C++ for-each循环，数组分配在堆上

Question

#include <bits/stdc++.h>
using namespace std;

int main(){
    ios::sync_with_stdio(0); cin.tie(0);
    auto arr = new int[5];
    // int arr[5] = {1, 2, 3, 4, 5};
    for (auto i: arr){
        cout << i << ' ';
    }
}

为什么这不起作用？我收到编译时错误提示。

C.cpp: In function 'int main()':
C.cpp:8:15: error: 'begin' was not declared in this scope
  for (auto i: arr){
               ^
C.cpp:8:15: note: suggested alternatives:
In file included from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/x86_64-pc-cygwin/bits/stdc++.h:94:0,
                 from C.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/valarray:1206:5: note:   'std::begin'
     begin(const valarray<_Tp>& __va)
     ^
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/valarray:1206:5: note:   'std::begin'
C.cpp:8:15: error: 'end' was not declared in this scope
  for (auto i: arr){
               ^
C.cpp:8:15: note: suggested alternatives:
In file included from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/x86_64-pc-cygwin/bits/stdc++.h:94:0,
                 from C.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/valarray:1226:5: note:   'std::end'
     end(const valarray<_Tp>& __va)
     ^
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/valarray:1226:5: note:   'std::end'

当我以注释方式初始化数组时，它工作正常。（明显地）所以，我认为问题出在新运营商身上。但是我不明白它是什么。

Answer 1

new int[5] 分配一个包含 5 个元素的数组，returns 分配一个指向第一个元素的指针。返回的类型是 rvalue int*

int foo[5] 是一个包含 5 个元素的数组。 foo 的类型是 lvalue int [5]

for 循环的范围要求它知道它正在迭代的对象的大小。它可以遍历数组，但不能遍历指针。考虑这段代码，

int foo[4];
for (int a : (int *)foo) {}

GCC 5.0 也会产生错误"error: ‘begin’ was not declared in this scope"，因为数组已衰减为指针。

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