请问这个问题怎么解决？

Question

#include<utility>
#include<string>
#include<memory>

using namespace std;

class StrVec
{
public:
    StrVec() :elements(nullptr), first_free(nullptr), cap(nullptr) {};
    StrVec(const StrVec&);
    StrVec& operator = (const StrVec&);
    ~StrVec();
    void push_back(const string&);
    size_t size() const { return first_free - elements; }
    size_t capacity() const { return cap - elements; }
    string* begin() const { return elements; }
    string* end() const { return first_free; }
private:
    static allocator<string> alloc;
    void chk_n_alloc() { if (size() == capacity()) reallocate(); }
    pair<string*, string*> alloc_n_copy(const string*, const string*);
    void free();
    void reallocate();
    string* elements;
    string* first_free;
    string* cap;
};

void StrVec::push_back(const string& s)
{
    chk_n_alloc();
    alloc.construct(first_free++, s);
}

pair<string*, string*> StrVec::alloc_n_copy(const string* b, const string* e)
{
    auto data = alloc.allocate(e - b);
    return { data,uninitialized_copy(b,e,data) };
}

void StrVec::free()
{
    if (elements) {
        for (auto p = first_free; p != elements;)
            alloc.destroy(--p);
        alloc.deallocate(elements, cap - elements);
    }
}

StrVec::StrVec(const StrVec& s)
{
    auto newdata = alloc_n_copy(s.begin(), s.end());
    elements = newdata.first;
    first_free = cap = newdata.second;
}

StrVec::~StrVec()
{
    free();
}

StrVec& StrVec::operator=(const StrVec& rhs)
{
    auto data = alloc_n_copy(rhs.begin(), rhs.end());
    free();        //释放左侧运算对象旧空间
    elements = data.first;
    first_free = cap = data.second;
    return *this;
}

void StrVec::reallocate()
{
    auto newcapacity = size() ? size() * 2 : 1;
    auto newdata = alloc.allocate(newcapacity);
    auto dest = newdata;
    auto elem = elements;
    for (size_t i = 0; i != size(); i++)
        alloc.construct(dest++, move(*elem++));
    free();
    elements = newdata;
    first_free = dest;
    cap = newdata + newcapacity;
}


int main()
{
    
}

用的是vs2019

Answer 1

在StrVec定义完以后，静态变量要进行申明
allocator<string> StrVec::alloc;
因为静态成员变量在全局数据区

满意还请点击采纳支持一下