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by jandrewrogers57 minutes ago |
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upvoteby drivebyhooting54 minutes ago|
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Your comment just made me realize contracts is different from concepts (which are basically bounded genetics, right…?)
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upvoteby jandrewrogers33 minutes ago|
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C++ concepts evaluate the properties of types at compile-time. You can inject arbitrary constraints on type properties anywhere in the code including conditional codegen. It is a way of asking if a type “can express this complex set of abstract things or has this enumerated set of properties” with almost arbitrary detail. They are extremely powerful in terms of expressiveness and specialization of code. Insanely good for performance and reliability engineering.

One of the best features to be added to C++ in many years.

C++ contracts are an attempt to bring C asserts into the C++ age. The weakness of C++ contracts is that they are mostly syntactic sugar over C asserts. If you really care about this type of thing, you probably already have a custom C++ assertion framework that is at least as nuanced if not more.

The promise of C++ contracts is that they can aid formal verification, which would be great. As delivered, it can’t do much more than was already possible, which will inhibit adoption.

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upvoteby AlotOfReading15 minutes ago|
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    The weakness of C++ contracts is that they are mostly syntactic sugar over C asserts.
The sugar is actually one of my issues with them. The evaluation semantics are much more complicated / potentially slower than C asserts because of the sugar, and why you would have to pay those costs isn't obvious at first glance. Lisa Lippincott's excellent talk on the design of contracts [0] is what switched me from a sort of neutral support to disliking it. I think it's a stupid place for the national bodies to draw the line though. std::regex is acceptable, but a nothingburger like contracts tanks the vote?

[0] https://www.youtube.com/watch?v=yhhSW-FSWkE

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upvoteby jandrewrogers9 minutes ago|
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To be clear, I have a lot of issues with the contracts proposal. I probably wouldn’t use despite living out on the standards frontier. I don’t even use C asserts already because they are not expressive enough in important ways. This proposal kind of reifies the inadequacies.

I think the risk is that C++ contracts become an evolutionary dead-end that hang around forever in code bases because they were standardized. It wouldn’t be the first time this has happened.

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upvoteby stingraycharles50 minutes ago|
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Concepts are evaluated compile time and make a lot of TMP more elegant. Contracts are runtime check, akin to assertions.

Phrased differently, contracts are to concepts what assert is to static_assert.

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upvoteby rramadass38 minutes ago|
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Concepts are for Types at compile time while Contracts are for Values at runtime.
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upvoteby quotemstr46 minutes ago|
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Huh? Concepts are types of types. Contracts are fancier, structured runtime assertions. What does one have to do with the other? They're nothing alike.
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upvoteby taybin9 minutes ago|
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They both start with the letter C, have two syllables, and have/had a hard time getting into the standard. If you’re not paying close attention, it’s easy to get them confused.
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upvoteby jandrewrogers24 minutes ago|
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That’s exactly my point.

Contracts notionally offer the ability to lift some runtime assertions to compile-time. That is how it was sold. Except they don’t in practice.

Meanwhile, concepts + constexpr if used artfully deliver that in practice, including for many cases that contracts were supposed to solve.

I push all verification to compile-time to the extent possible.

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