trying to understand smart pointers...

mzimmers

...but I don't understand what's wrong with this code:

QString qs = "xxx";
QString *qsp;
std::unique_ptr<QString> qsp2;

qsp = &qs;
list.append(qsp); // works
list2.append(qsp2); // doesn't compile

The error message is:

C:\Users\michael.zimmers\Qt_projects\qscopedpointer\main.cpp:16: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = QString; _Dp = std::default_delete<QString>]'

This occurs with QScopedPointer as well. Is it not possible to put unique_ptr objects into a QList?

EDIT: I omitted a couple of rather important lines in my text sample above; this is what it should be (apologies):

QString qs = "xxx";

QList<QString *> list;
QList<std::unique_ptr<QString>> list2;

QString *qsp;
std::unique_ptr<QString> qsp2;

qsp = &qs;
// how to assign to qsp2?

list.append(qsp);
list2.append(qsp2);

Thanks...

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

Is it not possible to put unique_ptr objects into a QList?

Not when the QList contains a pointer of QStrings since a pointer to a QString and std::unique_ptr<QString> are two different types.

Mesrine

@mzimmers

I think appending to a list tries to make a copy of the smart pointer, but this by definition of the unique_ptr class deletes the object. Try doing

list2.append(std::move(qsp2));

or

list2.append(make_unique<QString>("qsp2 string"));

mzimmers

@Mesrine thanks for the suggestions, but that doesn't compile either (essentially the same error). I think you're right about the unique_ptr not permitting copies. That makes me wonder if I'm misusing the unique_ptr, or if there's a better construct for me to use.

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

thanks for the suggestions, but that doesn't compile either (essentially the same error). I think you're right about the unique_ptr not permitting copies. That makes me wonder if I'm misusing the unique_ptr, or if there's a better construct for me to use.

This is complete bs.
A QString* and QSharedPointer<QString> are two comletelty different types so are QList<QString*> and QList<QSharedPointer<QString>> - they can not be implicitly converted to each other.

SGaist

@mzimmers hi,

The first question is: why a pointer to a QString ? AFAIK, 99.99% of the time you do not need a pointer to a QString.

mzimmers

@Christian-Ehrlicher said in trying to understand smart pointers...:

A QString* and QSharedPointer<QString> are two comletelty different types so are QList<QString*> and QList<QSharedPointer<QString>> - they can not be implicitly converted to each other.

In my example, I have two lists:

QList<QString *> list;
QList<std::unique_ptr<QString>> list2;

and I'm trying to append to each:

list.append(qsp);
list2.append(qsp2);

So I don't understand your comment.

@SGaist I just used a QString to simplify my example. In my app, I need a list of an class that I'm subclassing. If I just maintain a list of the parent class, I can't add subclasses to the list (at least, I don't see how I can). I'm attempting to use a list of pointers to avoid this problem; if there's a better way, I'd love to hear about it.

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

So I don't understand your comment.

You're right - I did not see there is list and list2

QList needs an copyable type. Use std::vector - it works also with move-only types.

std::vector<std::unique_ptr<QString>> l;
l.push_back(std::make_unique<QString>(""));

mzimmers

@Christian-Ehrlicher I'd like to pursue using QList, if only for my education. I've added a struct to the exercise:

struct TestStruct {
	int i;
	TestStruct() {}
	TestStruct(TestStruct &ts) {i = ts.i;}
};
TestStruct myStruct;

std::unique_ptr<TestStruct> *uniquePtr;
QList<std::unique_ptr<TestStruct>> list2;
uniquePtr = &myStruct; // how to form this?
list2.append(uniquePtr); // how to form this?

It seems to be wrong to try to assign to a unique_ptr; I see an assignment operator in the docs, but there's also a proviso:
Copy assignment (4) to a unique_ptr type is not allowed (deleted signature).
So, again, this leaves me thinking I really don't understand how to use unique_ptrs. Can they be modified after they're constructed?

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

Can they be modified after they're constructed?

Yes, it's a normal pointer but you can't copy them - only moving is allowed. Your TestStruct is missing the move ctor and move operator.

list2.append(uniquePtr); // how to form this?

As I said - it's not possible with QList as a unique_ptr<T> is not copyable, only movable.

mzimmers

@Christian-Ehrlicher

sigh I new I should have taken that extra C++ class in night school...

OK, so I think I've added the move c'tor and move operator (though the docs are a little confusing; there's reference to a move assignment operator, which I'm not sure is the same thing). Code looks like this:

struct TestStruct {
	int i;
	TestStruct() {}
	TestStruct(TestStruct &ts) {i = ts.i;} // copy c'tor
	TestStruct(TestStruct &&ts) { i = ts.i; } // move c'tor
	TestStruct& operator=(TestStruct&& ts) { return *this; } // move operator
};
TestStruct myStruct;

std::unique_ptr<TestStruct> *uniquePtr = nullptr;
QList<std::unique_ptr<TestStruct>> list;
uniquePtr = &myStruct; // how to form this?
list.push_back(*uniquePtr); // how to form this?

