> 
> template < typename T >
> struct test
> {
>   test(std::string const& x = f<std::string>(T()))
>     : test_string(x) {}
> 
>   std::string test_string;
> };
> 
> int main()
> {
>   test<int> t;
> 
>   std::cout << t.test_string;
>   std::cout << boost::lexical_cast<std::string>(int());
> 
>   std::cin.get();
> }