2009/12/3 Jonathan Chambers` :

I'm trying to get the program to compile if the variant takes on the guise of an int (because I've implemented void operator()(int &i) const, but not compile if I make the variant take the guise of a char e.g. boost::variant var = 'a'; because I've not implemented the void operator()(char &c) const

Can anyone think of a way of doing this, maybe using SFINAE (Substitution Failure Is Not An Error)

I don't think SFINAE will help here, since you don't have a template. What about just declaring, but not defining, the version with a char parameter? Though since it's a runtime distinction, I'm not convinced this will be possible at compile-time, since it will want to compile the code for all the runtime possibilities. What's wrong with just throwing an exception in the char-parameter function in the visitor? Or, alternatively, why are you using a variant if you never want it to be a char?

Jonathan Chambers` wrote:

I'm trying to get the program to compile if the variant takes on the guise of an int (because I've implemented void operator()(int &i) const, but not compile if I make the variant take the guise of a char e.g. boost::variant var = 'a'; because I've not implemented the void operator()(char &c) const

The value (and thus actual type) of a variant is a runtime property; there cannot be a way to achieve what you want. Sebastian