QVector copy constructor

[Caius Aérobus]

Hello,
I wonder why QList and QVector have a so different behaviour regarding compilation rules. Let us consider the following piece of code :

class T
{
public:
  T(int i) {};
};
class C
{
public:
  C() {};
  QList<T> GetTs() {return QList<T>(this->Ts);};
private:
  QList<T> Ts;
};

class T
{
public:
  T(int i) {};
};
class C
{
public:
  C() {};
  QList<T> GetTs() {return QList<T>(this->Ts);};
private:
  QList<T> Ts;
};

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This code compiles well, but if I change it by replacing QList by QVector:

class T
{
public:
  T(int i) {};
};
class C
{
public:
  C() {};
  QVector<T> GetTs() {return QVector<T>(this->Ts);};
private:
  QVector<T> Ts;
};

class T
{
public:
  T(int i) {};
};
class C
{
public:
  C() {};
  QVector<T> GetTs() {return QVector<T>(this->Ts);};
private:
  QVector<T> Ts;
};

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I get a compilation error:

/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h: In member function ‘void QVector<T>::realloc(int, int) [with T = T]’:
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:297:   instantiated from ‘void QVector<T>::detach_helper() [with T = T]’
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:104:   instantiated from ‘QVector<T>::QVector(const QVector<T>&) [with T = T]’
error.cxx:10:   instantiated from here
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:427: error: no matching function for call to ‘T::T()’
error.cxx:4: note: candidates are: T::T(int)
error.cxx:3: note:                 T::T(const T&)

/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h: In member function ‘void QVector<T>::realloc(int, int) [with T = T]’:
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:297:   instantiated from ‘void QVector<T>::detach_helper() [with T = T]’
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:104:   instantiated from ‘QVector<T>::QVector(const QVector<T>&) [with T = T]’
error.cxx:10:   instantiated from here
/Library/Frameworks/QtCore.framework/Versions/4/Headers/qvector.h:427: error: no matching function for call to ‘T::T()’
error.cxx:4: note: candidates are: T::T(int)
error.cxx:3: note:                 T::T(const T&)

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The problem is due to the fact there exists no T::T() constructor and I can fix it by adding one:

class T
{
public:
  T() {};
  T(int i) {};
};
class C
{
public:
  C() {};
  QVector<T> GetTs() {return QVector<T>(this->Ts);};
private:
  QVector<T> Ts;
};

class T
{
public:
  T() {};
  T(int i) {};
};
class C
{
public:
  C() {};
  QVector<T> GetTs() {return QVector<T>(this->Ts);};
private:
  QVector<T> Ts;
};

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which is not really nice since I would like to prevent users from instanciating class T without explicitely transmitting the integer parameter.
So could anybody help me fixing the problem:
- why QList and Qvector do not yield to identical compilation results?
- is there a better solution, assuming I would like that C::GetTs() returns a copy of table Ts in a very efficient way, which is the reason I would like to make the copy in this way, as explained in the Qt doc ("This operation takes constant time, because QVector is implicitly shared. This makes returning a QVector from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and that takes linear time.")?

[jpn]

Are you sure you even need QVector? For most purposes, QList is the right class to use. Do you really need continuous memory?

[Caius Aérobus]

Are you sure you even need QVector? For most purposes, QList is the right class to use. Do you really need continuous memory?

generally either I process every item in a for loop or I use direct accesses using indices, so I suppose it works quicker on continuous tables...

[jpn]

I recommend reading Generic Containers documentation. Pay special attention to "algorithmic complexity".