制作变量大小的方法 [英] Method for making a variable size struct

问题描述

我需要制作一个包头，拖尾和可变长度的有效负载字段的数据包。到目前为止，我一直在使用一个矢量的有效载荷，所以我的结构设置如下：

  struct a_struct {
 hdr a_hdr; 
 vector< unsigned int> a_vector; 
 tr a_tr; 
}; 
  

当我尝试访问向量的成员时，我得到一个seg错误和一个sizeof的整个结构给我32（在我向矢量中添加了大约100个元素之后）

这是一个很好的方法吗？更好吗？

我发现这个帖子
变量大小的Struct C ++
他正在使用一个char数组，而我正在使用一个向量。

解决方案

其他SO答案中的解决方案是c特定的，依赖于c数组的特殊性，即使在c中，sizeof（）也不会帮助你找到一个变量大小的true大小的结构体，这本质上就是作弊，这是一种欺骗这在C ++中是不必要的。

你在做什么是好的，为了避免seg错误，请像C ++中的其他向量一样访问向量：

  a_struct a; 
 for（int i = 0; i <100; ++ i）a.a_ vector.push_back（i）; 
 cout<<< a.a_vector [22]<<终点//打印22 
  

I need to craft a packet that has a header, a trailer, and a variable length payload field. So far I have been using a vector for the payload so my struct is set up like this:

struct a_struct{
 hdr a_hdr;
 vector<unsigned int> a_vector;
 tr a_tr;
};

When I try to access members of the vector I get a seg fault and a sizeof of an entire structs give me 32 (after I've added about 100 elements to the vector.

Is this a good approach? What is better?

I found this post Variable Sized Struct C++ He was using a char array, and I'm using a vector though.

解决方案

The solution in the other SO answer is c-specific, and relies on the peculiarities of c arrays - and even in c, sizeof() won't help you find the "true" size of a variable size struct. Essentially, it's cheating, and it's a kind of cheating that isn't necessary in C++.

What you are doing is fine. To avoid seg faults, access the vector as you would any other vector in C++:

a_struct a;
for(int i = 0; i < 100; ++i) a.a_vector.push_back(i);
cout << a.a_vector[22] << endl; // Prints 22

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