31–40 of 75 posts
Regarding returning lambdas, of course it could be done even in c++11 via std::function.
Problems with std::function:- 1. It can not hold move only callable objects. 2. It heap allocate stored callable object if the object is large enough.
i was in a leetcode BS interview a while ago with a googler who thought i could not write code in c++ because i could not remember lambda syntax. i only use maybe 30% of c++ features in my software and have been doing this for over 20 years. it’s only recently i started using lambdas more, but still stay away from them because of the potential hidden allocations.
What kind of hidden allocations?
Internally, lambdas are function pointers + the context, and the context may or may not be dynamically allocated depending on how lambdas are used.
> Even if you don’t need to handle several types, this can be useful to avoid repetition and make the code more compact and readable. ... > namespace1::namespace2::namespace3::ACertainTypeOfWidget Deeply nested namespaces are problematic in themselves due to namespace resolution (e.g. see https://abseil.io/tips/130 ), but templates should never be an answer to "my type is too long to type out". You're hurting yoursel…
Eh, in some cases it's a wash.
std::vector> objects;
// ...
auto it = std::find_if(objects.begin(), objects.end(), [](auto& widget) { return widget->x == 1; });
Earlier quoted context omitted.
I've seen a variation of this bug several times in just the last year: myVariable = 1 if not something_unusual(): myVariablr = 2 return myVariablr This code will work just fine until something_unusual() returns True and then it crashes with UnboundLocalError: local variable 'myVariablr' referenced before assignment This really sucks when it happens during a long-running job. Say for example you're looping through an…
Dynamic languages often benefit from some tool assistance. Both your and GPs experience can happen at the same time: GP is probably using a linter or an IDE (like Pycharm) which will point out that "myVariablr" may be undefined.
You might get a warning that myVariabl_r_ is unused, but that's also not a given. The bar for being 'used' is pretty low. For example, in this
def main():
x = 10
l = []
l.append(x)
return 5
Earlier quoted context omitted.
Problems with std::function:- 1. It can not hold move only callable objects. 2. It heap allocate stored callable object if the object is large enough.
Can you point to better alternate ways or idioms?
Example: https://github.com/facebook/folly/blob/main/folly/docs/Funct...
Earlier quoted context omitted.
What kind of hidden allocations?
That are used to hold captured variables and values. Internally, lambdas are function pointers + the context, and the context may or may not be dynamically allocated depending on how lambdas are used.
Is there any flag in some c++ compiler that forces to you to use the newer features? For example, forbidding non-auto declarations.
Earlier quoted context omitted.
What kind of hidden allocations?
That are used to hold captured variables and values. Internally, lambdas are function pointers + the context, and the context may or may not be dynamically allocated depending on how lambdas are used.
Lambdas are essentially syntactic sugar for an anonymous struct holding references to/copies of captured variables and an operator() method with your code. Not 100% precise, but memory-wise that's how it works (and has to work). No dynamic memory allocation is involved here at all.
If you put a lambda (or any other callable) in a std::function, the std::function will copy that functor object, either into its own small-buffer-optimized in-place storage or (above a certain functor size) into heap-allocated memory.
Earlier quoted context omitted.
What kind of hidden allocations?
That are used to hold captured variables and values. Internally, lambdas are function pointers + the context, and the context may or may not be dynamically allocated depending on how lambdas are used.
Earlier quoted context omitted.
>> in Python the number of times something like this happens Anec-data but I’ve been writing python for 15+ years, I’ve contributed to various popular open source projects. I’ve never seen this in a code review. I’ve certainly never seen this kind of mistake released.
I've seen a variation of this bug several times in just the last year: myVariable = 1 if not something_unusual(): myVariablr = 2 return myVariablr This code will work just fine until something_unusual() returns True and then it crashes with UnboundLocalError: local variable 'myVariablr' referenced before assignment This really sucks when it happens during a long-running job. Say for example you're looping through an…