Study on function object 9 visit_each

The boost: visit_each template is defined in the visit_each.hpp file.

namespace boost {  template<typename Visitor, typename T>  inline void visit_each(Visitor& visitor, const T& t, long)  {    visitor(t);  }  template<typename Visitor, typename T>  inline void visit_each(Visitor& visitor, const T& t)  {    visit_each(visitor, t, 0);  }}

This provides a simple mechanism. The constt & T parameter is the observed object, and the visitor & visitor parameter is the function object used to observe T. Therefore, the operator () function must be implemented, then it will be called within visist_each:

Visitor (t)

The third parameter long will be abolished in the future. Currently, only 0 can be passed.

The following is a simple example:

struct A {    A():y(3), z(5) {            }        int y;        int z;};template<class T>struct Visitor {    void operator()(T const& t) {        cout << t.y << endl;        cout << t.z << endl;    }};

int main(int argc, char** argv) {    A b;    Visitor<A> v;    visit_each(v, b, 0);    return 0;}

Running result:

35

There is a problem right away. Why does visit_each need this? This should be the implementation of the C ++ template of the visitor design mode. Do not design the visitor interface and specific subclass. You only need to design your own class and access the visitor function object of this class.

Classic Visitor Design Patterns can refer to a blog: http://blog.csdn.net/superbeck/article/details/5325991