It just means the template should use the default parameter(s). For example:
template <int N = 10>
class X { };
X<> x; // this is an X<10>
Clearly, it’s only possible when all the template parameters have defaults (or for variadic templates that have no mandatory parameters – see below)….
For boost::tokenizer specifically, the template’s:
template <
class TokenizerFunc = char_delimiters_separator<char>,
class Iterator = std::string::const_iterator,
class Type = std::string
>
class tokenizer;
It’s not relevant to the “meaning of tokenizer<> tok“ that the body of your question focuses on, but to address the more general question title “What is the meaning of empty “<>” in template usage?”…
As Shafik first mentioned, the my_template<> form can also be used with variadic templates such as std::tuple to specify an empty parameter pack:
// namespace std { template <class ...Args> class tuple; }
std::tuple<> t; // this tuple can't store anything :-.