Difference between STL Arrays and C arrays | Together C & C++ | Page 1

placid hawk Dec 8, 2022, 3:45 PM

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Is there a difference between how the following two arrays are implemented?
std::array<int,n> a
int a[n]
Which is better and why?

delicate burrowBOT Dec 8, 2022, 3:45 PM

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desert vault Dec 8, 2022, 4:00 PM

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std::array<T, N> is a convenience wrapper around a C style array of type T _private[N]

it prevents you from footgunning yourself.

Perfer it in all cases as it is just superior

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https://gcc.gnu.org/onlinedocs/gcc-4.6.3/libstdc++/api/a00752_source.html

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This is the implementation by g++'s libstdc++

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its a little annoying to read so I recommend putting it into an ide and using auto highlight

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however its only like 300 lines long and is definetly understdable

wooden tiger Dec 8, 2022, 4:14 PM

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C arrays decay easily when passed to functions (it can be prevented though) whereas std::array does not.
There are also certain constructs in modern C++ that cannot be applied to (decayed) plain C arrays, for example ranged-for (*).
So I am 100% with @desert vault ; prefer std::array over C arrays whenever/wherever possible.

(*) see: https://stackoverflow.com/questions/36973045/c-basics-ranged-based-for-loop-and-passing-c-style-arrays-to-functions

desert vault Dec 8, 2022, 4:30 PM

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https://godbolt.org/z/4xaKxdn7a

Compiler Explorer - C++ (x86-64 gcc 12.2)

int main()
{
std::cout << "hello worl\n";
int arr[3] {1, 2, 3};
int val {88};

std::cout << "arr 3: " << arr[6] << '\n';

std::array<int, 3> ar {11, 22, 33};
int val2 {888};

std::cout << "arrrrrrrL " << ar[12] << '\n';

try
{
    std::cout << "oopers" << ar.at(12);
}
catch (std::exception& e)
...

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example of reading out of bounds

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this can get real dicey if you are using packed structures. for example reading at arr[4] on a C compiler would gladly return 88

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https://godbolt.org/z/qndhee567

Compiler Explorer - C (x86-64 gcc 12.2)

int main()
{
int arr[3] = {1, 2, 3};
int val = {88};

if (arr[3] == 88)
{
    assert(!"oopers");
}

}

shell glade Dec 8, 2022, 5:57 PM

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placid hawk Is there a difference between how the following two arrays are implemented? `std...

and for these reasons, when passing std::array to some function, pass it as const std::array<T,N>& arr otherwise you will deep copy each element

spare mountain Dec 9, 2022, 5:48 AM

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which one has better performance tho?

pseudo delta Dec 9, 2022, 6:11 AM

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spare mountain which one has better performance tho?

they're the same

spare mountain Dec 9, 2022, 6:13 AM

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idk how it looks in assembly but id assume the array class takes more compiling time or something since it comes in a package

pseudo delta Dec 9, 2022, 6:31 AM

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sure, but the runtime performance will be identical

odd bane Dec 9, 2022, 7:48 AM

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spare mountain idk how it looks in assembly but id assume the array class takes more compiling ...

compilation time takes longer because it's a template*
but that means nothing about its runtime performance

opal karma Dec 9, 2022, 9:02 AM

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spare mountain which one has better performance tho?

imho the safety and usability benefits it gives makes it worth regardless

marsh fox Dec 9, 2022, 12:57 PM

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there's a big deal breaker with std::array

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you can't do this with std::array nooo

index[arr]

crude apex Dec 9, 2022, 12:58 PM

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?

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oh lol

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thank god you can't 😛

marsh fox Dec 9, 2022, 1:05 PM

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I think I saw stroustrup say in his site that he tried really hard to replicate the behavior kekw

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wasn't possible in the end though

desert vault Dec 9, 2022, 1:06 PM

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Lol

desert vault Dec 9, 2022, 1:06 PM

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spare mountain idk how it looks in assembly but id assume the array class takes more compiling ...

The difference is literal nano seconds.

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At compile time

crude apex Dec 9, 2022, 1:09 PM

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marsh fox I think I saw stroustrup say in his site that he tried really hard to replicate ...

i doubt that very much ^^

marsh fox Dec 9, 2022, 1:09 PM

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I'm gonna try to find it

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probably sarcastic anyways

crude apex Dec 9, 2022, 1:10 PM

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there are a few joke "interviews with bjarne" and stuff like that out there.

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tend to be published on april 1 😉

marsh fox Dec 9, 2022, 1:11 PM

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crude apex there are a few joke "interviews with bjarne" and stuff like that out there.

oooh TIL

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maybe it was one of those

crude apex Dec 9, 2022, 1:13 PM

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probably

spare mountain Dec 9, 2022, 5:59 PM

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i wonder if array template would have even faster runtime performance than c style array since the compiler will know exactly what you wanna do with the array and just write better machine code than it could’ve if you wrote something with a c style array

opal karma Dec 9, 2022, 6:03 PM

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spare mountain i wonder if array template would have even faster runtime performance than c sty...

how is that different from a c style array?

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it still knows the exact type

spare mountain Dec 9, 2022, 6:07 PM

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eh nevermind im just overthinking something like usual

shell glade Dec 9, 2022, 6:10 PM

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spare mountain eh nevermind im just overthinking something like usual

std::array's fields are one fixed length array T arr[N], so no, you can't get std::array to beat C arrays in performance

little herald Dec 9, 2022, 9:36 PM

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Also you can’t change the size of a std::array while you could reallocate a C style array

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Or can you call reallocate an std::array ?(sounds highly illegal regardless)

odd bane Dec 9, 2022, 9:37 PM

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a dynamically allocated C-"array" is more akin to a std::vector anyway

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a std::array is akin to a stack-allocated C-array, ie T[N]

tranquil linden Dec 9, 2022, 10:07 PM

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std::unique_ptr<T[]>:

delicate burrowBOT Dec 10, 2022, 10:46 AM

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#Difference between STL Arrays and C arrays