Comment by riazrizvi
3 days ago
It’s giving you an expression capability so that you can state your intent, in a standardized way, that other tooling can build off. But it’s recognizing that the degree of enforcement depends on applied context. A big company team might want to enforce them rigidly, but a widely used tool like Visual Studio would not want to prevent code from running, so that folks who are introducing themselves to the paradigm can start to see how it would work, through warnings, while still being able to run code.
This is not just expressing intent. The documentation clearly states that it's UB to violate them, so you need to be extra careful when using them.
Perhaps another helpful paradigm are traffic/construction cones with ‘do not cross’ messages. Sometimes nothing happens, other times you run into wet concrete, other times you get a ticket. They’re just plastic objects, easy to move, but you are not meant to cross them in your vehicle. While concrete bollards are a thing, they are only preferable in some situations.
I don't think this analogy fully respects the situation here. These pre/post condition are not just adding a warning to not do something, they also add a potentially bigger danger if they are broken. It's as if you also added a trap behind the construction cone which can do more damage than stepping on the wet concrete!
> documentation clearly states that it's UB to violate them
Only in "fast" mode. The developer has the choice:
> Compilation has two modes: “safe” and “fast”. Safe mode will insert checks for out-of-bounds access, null-pointer deref, shifting by negative numbers, division by zero, violation of contracts and asserts.
> The developer has the choice
The developer has the choice between fast or safe. They don't have a choice for checking pre/post conditions, or at least avoiding UB when they are broken, while getting the other benefits of the "fast" mode.
And all in all the biggest issue is that these can be misinterpreted as a safety feature, while they actually add more possibilities for UB!
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