Using placement new, malloc, and free

问题

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!

回答1:

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.

回答2:

The only thing you're missing is the call to all the strings' destructors. It's called ~basic_string (string is actually just a typedef for basic_string with some template parameters).

No, you don't need to call the vectors' destructors, because they're called by the compiler-provided destructor for your Word struct.

来源：https://stackoverflow.com/questions/7960556/using-placement-new-malloc-and-free