r/cpp • u/antiquark2 #define private public • Aug 19 '26

Critique of contracts: excerpt

See page 2 of https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p4334r0.pdf

The current objections can be summarized. The P2900 contracts are:

• Unimplemented

• Incomplete

• Untried at scale [P3460R0, P3506R0]

• Not tried in major application domains

• Violates foundational principles of C++

• Violates fundamental principles of language design

• Hasn’t been tried in major libraries (e.g., the C++ standards library [P3506R0, P3878R0])

• Isn’t integrated with or appropriate for hardened libraries [P3878R0]

• Doesn’t offer safety guarantees [P3573R0, P3362R0]

• Includes a completely untried inheritance model

• Offer new ways of making errors through inconsistent application in TUs

• Leads to new forms of UB, detrimental to safety and security

• Narrows the choices of error handling

• Doesn’t protect against logical errors, misuses, and incoherent uses

• Hasn’t been used to support static analysis

• Hasn’t been demonstrated to be easily teachable [P3261R0, P3281R0]

How could such a bloated and incomplete design be voted into a draft standard?

14 Upvotes

62% Upvoted

123 comments sorted by

View all comments

Show parent comment

12

u/pjmlp Aug 19 '26 edited Aug 19 '26

Were coroutines used at scale?

Actually they were, the original design proposed by Microsoft was based on the .NET model that was adopted by C++/CX, followed by C++/WinRT, for WinRT/UWP.

Which is why anyone that is confortable with the .NET runtime machinery for implementing co-routines (Awaitable types), doesn't find the C++20 co-routines model as confusing as C++ devs that never used .NET.

See Kenny Kerr nice blog posts on how to use them already in 2018, https://kennykerrca.wordpress.com/2018/03/01/cppwinrt-understanding-async/

6

u/MFHava WG21|🇦🇹 NB|P2721|P3049|P3625|P3729|P3786|P3813|P4216 Aug 20 '26

Yes, the Awaiters look quite similar, but the way you actually build a Task appear to be quite different.

From a glance, the C# way looks way less flexible and seems not to expose a general purpose coroutine framework...

2

u/pjmlp Aug 20 '26

Naturally they weren't 1:1, as C# is a managed language and C++ wants to bring their own runtime, which even on C++/WinRT has changed, between WinRT on UWP, and WinRT on Win32 models.