Object in parameter specification

问题

Why this sentence is valid in C++?

qi::rule<Iterator, std::string(), skipper<Iterator> > name;

Extracted from here:

• Boost::spirit::qi - mini C compiler tutorial - function.hpp

The definition of rule is (resumed) the following:

template <typename Iterator, typename T1, typename T2,
                             typename T3, typename T4>
struct rule : boost::proto::extends<bla, bla>, parser<bla, bla>
{
  bla, bla
};

Extracted from here:

• rule.hpp file

The rule definition expects a type, however I send it an object. It is possible?

回答1:

In this context, std::string() means the type of something that returns an std::string and has no parameters. For example,

std::string foo() { return "Foo!\n"; }

or an instance of

struct Foo
{
  std::string operator()() const { return "Foo!\n"; }
};

回答2:

You don't give it an object. std::string() is the type of a function that takes no arguments and returns an std::string.

来源：https://stackoverflow.com/questions/13472591/object-in-parameter-specification