Question

I’ve been messing around with Qt and C++ for a while, but I’ve run into this error and can’t seem to figure out why it crops up. There are a lot of other questions that have been answered out there with the const void* conversion error message, but I can’t really see how the explanations help in my case, so here goes:

I have a reimplementation ‘MyTypeManager’ of QList< MyType *const>, so a list of const pointers to non-const MyTypes. However, when a function in my reimplementation, addMyType is called

void MyTypeManager::addMyType(MyType *const var)
{
     this->append(var);
}

the following error(s) occur:

In file included from /usr/include/qt4/QtCore/QList:1:0,
             from ../qtsdlthread/mytypemanager.h:4,
             from ../qtsdlthread/mytypemanager.cpp:1:
/usr/include/qt4/QtCore/qlist.h: In member function ‘void QList<T>::node_construct(QList<T>::Node*, const T&) [with T = MyType* const]’:
/usr/include/qt4/QtCore/qlist.h:499:13:   instantiated from ‘void QList<T>::append(const T&) [with T = MyType* const]’
../qtsdlthread/mytypemanager.cpp:20:26:   instantiated from here
/usr/include/qt4/QtCore/qlist.h:359:58: error: invalid conversion from ‘const void*’ to ‘void*’
/usr/include/qt4/QtCore/qlist.h: In member function ‘void QList<T>::node_copy(QList<T>::Node*, QList<T>::Node*, QList<T>::Node*) [with T = MyType* const]’:
/usr/include/qt4/QtCore/qlist.h:666:9:   instantiated from ‘QList<T>::Node* QList<T>::detach_helper_grow(int, int) [with T = MyType* const]’
/usr/include/qt4/QtCore/qlist.h:497:48:   instantiated from ‘void QList<T>::append(const T&) [with T = MyType* const]’
../qtsdlthread/mytypemanager.cpp:20:26:   instantiated from here
/usr/include/qt4/QtCore/qlist.h:386:17: error: invalid conversion from ‘const void*’ to ‘void*’

20:26 in mytypemanager is the this->append line posted above.

Answer 1

From the documentation:

QList’s value type must be an assignable data type.

Alas MyType *const is not assignable. You have several remedies:

1. Make T a mutable pointer

2. Make T a pointer to your const pointer:

typedef MyType *const Element

void MyTypeManager::addMyType(Element var)
{
    Element* ptr2ptr = new Element(var);
    this->append(ptr2ptr);
}

But now you have 2 levels of memory management to worry about.

3. (Dangerous) Make T=MyType* and const-cast MyType *const into MyType *:

this->append(const_cast<MyType *>(var));

This will only work if you are certain var was originally created as a MyType* variable.