C++ for-each循环,在堆上分配数组

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？我收到编译时错误说这个.

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.(显然)所以,我认为问题在于新的运营商.但我不明白它是什么.

Answer 1

new int[5]分配一个包含5个元素的数组,并返回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]

循环范围要求它知道它迭代的对象的大小.它可以迭代数组,但不能迭代指针.考虑一下这段代码

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

GCC 5.0也会产生错误"错误:'begin'未在此范围内声明"因为数组已衰减为指针.