How is std::unique_ptr used?

Question

0.00/5 (No votes)

See more:

I have come to realise that one of the issues causing failure in my application recently is the improper use of smart pointer. I thought I had complete grasp of its usage until now!

Consider the code snippet from an application that consumed substantial amount of my time.

C++

int iSelCount = (int)SendMessage(GetDlgItem(hDlg, IDC_LIST2), 
                    LB_GETSELCOUNT, 0, 0);

unique_ptr<int[]> pStatus = make_unique<int[]>(iSelCount);

int iSelected = (int)SendMessage(GetDlgItem(hDlg, IDC_LIST2), 
                    LB_GETSELITEMS, 0, (LPARAM)pStatus.get());

if (iSelCount > iSelected)
    {
    AppErrMessage msg{ 
    AppErrMessage::ERM_TOO_MANY_STATUS_SELECTIONS };
    pair<wstring, wstring> err = GetAppErrMessage(msg);

    MessageBox(hDlg, err.first.c_str(), err.second.c_str(), MB_OK);

    return (INT_PTR)TRUE;
    }

Debugging shows that the smart pointer pStatus does not receive the various indices of items selected in the listbox.

Examples on the internet all show smart pointers being initialized with value at creation. None show their use for various other purpuses much later after their creation. How do I use a std::unique_ptrbin in the above situation. I wanted to use a vector, but it seemed ill-advised to so do. Can a vector be used in the same situation? Please help with sample code.

What I have tried:

The internet has no solution to this as far as my searching effort is concerned.

Posted 3-Nov-23 1:25am

Gbenbam

Updated 3-Nov-23 4:19am

merano99

v3

Add a Solution

Comments

Richard MacCutchan 3-Nov-23 8:05am

As far as I can see your cde should work. What is the value of iSelected on return?

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard MacCutchan · Accepted Answer · 2023-11-03T02:45:00

Solution 1

The problem you have is with this line:

C++

int iSelected = (int)SendMessage(GetDlgItem(hDlg, IDC_LIST2), LB_GETSELITEMS, 0, (LPARAM)pStatus.get());

The WPARAM item is required to contain the maximum size of the input buffer: see LB_GETSELITEMS message (Winuser.h) - Win32 apps | Microsoft Learn[^]. So it should be:

C++

int iSelected = (int)SendMessage(GetDlgItem(hDlg, IDC_LIST2), LB_GETSELITEMS, iSelCount, (LPARAM)pStatus.get());

Posted 3-Nov-23 2:45am

Richard MacCutchan

Comments

CPallini 3-Nov-23 8:58am

:thumbsup:
Good catch!

Richard MacCutchan 3-Nov-23 9:14am

Thanks; but I had to run the code to discover the problem.

CPallini 3-Nov-23 9:34am

A genuine experimental approach.
I was completely out of the way.

Richard MacCutchan 3-Nov-23 9:42am

I also needed to learn how to use unique_ptr, as I still think in C and struggle with all these new-fangled classes. :)

Rick York 3-Nov-23 16:48pm

unique_ptr is very, very handy. I try to use it whenever possible. One primitive way to use it is just to hold the pointer and eliminate the need for explicit deletion. I sometimes do this because it greatly simplifies code. You can have multiple return points and you don't have to remember to delete anything before each of them. I've been wrapping lots of resources in container classes just to have this automatic deletion capability.

What I meant earlier is you can construct an object in the usual way and then call the reset method of unique_ptr to save the pointer and that makes deleting the object automatic.

Richard MacCutchan 4-Nov-23 4:58am

You are right, I just need to use it more.

Gbenbam 3-Nov-23 16:09pm

Thanks. This is so embarassing. I am sure I have read through that document several times already. The problem is something so way, way, way too simple for my liking. I had started to think that visual studio is too buggy for my liking.

Richard MacCutchan 4-Nov-23 4:57am

Well, firstly this has nothing to do with Visual Studio. You need to understand that Visual Studio is an IDE: Integrated Development Environment. In simple terms it is just a glorified editor and file manager. When you get compilation errors in your code, they are produced by the compiler, which, although it runs within a Visual Studio window, is a completely separate application. The same with the linker that produces the final executable program. And finally, if your code does not work you can gauarantee that in 99.999% of cases the fault will be yours. Years of making simple mistakes has taught me that rule.