重新分配结构的std :: vector

Question

Let's say we have a std::vector of struct s:

struct abc {
    wstring a;     // length between 0 and 200, 100 on average
    long b;
    long c;
};

std::vector<abc> v;    // 1 - 10 millions of items will be there

What happens in case of a reallocation (for example because capacity is too small when push_back() , or if I suddenly increase the capacity myself with v.reserve(v.size() + 100000 )?

More precsily:

1. will all the data be potentially rewritten (ie all the wstrings are all rewritten, etc.)

or

2. a vector of struct kind-of internally "keeps record of pointers" to each individual struct, so if the vector is rewritten, only these pointers are rewritten, but not the actual data

?

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To make the question clearer, does it look like this

or like this:

or this one:

or another way?

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Additional informations for future reference:

struct abc { wstring a; int b; int c; }; 
wcout << sizeof(wstring);   // 32
wcout << sizeof(int);       // 4
wcout << sizeof(abc);       // 40, so this means there's probably no extra byte 
                            //     used by the "struct" envelope itself
wcout << sizeof(tuple<wstring, int, int>); // 40 too

Answer 1

std::vector stores abc objects, not pointers to them. Prior to C++11 expanding the capacity of the vector beyond what has been allocated (which may be greater than vector's size) would require copying the actual objects into the newly allocated array.

Starting with C++11 MoveInsertable concept has been introduced. Now the data is moved from the old location to the new location, which may be significantly cheaper in terms of both reallocations and CPU cycles. In particular, the wstring s inside abc s would not require copying the content of the string itself, which may be a rather big potential saving.