Experimental Physics and
Industrial Control System

Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov> · Mon, 12 Jun 2023 08:05:22 -0700

On 6/8/23 06:53, Wang, Lin via Tech-talk wrote:

Dear all,

In PVXS, when iterating over the tree structure of pvxs::Value with the approach similar to top() method in the following link,

_https://github.com/mdavidsaver/pvxs/blob/master/src/datafmt.cpp_

is it possible to know if the current field is the last node of the parent struct with pvxs::impl::FieldDesc or pvxs::impl::FieldStorage?

For example, I am wondering how to check "message" field is the last node of struct "alarm_t" when iterating.

struct "alarm_t" {
     int32_t severity = 2
     int32_t status = 1
     string message = "HIHI"
} alarm

The interface for iterating through the fields of a structure in brief is:

Value top(...); // some TypeCode::Structure

for(Value child : top.ichildren()) { ... }

Which is shorthand for something equivalent to:

{
    Value::IChildren range(top.ichildren());
    for(Value::IChildren::iterator cur(range.begin()), end(range.end()); cur!=end; ++cur) { Value child(*cur); ... }
}

cf. https://en.cppreference.com/w/cpp/language/range-for

One way to accomplish what you are asking with c++ iterators generally would be to track
both the "current" and "next" items.

{
    Value::IChildren range(top.ichildren());
    for(Value::IChildren::iterator end(range.end()), next(range.begin()), cur(next!=end ? next++ : end);
        cur!=end;
        cur=next, next!=end ? ++next : end
    ) { if(next==end) printf("last!\n"); }
}

Also, this is the same pattern could be used to eg. allow safe removal of 'cur' while iterating
an std::list.

You may come across other variations on this pattern, some using a while loop
instead.  This is ~equivalent functionally, and imo. ~equally ugly.

{
    Value::IChildren range(top.ichildren());
    Value::IChildren::iterator end(range.end()), next(range.begin());
    while(next!=end) {
        Value::IChildren::iterator cur=next++;
        if(next==end) printf("last!\n");
    }
}

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System