为什么将空结构插入保留向量会增加内存？

Question

EDITED The code was compiled with Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86 in VS2010 as 32bit and runs on Win7 64bit machine.

Program with empty main body takes 1KB of memory.

The sizeof(Bag)=32 . (=16 bytes of internal buffer for short strings + 4 byte of pointer to memory if string bigger than 16 bytes is added + other internal stuff of string).

After reserving 200,000 elements the vector v takes 6400KB of memory. Thus, total memory until now is 7400KB.

What I don't understand is why after inserting 100,000 elements (which is smaller than reserved capacity of v ) into v the memory usage increases to 14,800KB. If I replace string with int so the total usage memory will be as it supposed to be 1,800KB (= 1000KB + 200*4B)

1.    struct Bag
2.    {
3.       string s;      
4.    };

5.    vector<Bag> v;
6.    v.reserve(200000);

7.    for(int i = 0; i < 100000; ++i)
8.    {
9.      v.push_back(Bag());
10.   }

Answer 1

A std::string contains an additional allocated memory component to contain the contents of the string; the actual string object itself is only half the story. Some implementations have an optimization to eliminate the extra overhead for very short strings, but this is not guaranteed.

The amount of memory used by each allocation will depend on the minimum allocation characteristics for your platform.

Answer 2

reserve() function allocates memory for the bundle of Bag objects. This chunk of memory contains space for all data members of std::string . std::string allocates memory for keeping string data and has pointer to it as data member. The memory chunks for std::string data is addition space you observed. In case of int data members: it has no additional buffers so there is no extra memory allocated.

The following snippet shows that string has extra bytes allocated:

#include <string>
#include <iostream>

int main() {
        std::string s;
        std::cout << "[1] Buf: " << s.capacity() << std::endl;
        s = "Now it's contain some data";
        std::cout << "[2] Buf: " << s.capacity() << std::endl;
        return 0;
}

The output is as follows:

./a.out
[1] Buf: 22
[2] Buf: 47