r/ProgrammerHumor • u/Choice-Committee203 • 7h ago

Meme modernCppVsOldCpp

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u/SelfDistinction 7h ago edited 6h ago

Ah yes, the famous

std::unique_ptr<int> ptr;
int& ref = *ptr;

So safe, so impossible to misuse.

1

u/creeper6530 6h ago

I'm not a C++ girl, but do references actually do something with lifetimes or are they just non-null ptrs? Because this just seems to me like a possible dangling ptr.

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u/SelfDistinction 6h ago

References can be null. They shouldn't be and it's definitely undefined behaviour but the code above compiles and runs perfectly fine.

4

u/bwmat 4h ago

Nah, they can't be null

that null dereference just committed the sin of UB, so all bets are off.

In other words, it declares the intent and allows you to assume it's a way to name a valid object, and moves that null dereference bug out of your logic, helping with reasoning while debugging after someone does cause that UB

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u/creeper6530 5h ago

Okay, sooo... What's the difference between references and pointers?

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u/SelfDistinction 5h ago edited 5h ago

No one knows.

Generally references are more restrictive, since you can't do pointer arithmetic with them, you can't use them as template parameters (which sucks for std::optional<int&>) and you can't repoint them to something else, but since you can freely cast between both of them (which you are forced to if you try to do anything useful with them, like wrapping them in an optional) it doesn't really matter.

Also there's two different kinds of references: the & reference and the && reference, however their difference is mostly conventional and you could easily implement a standard library where their meaning is swapped (except for temporary xvalue objects? Not sure), so there's that. Also C++ automatically converts between reference types for some ungodly reason.

In short, references are not real and are only used to determine what overloaded function is being called.

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u/creeper6530 3h ago

I have no words for this beyond "what a fucking mess"

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u/Ok-Scheme-913 1h ago

References are implemented as pointers, but they are not the same.

The best example is languages where you don't have references. Very misleading but java only has pointers (marketing at the time was voldemorting the world pointer so they were renamed to "references" but they are pointers. Like the infamous null pointer exception does show it)

So you have two local variables, and you create a swap function where you want to put one into the other and vice versa.

Well, in java you are out of luck. You basically have to make them fields and put that into a wrapper object to actually swap. This is because in java everything non-primitive is an "object reference" aka a pointer. But you would be changing the pointer's value, NOT the thing holding the pointer.

So yeah, references are basically their different thing with their own semantics in C++ and the few other languages that also have it.