详解C++ STL中vector扩容机制

#include<iostream>
#include<vector>
using namespace std;
void fun(vector<int>&vec){
	vec.push_back(1);
    cout<<&vec[0]<<vec.size()<<" "<<vec.capacity()<<endl;
}
int main(){
    vector<int>vec;
    for(int i=0;i<10;i++) fun();
}

void push_back(const _Ty& _Val) { // insert element at end, provide strong guarantee
        emplace_back(_Val);
    }

    void push_back(_Ty&& _Val) { // insert by moving into element at end, provide strong guarantee
        emplace_back(_STD move(_Val));
    }

template <class... _Valty>
    decltype(auto) emplace_back(_Valty&&... _Val) {
        // insert by perfectly forwarding into element at end, provide strong guarantee
        auto& _My_data   = _Mypair._Myval2;
        pointer& _Mylast = _My_data._Mylast;
        if (_Mylast != _My_data._Myend) {
            return _Emplace_back_with_unused_capacity(_STD forward<_Valty>(_Val)...);
        }

        _Ty& _Result = *_Emplace_reallocate(_Mylast, _STD forward<_Valty>(_Val)...);
#if _HAS_CXX17
        return _Result;
#else // ^^^ _HAS_CXX17 ^^^ // vvv !_HAS_CXX17 vvv
        (void) _Result;
#endif // _HAS_CXX17
    }

template <class... _Valty>
    pointer _Emplace_reallocate(const pointer _Whereptr, _Valty&&... _Val) {
        // reallocate and insert by perfectly forwarding _Val at _Whereptr
        _Alty& _Al        = _Getal();
        auto& _My_data    = _Mypair._Myval2;//使用的长度
        pointer& _Myfirst = _My_data._Myfirst;//容器开始的位置
        pointer& _Mylast  = _My_data._Mylast;//容器末尾

        _STL_INTERNAL_CHECK(_Mylast == _My_data._Myend); // check that we have no unused capacity

        const auto _Whereoff = static_cast<size_type>(_Whereptr - _Myfirst);//插入元素的位置，这个函数insert也有在用，所以插入的位置不一定在尾部
        const auto _Oldsize  = static_cast<size_type>(_Mylast - _Myfirst);//容器大小

        if (_Oldsize == max_size()) {//长度超过数组的最大长度时报错
            _Xlength();
        }

        const size_type _Newsize     = _Oldsize + 1;
        const size_type _Newcapacity = _Calculate_growth(_Newsize);//调用扩容函数

        const pointer _Newvec           = _Al.allocate(_Newcapacity);//申请新的空间
        const pointer _Constructed_last = _Newvec + _Whereoff + 1;//计算新的末尾
        pointer _Constructed_first      = _Constructed_last;

        _TRY_BEGIN
        _Alty_traits::construct(_Al, _Unfancy(_Newvec + _Whereoff), _STD forward<_Valty>(_Val)...);//在新的空间添加新的元素
        _Constructed_first = _Newvec + _Whereoff;
		//下面是根据插入元素的位置判断如何拷贝
        if (_Whereptr == _Mylast) { // at back, provide strong guarantee
            _Umove_if_noexcept(_Myfirst, _Mylast, _Newvec);//移动，不能移动时拷贝
        } else { // provide basic guarantee
            _Umove(_Myfirst, _Whereptr, _Newvec);
            _Constructed_first = _Newvec;
            _Umove(_Whereptr, _Mylast, _Newvec + _Whereoff + 1);
        }
        _CATCH_ALL//拷贝出现异常的时候释放对应的内容
        _Destroy(_Constructed_first, _Constructed_last);
        _Al.deallocate(_Newvec, _Newcapacity);
        _RERAISE;
        _CATCH_END

        _Change_array(_Newvec, _Newsize, _Newcapacity);//更新数组信息
        return _Newvec + _Whereoff;//偏移到新元素的地址
    }

size_type _Calculate_growth(const size_type _Newsize) const {
        // given _Oldcapacity and _Newsize, calculate geometric growth
        const size_type _Oldcapacity = capacity();
        const auto _Max              = max_size();
        if (_Oldcapacity > _Max - _Oldcapacity / 2) {
            return _Max; // geometric growth would overflow
        }
        const size_type _Geometric = _Oldcapacity + _Oldcapacity / 2;
        if (_Geometric < _Newsize) {
            return _Newsize; // geometric growth would be insufficient
        }
        return _Geometric; // geometric growth is sufficient
    }

#include<iostream>
#include<vector>
using namespace std;

class A {
public:
    A(){}
    int* a = new int(1);
    ~A() { cout << this << "析构"<<endl; }
};

void fun(vector<A>& vec) {
    vec.push_back(A());
    cout << &vec[0] << " " << vec.size() << " " << vec.capacity() << endl;
}
int main() {
    vector<A>vec;
    for (int i = 0; i < 10; i++) fun(vec);
    vec.resize(5);
    cout << vec.capacity()<<endl;
}