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These days I am reading the C++ primer. The book says that, to get the length of a C-Style array, you can use the function std::begin and std::end to get two pointers and subtract them . However, I knew that I can also use sizeof(a)/sizeof(a[0]) if a is a C-Style array. So which way is better, or they just have little differences?

nada
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OkitaSan498
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  • This has been discussed in detail in [this SO question](https://stackoverflow.com/questions/4108313/how-do-i-find-the-length-of-an-array) – nada Oct 14 '19 at 06:50
  • The best way to get the size is to not throw it away in the first place. `const count = 5; int array[count];`. – Pete Becker Oct 14 '19 at 12:52

2 Answers2

5

The best way to go nowadays (C++17 and above) is std::size:

#include <iterator>
...
int arr[5];
static_assert(std::size(arr) == 5);

If you don't have access to C++17, it's pretty easy to roll your own too:

template <class T, size_t N>
constexpr size_t size(const T (&arr)[N]) { return N; }

Even better, just use std::array, and use it's size member:

std::array<int, 5> arr;
static_assert(arr.size() == 5);
Fatih BAKIR
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2

The method with std::begin and std::end is a better one because it is safer. So is std::size.

If you by mistake use the sizeof(a)/sizeof(a[0]) method on a pointer instead of an array, the compiler will likely tell you nothing, and you will get a meaningless result. The other methods will result in a compilation error, and you will know you are trying to do something wrong.

One drawback of the std::begin and std::end method is that the result is not a constant expression. The result of std::size is, so this should be a preferred method of getting the size of a C-style array.

n. m. could be an AI
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