What am I continuing to do wrong here?

Thanks...

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

What am I continuing to do wrong here?

You still use a QList - as I already told you QList needs a copyable type but std::unique_ptr<T> is not.

mzimmers

@Christian-Ehrlicher said in trying to understand smart pointers...:

QList needs a copyable type but std::unique_ptr<T> is not.

Ah, yes. And it appears that QScopedPointer is not either...pity.

So, if I use std::vector, and the make_unique that you mentioned above, I can modify what the unique_ptr references, and push it into the vector and it all works OK:

typedef std::unique_ptr<TestStruct> UniquePtr;
UniquePtr uniquePtr;
std::vector<UniquePtr> qVector;

uniquePtr = std::make_unique<TestStruct>(myStruct);

uniquePtr->i = 55;
qDebug() << uniquePtr->i;
qVector.push_back(std::make_unique<TestStruct>(*uniquePtr));

uniquePtr->i = 555;
qDebug() << uniquePtr->i;
qVector.push_back(std::make_unique<TestStruct>(*uniquePtr));

uniquePtr->i = 5555;
qVector.push_back(std::make_unique<TestStruct>(*uniquePtr));

qDebug() << qVector.at(0)->i << qVector.at(1)->i << qVector.at(2)->i;

Now: I want to reference elements in the vector, probably in a loop. Do I need to create a new unique_ptr for each loop iteration, since I can't do an assignment to it, or is there some other way to do this?

Christian Ehrlicher

@mzimmers said in trying to understand smart pointers...:

Do I need to create a new unique_ptr for each loop iteration, since I can't do an assignment to it, or is there some other way to do this?

I don't understand - you already access the elements (via at() ) and therefore can call the functions of the object (or modify the members of your struct)

mzimmers

@Christian-Ehrlicher I'd like to be able to do the following:

for (int i = 0; i < list.size(); i++) {
    p = list.at(i);
	p.this = that; 
	p.the_other(); // etc

Christian Ehrlicher

Why do you want to assign something to the local variable p? What's the point?

mzimmers

@Christian-Ehrlicher well, maybe I don't have to. The plan is to use this list in a list model I'm writing. I'll be making temporary variables of my struct, and copying to and from the list. (Plus QML access that I haven't even begun thinking about.) But...maybe I can do it all with newly-created unique_ptrs. I'll try and report back.

BTW: should I consider the use of QScopedPointers instead?

Christian Ehrlicher

Simply store the struct in the container.

Chris Kawa

@mzimmers said:

I'll be making temporary variables of my struct, and copying to and from the list.

The whole point of unique_ptr is that it is unique. It holds ownership of the object. You can't copy (temporary or otherwise) unique_ptr because then you would have two things owning the same object and that would just crash because of double delete.

You can move unique_ptrs, because it moves ownership of the object, so only one pointer still owns the object.

As others mentioned QList does not support move-only types because of implicit sharing. It needs to do copies underneath when a shared data detaches.

QScopedPointer is just a simplified version of std::unique_ptr. Switching one to the other doesn't change anything.

You can store unique_ptr in a std::vector, which does not do implicit sharing and supports move-only types:

std::vector<std::unique_ptr<QString>> pointers;
for (int i=0; i < 10; ++i)
{
    pointers.push_back(std::make_unique<QString>("Hello!"));
}

or

for (int i = 0; i < 10; ++i)
{
    auto ptr = std::make_unique<QString>("Hello!");
    pointers.push_back(std::move(ptr));

    // ptr does not point to the object here anymore, it's been moved from.
    // Code below will compile but is invalid and will likely crash at runtime:
    ptr->isEmpty();
}

You can then access these pointers like this:

for (int i = 0; i < 10; ++i)
{
    bool use_the_string = pointers.at(i)->isEmpty();
}

or get a reference to the pointer:

for (int i = 0; i < 10; ++i)
{
    const auto& ptr_ref = pointers.at(i);
    bool use_the_string = ptr_ref->isEmpty();
}

but you can't copy them:

for (int i = 0; i < 10; ++i)
{
   // This won't compile. You can't copy unique_ptrs
    auto ptr_copy = pointers.at(i);
}

If you want to copy the object (not the pointer!) you can do it like this:

for (int i = 0; i < 10; ++i)
{
    QString string_copy = *pointers.at(i);
}

JoeCFD

@mzimmers said in trying to understand smart pointers...:

uniquePtr = &myStruct; // how to form this?

This is not allowed. myStruct sits in stack and will be cleared when your app runs out of its scope. However, uniquePtr will destroy it again when uniquePtr is not used anymore.