Question about qvector.h code of Qt 4

[jacek]

Consider such code:

QVector<int> v;
...
v.append( v[0] );

QVector<int> v;
...
v.append( v[0] );

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What will happen if realloc() moves the data?

[Michiel]

So why pass t by reference in the first place then?

[jacek]

So why pass t by reference in the first place then?

Who knows? Maybe this problem was noticed after the interface was set or maybe to leave a possibility for more efficient implementation in future or for some other reason. You will have to ask the Trolls.

Anyway now it's just like if parameters were passed by value.

[karapas]

Thanks for the replies guys! jacek you are right! I did not notice that such a thing could happen in the first place! I 've just noticed that the same code exists in STL <vector> as well and there is an enlightening comment:

_Ty _Tmp = _Val;	// in case _Val is in sequence

_Ty _Tmp = _Val;	// in case _Val is in sequence

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Thanks again!

[Michiel]

Hm. Perhaps it is to ensure that the type has a public copy constructor? I'm not sure if a pass-by-value parameter would ensure the same thing.

[karapas]

Hm. Perhaps it is to ensure that the type has a public copy constructor? I'm not sure if a pass-by-value parameter would ensure the same thing.

I think that pass-by-value parameter does exactly the same thing i.e it calls the copy constructor to create a copy of the passed object.


I think that an optimazation would be to create the copy only when resize (i.e call realloc) is needed. This would possibly result in a bit larger code but it would probably be better not to needlessly invoke the copy constructor in case that large objects are stored inside the QVector (for example images)