To summarize, this is the complete instantiation scheme.

typedef bimap< LeftKeyType, RightKeyType [, SetTypeOfRelation ] [,Allocator] > bm;

{Side}KeyType can directly be a type. This defaults to set_of<{Side}KeyType>, or can be a {SetType}_of<Type> specification. Additionally, these two parameters can be tagged to specify other tags instead of the usual member_at::{Side} tags. The last two parameters are used to specify the set type of the relation and the allocator type.If you want to specify a custom allocator type while relying on the default value of SetTypeOfRelation, you can do so by simply writing bimap<LeftKeyType, RightKeyType, Allocator>. Boost.Bimap's internal machinery detects that the third parameter in this case does not refer to the relation type but rather to an allocator.

The following are the possible ways of instantiating a bimap:

bimap<LeftKeyType,RightKeyType>

bimap<LeftKeyType,RightKeyType,Allocator>

bimap<LeftKeyType,RightKeyType,SetOfRelationType>

bimap<LeftKeyType,RightKeyType,SetOfRelationType,Allocator>