How do I initiate a pointer list and push_back an item in it?

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how do I initiate a list-type pointer and push back a dynamically allocated item into the list? down is the general code, not that whole entire actual code.

What I have tried:

I have tried something like

C++

std::list<unsigned short>* Function::Function(unsigned short value) const{
    Type* temp = m_head;
    //initializing myList
    std::list* myList;
    while{
    if(conditional){
       (*myList).push_back(temp->getValue());
        }
}
    return myList;
}

it happens that if I do this it will tell me mylist is not initialized

BUT if i do something like

C++

std::list<unsigned short> *item = new std::list<unsigned short>;

it wont let me push back

C++

(*myList).push_back(item);

??

Posted 15-Jun-21 4:27am

Mango JellyBeans

Updated 15-Jun-21 5:25am

Shmuel Zang

v2

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Richard MacCutchan · Answer 1 · 2021-06-15T05:15:00

Solution 2

You are misusing pointer declarations and access. Use one of the following:

C++

// 1. A list created on the local stack, i.e. inside its enclosing function
//    Will be deleted automatically when it goes out of scope.
    std::list<unsigned short> myList;
    myList.push_back(item); // note access is by 'dot'


// 2. A pointer to a list created on the heap. NB must be deleted when no longer needed
    std::list<unsigned short>* pMyList = new std::list<unsigned short>;
    pMyList->push_back(item);  // note access is by 'arrow'

Posted 15-Jun-21 5:15am

Richard MacCutchan

Comments

Mango JellyBeans 15-Jun-21 13:58pm

lets say I have a value being passed into the function,
i created the
std::list<unsigned short="">* pMyList = new std::list<unsigned short="">;
how exactly would i assign the value being passed into the function in this allocated spot? it seems that if i try to push_back, it wouldnt let me even if i did object.value inside.

Mango JellyBeans 15-Jun-21 14:00pm

its the pointer to a list.

Richard MacCutchan 16-Jun-21 4:04am

Option 2 above is how you do it. But item must be an unsigned short value (not a pointer), because that is how you have declared your list.

k5054 · Answer 2 · 2021-06-15T05:25:00

Solution 3

Review the types of the various objects in your program. In the above, both myList and item are both pointers to a list of unsigned shorts. Since myList is expecting an object of unsigned short, trying to use it with an object of std::list<unsigned short>* is never going to work. What is the type of Type.getValue()? It should be unsigned short, in which case the following should work:

C++

Type* temp = m_head;
std::list<unsigned short>* myList = new std::list<unsigned short>;
while(condition)
{
   myList->push_back(temp->get_value());
}

Posted 15-Jun-21 5:25am

k5054

Comments

Mango JellyBeans 15-Jun-21 13:53pm

yeah so I tried
std::list<unsigned short="">* myList;
myList->push_back(temp->getValue());
before but then the IDE would just say my list was not initialized

k5054 15-Jun-21 15:16pm

the statement std::list<unsigned short>* myList; declares the variable myList but it does not initialize it. Assume, for the purposes of discussion, that the compiler gives myList the value NULL when declared, unless assigned at declaration time. What you have then is the equivalent of

 (NULL)->push_back(val)

. You need to assign a value to myList, either through new (or a smart pointer), or by assigning the pointer to an already existing list of a conforming type. Just declaring a pointer variable, and then trying to use it without pointing at something first is a programming error.