[Boost-users] [MultIndex] Iterator/Range validity when modify entries.

8 Sep 2016

      Hi guys,

While trying to implementing a new feature, I came across a question 
concerning iterator/range validity of boost::multi_index.

To understand the question, here is my setup:

<code>

// Database entry.
struct Entry { int key1; int key2; };
// Access-keys.
struct Index1{};
struct Index2{};

using namespace boost::multi_index;
using namespace boost;

// The multi_index database.
using Database = multi_index_container<
   Entry,
   indexed_by<
     hashed_non_unique<
       tag<Index1>,
       member
     >,
     hashed_non_unique<
       tag<Index2>,
       member
     >
...
;
Database db;
/* ... Here db is getting filled. ... */

</code>

I obtain a range of entries from that database with the same value for 
key1 through the first index and want to modify each single entry of 
that range. However, I will only modify key2 (which is only used as key 
in the other index)!

My main question is now:
Does modifying an entry invalidate my range although I do not modify the 
member that was used as key for obtaining the range?

The code in question would possibly look like this:

<code>

auto& index1 = db.get<Index1>();
auto range = index1.equal_range(42);

for (auto it = range.first; it != range.second; ++it)
{
   index1.modify(it, [](Entry& entry)
     {
       entry.key2 = 123;  // <---- Does this invalidate my range?
                          // entry.key1 is not getting modified!
     });
}

</code>

My follow-up question is then:
If the range is indeed invalidated, what would be a working solution for 
that problem?

Incrementing the loop-iterator by hand before modifying the entry (and 
not letting the loop do it)?
So, would the following be a valid solution?

<code>

auto& index1 = db.get<Index1>();
auto range = index1.equal_range(42);

if (range.first != range.second)
{
   auto it = range.first;
   auto end = --range.second;
   do {
     index1.modify(it++, [](Entry& entry)
       {
         entry.key2 = 123;  // <---- Does this invalidate my range?
                            // entry.key1 is not getting modified!
       });
   while (it != end);
}

</code>

Of course, neither do I want to iterate over an entry twice (and thereby 
modify it twice) nor do I want miss an entry.

And one last question:
Does this problem or its solution apply to all index-types or just to 
hashed indexes?

Thanks for the clarification.
Deniz

PS: Oh and by the way: Boost.MultiIndex is a really cool library. :-)

[Boost-users] [MultIndex] Iterator/Range validity when modify entries.

Deniz Bahadir