[MS] The MIDL compiler still has trouble with double greater-than signs, sadly - devamazonaws.blogspot.com

The parser for the Microsoft MIDL compiler has long suffered from the problem of the double greater-than sign. This problem plagued C++ until C++11 added a special rule:¹

[temp.names] (4): When parsing a template-argument-list, the first non-nested > is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.

Microsoft's MIDL compiler predates C++11, and its parser treats two consecutive greater-than signs as a bitwise shift operator. This means that you cannot write

Windows.Foundation.IAsyncOperation<
    Windows.Foundation.Collections.IVector<Int32>>

when you want an asynchronous operation that produces a vector of 32-bit integers. The double-greater-than is interpreted as a shift operator, and you get a weird MIDL error message because a shift operator is not allowed there.

You must explicitly insert a space to force it to be parsed as two greater-than signs.

Windows.Foundation.IAsyncOperation<
    Windows.Foundation.Collections.IVector<Int32> >

Larry Osterman tells me that multiple people have attempted to fix this and allow the compiler to break up a shift operator into two greater-than signs "when appropriate".

All such attempts have failed.

I wonder if there is a comment in the source code similar to this legendary one:

// 
// Dear maintainer:
// 
// Once you are done trying to 'optimize' this routine,
// and have realized what a terrible mistake that was,
// please increment the following counter as a warning
// to the next guy:
// 
// total_hours_wasted_here = 42
// 

¹ The controversy is still not over, though, because the standard does not define what "non-nested" means. Issue 579 gives as an example the sequence

X<a ? b > c : d>

Is the first > a "nested" greater-than sign? This issue, opened in 2006, is still outstanding!

Post Updated on June 23, 2025 at 03:00PM
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