Re: Map of objects
On 2 Nov., 16:51, Andrea Crotti wrote:
Ok well it was wrong what I wrote before
Now the first part works perfectly
if (trace.count(index) == 0) {
RingBuffer<PadNodeID> empty(GLOBALS::trace_length);
trace.insert(make_pair(index, empty));
}
What is trace? What is RingBuffer? I'm /assuming/ that trace refers to
an object of type std::map<some_unknown_key_type,RingBuffer<PadNodeID>
.
But then I want to actually add the element to the ringbuffer with
RingBuffer<PadNodeID> tmp = trace[index];
tmp.push(node);
Two issues: 1st, if you want to use operator[] on a map, the
mapped_type -- in your case RingBuffer<PadNodeID> -- has to be default
constructible. This is because the operator might have to default
construct such an object in case there is no such key/value pair yet.
2nd, this line is a copy-initialization. The RingBuffer object would
be copied so that tmp refers to a copy. Any modifications of the
object tmp refers to won't change the object that is stored on the
map. It seems you're not aware of the differences between C++ and,
say, Java in this respect. Better grab a decent C++ book and learn the
language properly.
If you're sure that such a RingBuffer objects already exists, you can
use find. Also, if you want to keep a reference to the RingBuffer, you
have to say so:
RingBuffer<PadNodeID> & tmp = trace.find(index)->second;
But why I don't get it, why should it want again the size constructor
when I'm taking something that should be ALREADY a created object?
Doesn't matter. The operator[] implementation works like this:
iterator it = this->find(key);
if (it==this->end()) {
return this->insert(value_type(key,mapped_type())->second;
} else { // ^^^^^^^^^^^^^
return it->second;
}
Even if the object exists in /some situation/ at /runtime/, we still
need the compiler to translate the first branch as well for the cases
where the object doesn't exist. And the first branch requires
mapped_type to be default constructible. That's simply a precondition
for this operator. If this precondition is not satisfied, you can't
use the operator -- regardless of whether the object already exists or
not.
Cheers!
SG