Setting properties in C++/WinRT is done by a function call, but you need to call the function the right way

Raymond Chen

For people transitioning to C++/WinRT from other languages, a common beginner mistake is setting properties incorrectly. Here are the ways of accessing a Windows Runtime property in various languages.

Notice the odd one out in the above list.¹ Everybody uses the assignment operator to set a property, except C++/WinRT, which uses a function call.

If you’re used to the other languages, you may incorrectly interpret the rule that “property access is a function call” People transitioning to C++/WinRT from other languages are tempted to write

o.ActiveWidget() = newValue;

in a misguided attempt to set the property to a new value. What this actually does is fetch the current value of the property, and then assign the new value to the temporary. It’s a lot of work with nothing to show for it, and then you scratch your head wondering why the attempt to update the property appears to have been ignored.

I tried to update C++/WinRT to deny assignment to most types of rvalues, rendering the above a compile-time error, but the change had to be reverted for compatibility reasons.

There’s no real value to creating a temporary only to assign a new value to it. You may as well just create the temporary with the desired value, and then use it.

// Pointless
DoSomething(o.ActiveWidget() = newValue);

// Just do this
DoSomething(Widget(newValue));

// Or possibly even just
DoSomething(newValue);

¹ Rust/WinRT also uses function calls to access properties:

However, it doesn’t suffer from the same problem as C++/WinRT, because Rust disallows assignment to rvalues (known as value expressions in Rust-speak).

// error E0070: Invalid left-hand side of assignment
o.active_widget() = new_value;

The problem of assigning to a temporary is peculiar to C++.

Raymond Chen

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