Question

While working on a project I came upon this code, which I’m trying to figure out:

enum Attributes { ACTIVE, COMPLETE, POSITION }
template<Attributes NN,typename TT>  
TT& Set(TT&& t) 
{ 
    return typeList.get<NN>()=t; //typeList is a boost tuple
}

This is called later on with

object.Set<ACTIVE>(true);

There’s only one template parameter in there!
How is possible to specify a template with two parameters, and then call it with only one? I would think the Set method supposed to take 2 template parameters (Attributes and typename), like an std::map.

Answer 1

When calling a function template template parameters, which are the type of the function arguments can be automatically deduced from the type of the arguments, the function is called with:

object.Set<ACTIVE>(true); //same as object.Set<Active,bool>(true);
object.Set<ACTIVE>(5);    //same as object.Set<Active,int>(5);

This is the reason one can use template functions from the standardlibrary, like std::max or std::copy without explicitely mentioning the types of the arguments.