How to declare nested templates in idl 3.0

Question

I have what I hope is a basic question, but I've been unsuccessful finding any documentation or examples on MSDN or the internet in general.

I'm implementing a windows runtime component in C++/WinRT. I have an asynchronous method that scans for the presence of a specific type of device and returns a vector of strings (device IDs) of devices found. The C++ signature of the function is:

static IAsyncOperation<IVector<winrt::hstring>> ScanInterfacesAsync();

I have been unsuccessful in getting an authoring an IDL that projects this function to the runtime. My best guess:

static IAsyncOperation<IVector<String>> ScanInterfacesAsync();

Fails with an error:

error MIDL2025: [msg]syntax error [context]: expecting > or , near ">>"

I've tried a few other things but I can tell I just fundamentally don't understand how IDL works with templates. And I can't find any examples of a templated return type in any of the online IDL documentation.

Is this possible? What's the right syntax? Any pointers to documentation I've overlooked? Thanks.

Answer 1

The error message seems to indicate, that MIDL 3.0 uses a greedy parser. It interprets the two consecutive closing angle brackets (>) as a single right-shift operator (>>), much like C++ did prior to C++11 (see right angle bracket).

To work around this, MIDL 3.0 source needs to introduce arbitrary whitespace (e.g. a single space character) in between consecutive closing angle brackets so that it gets interpreted as two distinct tokens as opposed to a single token. The following should compile:

static IAsyncOperation<IVector<String> > ScanInterfacesAsync();