Why do std::begin() and std::end() work with fixed arrays, but not with dynamic arrays?

Question

I can't understand why the std::begin() function doesn't work when it is given an int * arr pointer, but it works with an int arr[] array.

This code doesn't work:

int *arr = new int[5]{ 1,2,3,4,5 };

if (find(begin(arr),end(arr),5)!=end(arr))
{
    cout << "found";
}

This code does work:

int arr2[5] = { 1,2,3,4,5 };

if (find(begin(arr2),end(arr2),5)!=end(arr2))
{
    cout << "found";
}

Answer 1

It doesn't work because int *arr isn't an array, it's a pointer. It happens to point to the first element of an array, but it's pointing to the element, not the array. Once it's converted to a pointer, the length information is lost.

迂腐注：这不是完全丢失，因为它实际上是隐藏在实现特定的方式使得分配器知道多少，当你以后释放delete[]它，但是这不暴露任何东西，但分配器，和它往往不等于实际由于分配器对齐要求，请求的大小不可用，因此此处没有用。