Let's say if using - std::map tmap; Now you can put anything in tmap, which could be primitive types or vector or another map. Then you want to implement ostream& operator << (ostream& os, const tmap& _map) { // Over here we need to identify - // if iterator->second is primitive type, vector, map or unknown type. // Depending up on that we can decide if we need to iterate through // another vector or simply use << for primitive type. // That's why is_primitive or other type_traits functions may help. // Ideally we need switch statement like following switch(type(_map)) { case int : case double : case long : return os<< _map; case vector : {// interate through vector here. } .. .. // and so on. } } -----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Christian Henning Sent: Saturday, October 08, 2005 6:10 PM To: boost-users@lists.boost.org Subject: Re: [Boost-users] boost::any Question I'm not sure why you would want to use traits here. I think traits are used for compile-time if...else tests on types. Why not use a static numerical member in classes that you put in your map and so you could use a switch statement to figure out what type you are using? There are probably better ways to do that. RTTI ain't! Christian On 10/7/05, Piyush Kapadia wrote:

I am new user of Boost, I tried to use boost::variant first, but in comparison to boost::any performance of boost::variant appears to be

around

4 times slower, so settled for boost::any.

Also boost::any has cleaner interface but I don't know how to query current type information for boost::any ?

My code looks like following

map result_table;

result_table[1] = 543.434;

vector<int> vec;

.. Some vec population code;

result_table[2] = vec;

Now I need to develop a stringify function for result_table;

I can fetch the results back as

boost::any value = result_table[2];

But I need to know the type of value, so that I can treat it accordingly to stringify;

I know there exists type_traits, but very limited examples of type_traits is provided on Boost documentation and particularly, I am clueless about how to use type_traits in above context to find out what is the current type returned by result_table[int]

Also I want to know if Boost has any Hash map implementation that can be superior in performance compare to regular std::map ??

Thanks,

Piyush _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

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I took a look at the examples in the Boost::Any documentation and found this: bool is_int(const boost::any & operand) { return operand.type() == typeid(int); } bool is_char_ptr(const boost::any & operand) { try { any_cast(operand); return true; } catch(const boost::bad_any_cast &) { return false; } } bool is_string(const boost::any & operand) { return any_caststd::string(&operand); } Is that what you're looking for? Christian

typeid() gives you a reference to a type_info object. Here you could ask for the typename name() and using a map you could use a switch statement. This is very slow since the string lookup, but it works. Another solution would be a thin wrapper class around all of your types that has a static int member and pointer to the real object. Here, you simply use the static and in the switch statement. I don't know if there is more boost-like way available? Perhaps someone else will have a higher performance and cleaner solution. Christian On 10/10/05, Piyush Kapadia wrote:

True that I just need limited type supported and not entire universe of types, I have also seen following, but it would mean to put it in series of if(is_int), elseif(is_char_ptr) kind of statements, switch statement would be more cleaner, that why I thought type_traits may offer cleaner solution.

Performance is also criteria.

On 10/10/05, Christian Henning wrote:

...

I took a look at the examples in the Boost::Any documentation and found

this:

...

bool is_int(const boost::any & operand) { return operand.type() == typeid(int); }

bool is_char_ptr(const boost::any & operand) { try { any_cast(operand); return true; } catch(const boost::bad_any_cast &) { return false; } }

bool is_string(const boost::any & operand) { return any_caststd::string(&operand); }

Is that what you're looking for?

Christian

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