循环遍历 C++ 数组的正确方法

Lucas
2013-11-27  
312683  
    
11  

最近我发现了很多例子，其中大部分都是关于 C++ 98 的，反正我已经创建了我的简单数组和一个循环（ codepad ）：

#include <iostream>
using namespace std;

int main ()
{
   string texts[] = {"Apple", "Banana", "Orange"};
   for( unsigned int a = 0; a < sizeof(texts); a = a + 1 )
   {
       cout << "value of a: " << texts[a] << endl;
   }

   return 0;
}

输出：

value of a: Apple
value of a: Banana
value of a: Orange

Segmentation fault

它工作正常，除了最后的分段错误。

我的问题是，这个数组/循环是否做得很好？我正在使用 C++ 11，所以想确保它符合标准并且不能做得更好？

c++ ,  arrays ,  loops ,  c++11
stack overflow more modern way of looping through C++ arrays
原文答案
author avatar
Nicu Stiurca
2013-11-27
119
     原文

接受的答案

在C/C ++ sizeof 中。始终给出整个对象中的字节数，并且数组被视为一个对象。注意： sizeof 一个指针 - 对数组或单个对象的第一个元素 - 示出 pointer 的大小，而不是指向的对象。无论哪种方式， sizeof die not 给出数组中的元素数（其长度）。为了获得长度，您需要除以每个元素的大小。例如。，

for( unsigned int a = 0; a < sizeof(texts)/sizeof(texts[0]); a = a + 1 )

至于C ++ 11方法，最好的方法可能是

for(const string &text : texts)
    cout << "value of text: " << text << endl;

这使编译器可以弄清楚您需要多少迭代。

正如其他人指出的那样， std::array 在C ++ 11中优先于RAW数组。但是，没有其他答案解决了为什么 sizeof 失败的原因，所以我仍然认为这是更好的答案。


答案:

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Mark Garcia
2013-11-27    30      原文 
string texts[] = {"Apple", "Banana", "Orange"};
for( unsigned int a = 0; a < sizeof(texts); a = a + 1 )
{
    cout << "value of a: " << texts[a] << endl;
}

Nope. Totally a wrong way of iterating through an array. sizeof(texts) is not equal to the number of elements in the array!

The modern, C++11 ways would be to:

  • use std::array if you want an array whose size is known at compile-time; or
  • use std::vector if its size depends on runtime

Then use range-for when iterating.

#include <iostream>
#include <array>


int main() {
    std::array<std::string, 3> texts = {"Apple", "Banana", "Orange"};
    // ^ An array of 3 elements with the type std::string

    for(const auto& text : texts) {   // Range-for!
        std::cout << text << std::endl;
    }
}

Live example

You may ask, how is std::array better than the ol' C array? The answer is that it has the additional safety and features of other standard library containers, mostly closely resembling std::vector. Further, The answer is that it doesn't have the quirks of decaying to pointers and thus losing type information, which, once you lose the original array type, you can't use range-for or std::begin/end on it.

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kfsone
2013-11-27    14      原文

sizeof tells you the size of a thing, not the number of elements in it. A more C++11 way to do what you are doing would be:

#include <array>
#include <string>
#include <iostream>

int main()
{
    std::array<std::string, 3> texts { "Apple", "Banana", "Orange" };
    for (auto& text : texts) {
        std::cout << text << '\n';
    }
    return 0;
}

ideone demo: http://ideone.com/6xmSrn

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Nuno Rafael Figueiredo
2016-07-22    4      原文

Add a stopping value to the array:

#include <iostream>
using namespace std;

int main ()
{
   string texts[] = {"Apple", "Banana", "Orange", ""};
   for( unsigned int a = 0; texts[a].length(); a = a + 1 )
   {
       cout << "value of a: " << texts[a] << endl;
   }

   return 0;
}
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SPIDERMAN
2019-11-26    3      原文

you need to understand difference between std::array::size and sizeof() operator. if you want loop to array elements in conventional way then you could use std::array::size. this will return number of elements in array but if you keen to use C++11 then prefer below code

for(const string &text : texts)
    cout << "value of text: " << text << endl;
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Phidelux
2018-02-01    2      原文

If you have a very short list of elements you would like to handle, you could use the std::initializer_list introduced in C++11 together with auto:

#include <iostream>

int main(int, char*[])
{
    for(const auto& ext : { ".slice", ".socket", ".service", ".target" })
        std::cout << "Handling *" << ext << " systemd files" << std::endl;

    return 0;
}
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bvj
2013-11-27    1      原文

sizeof(texts) on my system evaluated to 96: the number of bytes required for the array and its string instances.

As mentioned elsewhere, the sizeof(texts)/sizeof(texts[0]) would give the value of 3 you were expecting.

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Harmen
2019-11-26    1      原文

How about:

#include <iostream>
#include <array>
#include <algorithm>

int main ()
{
    std::array<std::string, 3> text = {"Apple", "Banana", "Orange"};
    std::for_each(text.begin(), text.end(), [](std::string &string){ std::cout << string << "\n"; });

    return 0;
}

Compiles and works with C++ 11 and has no 'raw' looping :)

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Muhammad Ishtiaq Hussain
2020-01-08    0      原文

You can do it as follow:

#include < iostream >

using namespace std;

int main () {

   string texts[] = {"Apple", "Banana", "Orange"};

   for( unsigned int a = 0; a < sizeof(texts) / 32; a++ ) { // 32 is the size of string data type

       cout << "value of a: " << texts[a] << endl;

   }


   return 0;

}
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BIOHAZARD
2022-05-28    0      原文

感觉像是非法的，但这有效：

所以基本上它是动态多维数组迭代终止情况，它与一维解决方案有点不同，最后一个元素是-1，它是循环的停止值（我是 C++ 新手，但我喜欢这种方法）

int arr[][3] = {{164, 0, 0}, {124, 0, 0}, {92, 4, 0}, {68, 4, 0}, -1};

for(int i = 0; arr[i][0]!=-1; i++)
{
    cout << i << "n";
}

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