Question

I’m using Visual Studio 2010 sp1. with c++ and boost 1.47 in Windows 7

I have the following code:

typedef boost::variant<unsigned int,int,long,double,bool,string,istringstream,int32_t,
                                 uint32_t,int64_t,uint32_t,sql::SQLString> SQLVariant;

I have also try replacing istringstream for istream. I do get the following error :

Error   13  error C2249: 'std::basic_ios<_Elem,_Traits>::operator =' : 
no accessible path to private member declared in virtual base 'std::basic_ios<_Elem,_Traits>'
c:\program files (x86)\microsoft visual studio 10.0\vc\include\istream  860

If I declared istringstream i; I don’t get the error. I do get similar error if I declare istream i2;

What am I doing wrong?

The reason that I need the istream or a derived class, it’s for a sql library that I’m working on. In specific, for the mysql implementation I need a istream or istringstream to call setBlob…

I have found a temporary worked around, allowing to pass a string and the initializing the stringstream with the string. However, it will be nice if my variant type could take istreamstream or better yet, istream

Thanks,
Frank

Answer 1

Quoting from Boost.Variant:

The requirements on a bounded type are as follows:

• CopyConstructible [20.1.3].
• Destructor upholds the no-throw exception-safety guarantee.
• Complete at the point of variant template instantiation. (See boost::recursive_wrapper for a type wrapper that accepts incomplete types to enable recursive variant types.)

std::istringstream is not copy-constructible.