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

23 Jul 2007

      Raider wrote:
...
...
...
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.
It's not about std::vector, it's about 
boost_foreach_is_lightweight_proxy. I used std::vector only to make 
example code small. If you think std::vector make sence, you can change 
"typedef std::vectorstd::string vec;" line by the following:
typedef vector<string> sv;
sv global;
class vec
{
public:
     vec(sv& v = global) : v_(v) {}
     typedef sv::iterator iterator;
     typedef sv::const_iterator const_iterator;
     iterator begin() { return v_.begin(); }
     iterator end() { return v_.end(); }
     const_iterator begin() const { return v_.begin(); }
     const_iterator end() const { return v_.end(); }
     void push_back(const string& s) { v_.push_back(s); }
private:
     sv& v_;
};
It's a *real* _lightweight_ proxy, but still getting a compiler error!
Better, but you have proxied a non-const vector, and you're presenting a 
const interface. Try this:

   class vec
   {
   public:
        vec(sv& v = global) : v_(v) {}
        typedef sv::iterator iterator;
        typedef sv::iterator const_iterator;
        typedef sv::const_iterator const_iterator;
        iterator begin() const { return v_.begin(); }
        iterator end() const { return v_.end(); }
        void push_back(const string& s) const { v_.push_back(s); }
   private:
        sv& v_;
   };

The const-ness of the proxy itself shouldn't matter. Only the const-ness 
of the proxied object should matter.

HTH,

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

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