What is the meaning of empty "<>" in template usage?

Question

What is the meaning of tokenizer<> tok(s) line in code below? I know that <> is used while working with templates, but according to my understanding <> should not be empty- it should contain type definition.

    using namespace std;     using namespace boost;     string s = "This is, a te\x1Dst";     cout<<s<<endl;     tokenizer<> tok(s);     for(tokenizer<>::iterator beg=tok.begin(); beg!=tok.end();++beg){         cout << *beg << "\n"; } 

Answer 1

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; 

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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 :-.