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by 20k5 hours ago |
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upvoteby Jtsummers5 hours ago|
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> 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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upvoteby 20k3 hours ago|
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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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upvoteby Jtsummers3 hours ago|
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> 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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upvoteby 20k2 hours ago|
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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

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upvoteby 1 hours ago|
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upvoteby deepsun5 hours ago|
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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.

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upvoteby WalterBright4 hours ago|
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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.

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