制作可变大小的结构体的方法 [英] Method for making a variable size struct

问题描述

我需要制作一个包含标题，预告片和可变长度有效字段的数据包。到目前为止，我一直使用一个向量的有效负载，所以我的结构是这样设置：

  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错误和一个整数structs的sizeof给我32（在我向向量中添加了大约100个元素之后）。

这是一个好方法吗？ b $ b

我发现这篇文章
http://stackoverflow.com/questions/ 688471 / variable-sized-struct-c
他使用的是char数组，我使用的是一个向量。

解决方案

另一个SO答案中的解决方案是c特定的，并依赖于c数组的特性 - 甚至在c中，sizeof（）将无法帮助你找到真

你所做的是很好的，为了避免seg错误，像C ++中的任何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]<< endl; // Prints 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 http://stackoverflow.com/questions/688471/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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