This is the biggest problem with contracts, there's simply no way to actually say "please always execute this check". A lot of people replying seem to think that the critique is with the concept of contract based programming, but the C++ version of contracts is just weird

The programming model for C++ contracts as described by the authors, and spec, is that they are 'ghost code'. You must never assume that a contract check is actually executed, even if you've asked your compiler to execute your contract checks. This is because ABI/ODR problems can and will cause your contract checks to be deleted, as well as the 'observe' contract mode which is a strange one

So its hard to know what the use case for them is as a result. You can't use them for safety checks, because you must assume they won't be run. Hilariously, they actually introduce safety problems vs asset: Eg take two pieces of code:

    assert(p != nullptr);

    assert(p->some_value());

    contract_assert(p != nullptr);

    contract_assert(p->some_value());

People mistakenly transform #1 into #2 unconditionally, not realising that this is actually a misuse of the contracts feature. It can, and will lead to undefined behaviour - while being perfectly well defined with asserts

I have a feeling that in 10 years the advice will be to never use contracts in anything approaching code that needs safety, as they're generally less safe than using an assert for no real reason. So what's their use case?

▲ Jtsummers 4 hours ago | parent | next [-]

> You can't use them for safety checks, because you must assume they won't be run.

This is also true of assert, though. It can be compiled out so shouldn't be used for anything that must run.

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20k 2 hours ago | parent [-]

Its pretty easy for me to guarantee that my asserts will all always be run via NDEBUG. I can't make that same guarantee for contracts

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Jtsummers 2 hours ago | parent [-]

> Its pretty easy for me to guarantee that my asserts will all always be run via NDEBUG.

That's the opposite of what NDEBUG does. https://en.cppreference.com/cpp/error/assert

Critical bit:

> 1) If NDEBUG is defined as a macro name at the point in the source code where <cassert> or <assert.h> is included, the assertion is disabled: assert does nothing.

It also is impossible to guarantee it unless you're the only one compiling the program. If you've built a library that others can build and relied on asserts executing for your library to be correct, you wrote a bad library.

EDIT: Neither asserts nor these C++26 contract features should be used in a way that requires that they be executed. If you do, you're using them wrong because it is valid for someone to disable them as runtime checks. If the code must run, you should use some other mechanism, I'd recommend an if statement.

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20k an hour ago | parent [-]

Of course? You simply undefine NDEBUG in your library

Its trivially easy to guarantee that an assert always runs, and its well defined behaviour according to the spec. There's no reason not to if you want to use asserts in a way that guarantees that they always fire, which is a supported use case. Its common practice

There's no equivalent for contracts

	▲	13 minutes ago \| parent [-]
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▲ deepsun 4 hours ago | parent | prev [-]

Interestingly, Java had exactly that in mind when introduced `assert` keyword in very early versions (2002). It's equivalent to an `if` that can throw a runtime AssertionException, but only if the program is launched with `-enableassertions` flag.

The idea is that you use `-enableassertions` only during development, and your program should never rely on it. But it helps IDEs and static analyzers to have a contract about code.

	▲	WalterBright 3 hours ago \| parent [-]
		D has a keyword prefix `debug`. So, one can write things like: `... debug if (x) printf("x: %d\n", x); ...` So, you can throw the -debug switch on the compiler and the debug code is enabled. For asserts, `debug assert(lookMa);` and the check is only run when the -debug switch is done. It works remarkably well. Another interesting property of `debug` is it turns off the safety checks in the target statement that follows it. Because, sometimes you just gotta do some unsafe tricks to figure out where your code went wrong.