Re: [boost] variant assignment

From: Eric Friedman (ebf_at_[hidden])
Date: 2006-06-04 02:57:24

Hi Arun,

Thank you for noting and taking the time to track down this issue! I 
believe you are correct. I just wrote the following simple test:

    #include <iostream>
    #include "boost/variant.hpp"

    static int copyCount = 0;
    static int assignCount = 0;

    struct MyType
    {
       MyType() {}
       MyType(const MyType&) { ++copyCount; }
       MyType& operator=(const MyType&) { ++assignCount; return *this; }
    };

    int main()
    {
       boost::variant<MyType> var;
       var = MyType();

       std::cout
          << "copied: " << copyCount
          << "; assigned: " << assignCount
          << std::endl;
    }

The expected result would be

    copied: 0; assigned: 1

but due to the defect you noted, the actual result in fact is

    copied: 1; assigned: 1

For GCC, the fix is exactly as you suggest. In the next day or two I 
will make the change so that it can be tested with other compilers. As 
you can see in the code, there is already a workaround for VC6, so you 
never know what might break :)

Eric

arun.s wrote:
> Im trying to initialize boost  variant  ( from boost_1_33_1) as below in
> MSVC8
> 
>    variant<char* , int> v ;
>    v =  2;
> 
> I find that the the direct_assigner always fails and returns false in assign
> method of variant.
> 
> template <class ....
> class variant
> {
> ...........
>     template <typename T>
>     void assign(const T& rhs)
>     {
>         // If direct T-to-T assignment is not possible...
>         detail::variant::direct_assigner<const T> direct_assign(rhs); ///
> ---------------(1)
>         if (this->apply_visitor(direct_assign) == false)<---------
> 
> 
> the variant template arguments
> 
> digging deeper I found that the reason is in direct_assigner visitor
> interface
> 
> template <typename T>
> class direct_assigner
>     : public static_visitor<bool>
> {
> 
> private: // representation
> 
>     T& rhs_;                    //  <---------------------------------------
> (2)
> 
> ....
> ...
> 
> #if !BOOST_WORKAROUND(BOOST_MSVC, <= 1200)
> 
> public: // visitor interface
> 
>     bool operator()(T& lhs) //<--------------------(3) is never called
>     {
>         lhs = rhs_;
>         return true;
>     }
> 
>     template <typename U>
>     bool operator()(U&)
>     {
>         return false; ////<---------we always hit this
>     }
> 
> The operator() in statement (3) is never called since T is of const type (
> as set in statement (1) above in assign method of variant).
> bit variant template argument is not const.
> 
> Shouldnt 'const' be prepended to statement (2) instead of prepending it in
> (1)?
> 
> Thanks,
> Arun
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