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comment was wrong

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This commit is contained in:
dwight 2010-11-30 08:21:50 -05:00
parent e183f5d144
commit 47b43cea41
2 changed files with 19 additions and 14 deletions
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31
bson/bsonobj.h
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@ -84,21 +84,26 @@ namespace mongo {
~BSONObj() { /*defensive:*/ _objdata = 0; }
/**
A BSONObj can use a buffer it "owns" or one it does not. If the BSONObj owns the buffer, it will free()
it during ~BSONObj() destruction.
A BSONObj can use a buffer it "owns" or one it does not.
OWNED CASE
If the BSONObj owns the buffer, the buffer can be shared among several BSONObj's (by assignment).
it during ~BSONObj() destruction. In this case the buffer is basically implemented as a shared_ptr.
Since BSONObj's are typically immutable, this works well.
You can specify ownership with the ifree parameter in the constructor.
UNOWNED CASE
A BSONObj can also point to BSON data in some other data structure it does not "own" or free later.
For example, in a memory mapped file. In this case, it is important the original data stays in
scope for as long as the BSONObj is in use. If you think the original data may go out of scope,
call BSONObj::getOwned() to promote your BSONObj to having its own copy. If you are not sure about
ownership but need the buffer to last as long as the BSONObj, call getOwned(). getOwned() is a
no-op if the buffer is already owned. If not already owned, a malloc and memcpy will result.
Most ways to create BSONObj's create 'owned' variants. Unowned versions can be created with:
(1) specifying true for the ifree parameter in the constructor
(2) calling BSONObjBuilder::done(). Use BSONObjBuilder::obj() to get an owned copy
If you are not sure about ownership but need the buffer to last as long as the BSONObj, call getOwned().
getOwned() is a no-op if the buffer is already owned. If not already owned, a malloc and memcpy will result.
On assignment of a BSONObj, the buffer pointer moves to the lvalue bsonobj, and is set to null on the rvalue.
This is a lot like auto_ptr semantics. The owned state transfers too.
Why would it not be owned? To avoid a memcpy. For example if the source data exists in say, a memory mapped file,
we can reference it directly.
@return true if the buffer is owned by *this.
@return true if this is in owned mode
*/
bool isOwned() const { return _holder.get() != 0; }

2
bson/bsonobjbuilder.h
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@ -529,7 +529,7 @@ namespace mongo {
would like the BSONObj to last longer than the builder.
*/
BSONObj done() {
return BSONObj(_done());
return BSONObj(_done(), /*ifree*/false);
}
// Like 'done' above, but does not construct a BSONObj to return to the caller.