使用placement new，malloc和free [英] Using placement new, malloc, and free

问题描述

基本上，我有一个内存块分配使用malloc，我想开始放置对象使用placement new。我知道这些对象的析构函数必须在我删除它们时被显式调用，但我想确保我完全理解这一点，并且我正在开发它。我几乎肯定会废除这种方法，并以更直接的方式去处理事情，但我想我只是为了理解的缘故。

我有包含以下私有成员变量的类WordIndex：

  struct Word 
 {
 std :: string字; 
 std :: vector< unsigned int>指数; 
}; 
 
 Word * m_words; 
 std :: string * m_lines; 
 

使用malloc / realloc为两个指针分配/重新分配内存，并使用以下内容创建对象代码（它们不会覆盖任何预先存在的对象）：

  new（& m_words [pos]）Word 
 new（& m_lines [m_numoflines]）std :: string（）; 
  

我有一个析构函数，如下，但我几乎100％肯定， ，但运行时没有显示任何错误。

  WordIndex ::〜WordIndex（）
 {
 unsigned int i; 
 for（i = 0; i  {
 m_words [i]。〜Word（）; 
} 
 
 free（m_words）; 
免费（m_lines）; 
} 
  

我的问题如下：

1. 考虑到Word结构有一个向量，是否足够简单地调用〜Word，或者我需要单独为向量单独调用析构函数？


2. 我需要为字符串调用析构函数（对于m_lines和Word.word），如果是这样，我该如何做？它似乎没有像〜string这样的函数，我可以调用。


3. 我错过了什么吗？

提前感谢！

解决方案

1. 考虑Word结构有一个向量，是否足够简单地调用〜Word，或者我需要调用析构函数为向量
   单独预先？

你可以为你的<

如果你想要的话，
1. 我需要为字符串调用析构函数（对于m_lines和Word.word），如果是这样，我该如何做？
   似乎没有像我可以调用的〜string这样的函数。

是的，你做。析构函数应该在那里，它 typedef ed从 basic_string ，所以你应该调用〜 basic_string（）。

string的析构函数。

此外，请确保您正确计算 malloc 的空间，不显示该代码，它应该是什么。

Basically, I have a block of memory allocated using malloc that I want to start placing objects into using placement new. I know that the destructors for these objects will have to be explicitly called when I'm deleting them, but I want to make sure that I understand this completely, and that I'm going about it the right way. I'm almost certainly going to scrap this approach and go about things in a more straightforward manner, but I figured that I'd ask just for understanding's sake.

I have a class WordIndex that contains the following private member variables:

struct Word
{
    std::string word;
    std::vector<unsigned int> index;
};

Word* m_words;
std::string* m_lines;

Memory is allocated/reallocated for both pointers using malloc/realloc, and objects are being created using the following code (they're not overwriting any preexisting objects):

new (&m_words[pos]) Word();
new (&m_lines[m_numoflines]) std::string();

I have a destructor as follows, but I'm almost 100% sure that it's not doing things correctly, despite not showing any errors when running.

WordIndex::~WordIndex()
{
    unsigned int i;
    for( i = 0; i < m_numofwords; i++)
    {
        m_words[i].~Word();
    }

    free(m_words);
    free(m_lines);
}

My questions are as follows:

1. Considering the Word struct has a vector, is it enough to simply call ~Word, or do I need to call the destructor for the vector separately beforehand?
2. Do I need to call a destructor for a string (both for m_lines and for Word.word), and if so, how do I do that? It doesn't appear to have a function like ~string that I can call.
3. Have I missed anything else? I'm fairly certain that I have.

Thanks in advance!

解决方案

1. Considering the Word struct has a vector, is it enough to simply call ~Word, or do I need to call the destructor for the vector separately beforehand?

You can write a destructor for your struct if you want, but in this case it's not necessary.

1. Do I need to call a destructor for a string (both for m_lines and for Word.word), and if so, how do I do that? It doesn't appear to have a function like ~string that I can call.

Yes, you do. The destructor should be there, it's typedefed from basic_string, so you should be calling ~basic_string() on it.

1. Have I missed anything else? I'm fairly certain that I have.

The destructor for string.

Also, make sure you calculate the space for malloc correctly, you don't show that code, but verify its what its supposed to be.

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