QVector::append() and copy-constructor

dream_captain

Can anybody explain why copy-constructor is being called twice here?

MyClass.h

#include <QDebug>
class MyClass
{
public:
    MyClass(int a) :
        mA(a)
    {
        qDebug() << "parameterized constructor";
    }

    MyClass()
    {
        qDebug() << "default constructor";
    }

    MyClass(const MyClass &other)
    {
        qDebug() << "copy-constructor" << other.mA;
    }

private:
    int mA;
};

main.cpp

#include <MyClass.h>
...
 MyClass myclass(10);
 QVector<MyClass> qvec;
 qvec.append(myclass);
...

Output:

parametrized constructor
copy-constructor  10
copy-constructor  19182872

mrjj

Hi

qvec.append(myclass);

This will copy it to the one/new spot in the list
First new spot in list is created , then your class is copied to it.

for std::vector they made emplace to solve that
http://www.cplusplus.com/reference/vector/vector/emplace/

we say its constructed in place and hence no copy is needed.

I do not know if QVector can do the same.

Taz742

@dream_captain
i dont know what happens... but i noticed.
if i call constructor for example

    MyClass myclass(10);
    QVector<MyClass> qvec;
    qvec.push_back(MyClass(myclass));

    parameterized constructor
    copy-constructor 10
    copy-constructor 1960650240
    copy-constructor 10419688

    MyClass myclass(10);
    std::vector<MyClass> qvec;
    qvec.push_back(myclass);

    parameterized constructor
    copy-constructor 10

aha_1980

@dream_captain: If your class is movable [1], than you can use std::move from C++11 to optimize:

int main()
{
    MyClass myclass(10);
    QVector<MyClass> qvec;
    qvec.append(std::move(myclass));
}

which gives the following output:

parameterized constructor
copy-constructor 10

Voilà :)

Note that the most Qt types are implicitely shared, so even calling the copy constructor is very cheap for them (effects for example QStringList::append())

[1] https://stackoverflow.com/questions/3106110/what-are-move-semantics

SGaist

Hi,

Because when you add custom copy constructor you "destroy" the implicitly created move constructor/copy assignment operator.

Modify your class like that:

class MyClass
{
public:
    MyClass(int a) :
        mA(a)
    {
        qDebug() << "parameterized constructor";
    }

    MyClass() :
        mA(0)
    {
        qDebug() << "default constructor";
    }

    MyClass(const MyClass &other) :
     mA(other.mA)
    {
        qDebug() << "copy-constructor" << mA;
    }

    MyClass(MyClass &&) = default;
    MyClass & operator= (const MyClass &) = default;

private:
    int mA;
};

And you should be good again.

See C++ move constructor and C++ copy assignment operator.

dream_captain

@SGaist it doesn't work with old visual studio 2012 compiler and Qt 5.5.1, but works well with vs2015 and qt 5.8.0. Seems like std::move support in msvc2012 is an issue.

SGaist

Because VS2012 has incomplete C++11 support. See this page.

cdietz

Related issue: https://bugreports.qt.io/browse/QTBUG-49250