这是由内存碎片造成的吗？如果是的话，为什么会这样呢？

Question

环境：

内存限制：512 MB

运行以下代码，将顺利完成：

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>

using namespace std;

int main() {
    long long N;

    vector<long long> set3;
    for(long long i = 1; i < 20000000; ++i) {
        set3.push_back(i);
    }
    cout << "count:" << set3.size() << " capacity:" << set3.capacity() << endl;

    vector<long long> set5;
    for(long long i = 1; i < 12000000; ++i) {
        set5.push_back(i);
    }
    cout << "count:" << set5.size() << " capacity:" << set5.capacity() << endl;

    return 0;
}

如果我交换set3和set5，它将因异常而失败：

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc

以下是失败的代码：

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>

using namespace std;

int main() {
    long long N;

    vector<long long> set5;
    for(long long i = 1; i < 12000000; ++i) {
        set5.push_back(i);
    }
    cout << "count:" << set5.size() << " capacity:" << set5.capacity() << endl;

    vector<long long> set3;
    for(long long i = 1; i < 20000000; ++i) {
        set3.push_back(i);
    }
    cout << "count:" << set3.size() << " capacity:" << set3.capacity() << endl;

    return 0;
}

为什么吗

这是另一个成功案例，我们甚至可以通过vector :: assign

成功分配370 MB

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>

using namespace std;

int main() {
    long long N;

    vector<long long> set5;
    for(long long i = 1; i < 12000000; ++i) {
        set5.push_back(i);
    }
    cout << "count:" << set5.size() << " capacity:" << set5.capacity() << endl;


    vector<long long> set3;
    //set3.assign(49807360, 0); // fail: 49807360 * 64 / 8 / 1024 / 1024 == 380 MB
    set3.assign(48496640, 0); // succeed: 48496640 * 64 / 8 / 1024 / 1024 == 370 MB
    /*
     for(long long i = 1; i < 20000000; ++i) {
     set3.push_back(i);
     }
     */
    cout << "count:" << set3.size() << " capacity:" << set3.capacity() << endl;

    return 0;
}

为什么矢量的push_back（导致内存从128MB增加到256MB）的内存分配在384 MB上失败;而在相同的384 MB上vector.assign（ 370MB ，远大于256 MB）会成功吗？

这是内存碎片问题吗？

在成功案例中：

Allocate for set3: (by vector::push_back) 
    256 MB out of 512 MB ---- Good; 
then allocate for set5: (by vector::push_back) 
    128 MB out of the remaining 256 MB ---- Good;

在失败的情况下：

Allocate for set5: (by vector::push_back) 
    128 MB out of 512 MB ---- Good; 
then allocate for set3: (by vector::push_back) 
    256 MB out of the remaining 384 MB ---- Fail.

在另一个成功案例中：

Allocate for set5: (by vector::push_back) 
    128 MB out of 512 MB ---- Good; 
then allocate for set3: (by vector::assign) 
    370 MB out of the remaining 384 MB ---- Good.