Re: [Boost-users] Can Lambda dynamic create template parameter？

From: Kirit Sælensminde (kirit.saelensminde_at_[hidden])
Date: 2007-12-15 00:53:42

Guo Xu wrote:
> I want to implement the grammar like this,
> to create template parameter dynamicly,
>  
> set< int, ( _1 > _2 ) > intset;
>  
> can lambda do it ,and how to ?

Templates are compile time entities so the parameters have to be fully 
specified at compile time. For the specific example you have 
std::greater should work for you:

set< int, std::greater< int > > intset;

More generally, you can write your own custom comparators and you can 
write one of those that takes a lambda at run time to use.

There's an example comparator using Boost.Regex at 
http://www.boostcookbook.com/Recipe:/1235053 which shows the general 
idea. The important points are that it derives from 
std::binary_function<> and implements an operator() which returns a bool 
and takes two instances of the value type that are being compared.

To use a lambda, you'd want a constructor that took a boost::function< 
bool ( int, int ) > and stored it for future use. Changing the lambda 
after the set has started to be populated would almost certainly be a 
terrible idea.

K

-- 
http://www.kirit.com/