Understanding the lifecycle of winrt::hstring with ABI handling

Question

Hello,

I'm trying to understand the lifecycle of winrt::hstring when working with the ABI and managing reference counts.

Specifically, I want to store an winrt::hstring in a void* pointer, and to do so, I'm using the copy_to_abi API. This increases the reference count of the internal string, preventing it from being destroyed while the void* pointer is in use.

Once I'm done with the void* pointer, I need to safely decrement the reference count to destroy the winrt::hstring. However, I couldn't find any direct method to decrement the reference count. So, here's what I am doing:

void Func1()
{
    void* ptr = nullptr;

    { // Start Block1

        winrt::hstring myString = L"Hello, WinRT!";

        copy_to_abi(myString, ptr); // Increase the ref count

    } // End Block1

    // ... Consume ptr

    winrt::hstring myString2(ptr, winrt::take_ownership_from_abi);

} // Decrease the ref count since myString2 got out-of-scope

Explanation:

Inside Block 1, a winrt::hstring (myString) is created and its pointer is stored in ptr using copy_to_abi, which increases the reference count.
After Block 1 ends, I use take_ownership_from_abi to create a new winrt::hstring (myString2) from ptr, which takes ownership of the string and manages the reference count.
When myString2 goes out of scope, the reference count is decremented, and the winrt::hstring should be destroyed if no other references exist.

My question is: Is this approach correct for managing the lifecycle of winrt::hstring in this case? Will this work as expected, ensuring that the string is properly destroyed when no longer needed?

Thanks!

Accepted Answer

Hello @Shyam Butani ,

Yes, it is valid to put the pointer into a winrt::hstring via “take ownership from abi” and then allowing the winrt::hstring to destruct.

You can use detach_abi to avoid bumping the reference count:

int main()
{
    hstring str = L"hello world";
    void* ptr = detach_abi(str);
    hstring str2(ptr, take_ownership_from_abi);
    assert(str == L"");
    assert(str2 == L"hello world");
}

Another valid way is to call WindowsDeleteString((HSTRING)ptr), which is the OS’s ABI method for managing the lifetime ABI HSTRINGs.

Thank you.

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