4 ms·

What is wrong about C++ exceptions?

ajross 1y ago
"Nothing", in principle. But they're bug factories in practice. It's really easy to throw "past" cleanup code in a language where manual resource management remains the norm. It's not that they can't be used productively. It's that they probably do more harm than good on balance. And I think async mania is getting there. It was a revelation when node showed it to us 15 years ago. But it's evolved in bad directions, IMHO.
- baq 1y ago
  Yeah node showed that a native async single threaded runtime can be performant. Seems like this knowledge was lost to the world somewhere along windows vista; everyone who has had to ever develop in the cooperative world of early winapi could tell you that easily.
- efdee 1y ago
  C# had async/await long before Javascript/node. Not that big a revelation ;-)
  - ajross 1y ago
    .NET wasn't the first either. Lisps were doing continuations in the 70's. But "invented" and "revealed" are different verbs for a reason. The release of node.js and it's pervasively async architecture changed the way a lot of people thought about how to write code. For the better in a few ways. But the resulting attempt to shoe-horn the paradigm into legacy and emerging environments that demanded it live in a shared ecosystem with traditional "blocking" primitives and imperative paradigms has been a mess.
    - mrsmrtss 1y ago
      I think you're underestimating the role of .NET in this. It was .NET that popularized this concept for the masses, and from there it spread to other languages including JavaScript, which also borrowed the exact same async/await keywords from C#.
KerrAvon 1y ago
For one thing, they’re expensive and viral. “Zero overhead” implementations don’t take into account the need for unwind tables. For every function/method that might be thrown across. They’re disabled in a lot of production environments for this reason.
- amelius 1y ago
  But if you explicitly handle exceptions using IF statements then that's overhead too, right?
  - arbitrandomuser 1y ago
    yes but i think branch prediction essentialy makes them zero overhead
    - neonz80 1y ago
      That's a different type of overhead than having unwind tables. With exceptions you wouldn't need a branch after each function call at all.
      - amelius 1y ago
        But a branch that is (almost) never taken has an overhead close to the overhead of a NOP instruction, which may be negligible on modern architectures.
        neonz80 1y ago
        The CPU can not remember an infinite number of branches. Also, many branches will increase code size. With exceptions the unwind tables and unwind code can be placed elsewhere and not take up valuable L1 cache.
        amelius 1y ago
        > The CPU can not remember an infinite number of branches. I suspect a modern CPU has a branch instruction saying "This branch will never be taken except in exceptions, so assume this branch is not taken". But I must admit I haven't seriously looked at assembly language for some time. (EDIT: yes, modern CPUs including x86 and ARM allow the programmer/compiler to hint if a branch is expected to be taken). > Also, many branches will increase code size. I'd like to see some data on that. Of course branches take code size, but how much is that percentage-wise? I suspect not much.
jandrewrogers 1y ago
There are cases in systems-y code where it is not safe to unwind the stack in the ordinary way and it is difficult to contain the side-effects. These can be non-obvious and subtle edge cases that are often difficult to see and tricky to handle correctly. C++ today is primarily used in code contexts where these kinds of issues can occur. This is why it is a standard practice to disable exceptions at build time i.e. -fno-exceptions. With the benefit of hindsight, explicit handling and unwinding has proven to be safer and more reliable.
- amelius 1y ago
  But you can implement exceptions by using the same IF statement approach you would use for manual error handling. No need for unwinding tables and such if that optimization is a bridge too far for your specific target platform.

"Nothing", in principle. But they're bug factories in practice. It's really easy to throw "past" cleanup code in a language where manual resource management remains the norm. It's not that they can't be used productively. It's that they probably do more harm than good on balance. And I think async mania is getting there. It was a revelation when node showed it to us 15 years ago. But it's evolved in bad directions, IMHO.

baq 1y ago
Yeah node showed that a native async single threaded runtime can be performant. Seems like this knowledge was lost to the world somewhere along windows vista; everyone who has had to ever develop in the cooperative world of early winapi could tell you that easily.
efdee 1y ago
C# had async/await long before Javascript/node. Not that big a revelation ;-)
- ajross 1y ago
  .NET wasn't the first either. Lisps were doing continuations in the 70's. But "invented" and "revealed" are different verbs for a reason. The release of node.js and it's pervasively async architecture changed the way a lot of people thought about how to write code. For the better in a few ways. But the resulting attempt to shoe-horn the paradigm into legacy and emerging environments that demanded it live in a shared ecosystem with traditional "blocking" primitives and imperative paradigms has been a mess.
  - mrsmrtss 1y ago
    I think you're underestimating the role of .NET in this. It was .NET that popularized this concept for the masses, and from there it spread to other languages including JavaScript, which also borrowed the exact same async/await keywords from C#.

KerrAvon 1y ago

For one thing, they’re expensive and viral. “Zero overhead” implementations don’t take into account the need for unwind tables. For every function/method that might be thrown across. They’re disabled in a lot of production environments for this reason.

amelius 1y ago
But if you explicitly handle exceptions using IF statements then that's overhead too, right?
- arbitrandomuser 1y ago
  yes but i think branch prediction essentialy makes them zero overhead
  - neonz80 1y ago
    That's a different type of overhead than having unwind tables. With exceptions you wouldn't need a branch after each function call at all.
    - amelius 1y ago
      But a branch that is (almost) never taken has an overhead close to the overhead of a NOP instruction, which may be negligible on modern architectures.
      - neonz80 1y ago
        The CPU can not remember an infinite number of branches. Also, many branches will increase code size. With exceptions the unwind tables and unwind code can be placed elsewhere and not take up valuable L1 cache.
        amelius 1y ago
        > The CPU can not remember an infinite number of branches. I suspect a modern CPU has a branch instruction saying "This branch will never be taken except in exceptions, so assume this branch is not taken". But I must admit I haven't seriously looked at assembly language for some time. (EDIT: yes, modern CPUs including x86 and ARM allow the programmer/compiler to hint if a branch is expected to be taken). > Also, many branches will increase code size. I'd like to see some data on that. Of course branches take code size, but how much is that percentage-wise? I suspect not much.

jandrewrogers 1y ago

There are cases in systems-y code where it is not safe to unwind the stack in the ordinary way and it is difficult to contain the side-effects. These can be non-obvious and subtle edge cases that are often difficult to see and tricky to handle correctly. C++ today is primarily used in code contexts where these kinds of issues can occur. This is why it is a standard practice to disable exceptions at build time i.e. -fno-exceptions. With the benefit of hindsight, explicit handling and unwinding has proven to be safer and more reliable.

amelius 1y ago
But you can implement exceptions by using the same IF statement approach you would use for manual error handling. No need for unwinding tables and such if that optimization is a bridge too far for your specific target platform.