Re: [Boost-users] boost::any and const pointers

On Dec 2, 2009, at 11:24 AM, Dominique Devienne wrote:

On Wed, Dec 2, 2009 at 10:55 AM, James C. Sutherland wrote:

...

int main() { Properties p; int i=1; set(p,"int",i); set(p,"int*",&i);

int* j = get(p,"int*"); // I don't want this to compile const int* k = get(p,"int*"); // I want this to compile fine. }

Currently this whole code compiles without trouble, but I don't want to allow a non-const pointer to be extracted. Any ideas of how to accomplish this?

Thanks not possible AFAIK. The first compiles alright, but will fail at runtime because the boost::any was constructed using a const int*. By design all extractions are checked at runtime only, and the typeid()/std::type_info of what you put in and what you try to get it as must be the same. Given your code, you can't even do get<int>(p, "int"), because a const int was put in, not an int (see your set() impl), and typeid(int) != typeid(const int). --DD

Actually, this runs just fine (at least on my mac running g++ 4.2.1). Also, the following compiles and runs fine: int m = get<int>(p,"int"); This is because you cannot force const semantics on a copy. I came up with a way to get const references out (using boost::ref), but can't get const pointers guaranteed. I fear that you are right about this not being possible.