模板副本构造函数给出错误

Question

I try to create a copy constructor to a list. The list has all variables as private members. Is there something special about a constructor in a template that does so this doesn't work?

I get these errors:

   1>          consoleapplication1\consoleapplication1\circulardoubledirectedlist.h(52) : while compiling class template member function 'CircularDoubleDirectedList<int>::CircularDoubleDirectedList(const CircularDoubleDirectedList<int> &)'
    1>          consoleapplication1\consoleapplication1\testdeepcopyingoflist.cpp(25) : see reference to function template instantiation 'CircularDoubleDirectedList<int>::CircularDoubleDirectedList(const CircularDoubleDirectedList<int> &)' being compiled
    1>          consoleapplication1\consoleapplication1\testdeepcopyingoflist.cpp(24) : see reference to class template instantiation 'CircularDoubleDirectedList<int>' being compiled
    1>consoleapplication1\consoleapplication1\circulardoubledirectedlist.h(57): error C2039: 'dir' : is not a member of 'CircularDoubleDirectedList<int>'
    1>consoleapplication1\consoleapplication1\circulardoubledirectedlist.h(62): error C2662: 'void CircularDoubleDirectedList<int>::changeDirection(void)' : cannot convert 'this' pointer from 'const CircularDoubleDirectedList<int>' to 'CircularDoubleDirectedList<int> &'
    1>          Conversion loses qualifiers
    1>consoleapplication1\consoleapplication1\circulardoubledirectedlist.h(68): error C2662: 'void CircularDoubleDirectedList<int>::moveCurrent(void)' : cannot convert 'this' pointer from 'const CircularDoubleDirectedList<int>' to 'CircularDoubleDirectedList<int> &'
    1>          Conversion loses qualifiers
    1>consoleapplication1\consoleapplication1\circulardoubledirectedlist.h(71): error C2662: 'void CircularDoubleDirectedList<int>::changeDirection(void)' : cannot convert 'this' pointer from 'const CircularDoubleDirectedList<int>' to 'CircularDoubleDirectedList<int> &'
    1>          Conversion loses qualifiers

My Code:

template <typename T>
CircularDoubleDirectedList<T>::CircularDoubleDirectedList(const CircularDoubleDirectedList<T>& other){

    bool changeDir;

    if (other.getCurrentDirection == ICircularDoubleDirectedList::FORWARD){
        changeDir = false;
    }
    else{
        changeDir = true;
        other.changeDirection();
    }

    int size = other.size();
    for (int i = 0; i < size; i++){
        this->addAtCurrent(other.getElementAtCurrent());
        other.moveCurrent();
    }
    if (changeDir){
        other.changeDirection();
    }
    this->currentDirection = other.getCurrentDirection();

}

Answer 1

Your copy constructor (rightly) has a const reference parameter. However, it looks like you're trying to call non- const members on the argument:

other.changeDirection(); // this looks like a non-const operation
....

other.moveCurrent();     // this looks like a non-const operation

etc. It doesn't make much sense for a copy constructor to modify the thing being copied from , but if you really need that then you need the parameter to be non- const reference.

Alternatively, if those member functions don't modify the object, make them const .

Note that this has nothing to do with templates.

Answer 2

The first error doesn't seem to come from this code: there's no use of the name dir in the code.

The remaining errors are because you're trying to call non-const member functions on other , which is const . Either rethink what the constructor should do (since modifying its argument makes it rather confusing), or remove const from the parameter to allow modification.