Re: [Boost-users] [foreach] boost_foreach_is_lightweight_proxy doesn't work

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-23 14:06:39

Raider wrote:
> Hi, i'm trying to modify rvalue container using Boost.ForEach:
> 
> #include <string>
> #include <vector>
> #include <boost/foreach.hpp>
> 
> typedef std::vector<std::string> vec;
> 
> inline boost::mpl::true_ *
> boost_foreach_is_lightweight_proxy(vec *&, boost::foreach::tag)
> { return 0; }
> 
> vec GetVector()
> {
>      vec v;
>      v.push_back("hello");
>      v.push_back("world");
>      return v;
> }
> 
> int main()
> {
>      // compiles ok
>      GetVector().push_back("oh god, it's writable!");
> 
>      // next line gives "unable to convert `const string' to `string&'"
>      BOOST_FOREACH(std::string& String, GetVector())
>          String.clear();
> 
>      return 0;
> }
> 
> It doen't compiles (MCVC 8.0) - it looks like 
> boost_foreach_is_lightweight_proxy is ignored. What's wrong?

It's not being ignored --- std::vector is simply not a lightweight 
proxy. You're lying to BOOST_FOREACH, and it's found you out. :-) The 
docs say this about lightweight proxy types:

> A pair of iterators is an example of a lightweight proxy. It does not store the values of the sequence; rather, it stores iterators to them. This means that iterating over a copy of the proxy object will give the same results as using the object itself.

Iterating over a copy of a std::vector isn't the same thing as as 
iterating over the original vector because the elements have been 
copied. When BOOST_FOREACH has to make a copy of a sequence (and it 
always copies lightweight proxies), it treats the copy as const because 
it's a temporary object. That's fine for *real* proxies because the 
const-ness of the proxy doesn't affect the constness of the referred-to 
elements.

I think BOOST_FOREACH is doing the right thing here.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com
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