Is there any way to convert a array pointer back into a regular array? [duplicate]

Question

I am trying to pass an array through a function but when I try to get the length of the array it gives me the length of the pointer. Is there any way to convert the array pointer back into a regular array?

float arr[] = {10, 9, 8]

void func(float arr[])
{
   // now I want to figure out the size of the array 
   int lenArr = sizeof(arr) / sizeof(arr[0]); // this will get the size of the pointer to the array and not the actual array's size
}

With C-style arrays, always pass the size along with the array, which will decay to a pointer. Or scrap that type and use std::array. — sweenish
– sweenish, Commented Jan 4, 2022 at 21:24
@Immanuel Charles Declare the parameter as having a referenced type. That is pass the array by reference. — Vlad from Moscow
– Vlad from Moscow, Commented Jan 4, 2022 at 21:26
If you must pass in a C-style array (which will be a pointer to the first element of the array), also pass in the std::size_t size of the number of elements in the array. — Eljay
– Eljay, Commented Jan 4, 2022 at 21:28
Outside of this function. How is your array declared and how would you determine its length? — John
– John, Commented Jan 4, 2022 at 21:32
The size calculation only works before passing the array into a function as parameter (that is why some commenters suggested an additional size parameter). Using C arrays is in general not recommended for C++. Especially because of those limitations, which could lead to errors or security issues, if the function accesses the array behind the end. Try std::vector<float> — Sebastian
– Sebastian, Commented Jan 4, 2022 at 22:01

Loki Astari · Accepted Answer · 2022-01-04 21:49:00Z

4

You can declare the parameter a reference to an array.

void func(float (&arr)[10])
{
    // but you have to know the size of the array.
}

To get around having to know the size, you can template on size

template<int Size>
void func(float (&arr)[Size])
{
    // Now the size of the array is in "Size"
    // So you don't need to calcualte it.
}

answered Jan 4, 2022 at 21:49

Loki Astari

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2 Comments

Sebastian Over a year ago

One should mention that the size has to be determined/-able at compile time even with the template solution

Loki Astari Over a year ago

@Sebastian True. But also true for the C style array being passed in the OP question.

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