在类外部定义类成员记得加类作用域！切记

#include<iostream>
#include<algorithm>
#include<memory>//the hearder file of shared_ptr
#include <vector>
#include <list>
#include <map>
#include <string>
#include <set>
#include <utility>

using namespace std;


class StrVec {
public:
    StrVec() ://allocator成员进行默认初始化
        elements(0), first_free(0), cap(0) {}
    StrVec(const StrVec&);
    StrVec& operator=(const StrVec&);
    ~StrVec();

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

    std::string *elements;
    std::string *first_free;
    std::string *cap;
};

allocator<string> StrVec::alloc;

void StrVec::push_back(const string& s) {
    chk_n_alloc();//检查是否有可用空间
    alloc.construct(first_free++, s);
}

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

void StrVec::free() {//销毁元素并且释放内存
    if (elements) {//检查所要释放的内存非空
        for (string* p = first_free;p != elements;/*空哦*/)
            alloc.destroy(--p);//逆序销毁
        alloc.deallocate(elements, cap - elements);
    }
}

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

StrVec& StrVec::operator=(const StrVec& v) {
    auto newdata = alloc_n_copy(v.begin(), v.end());

    free();
    elements = newdata.first;
    cap = first_free = newdata.second;
    return *this;
}

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

void StrVec::reallocate() {
    auto newcapacity = size() ? size() * 2 : 1;

    auto newdata = alloc.allocate(newcapacity);
    auto dest = newdata;
    auto ele = elements;//elements不可改变，下面size()和free()都将用到

    for (size_t i = 0;i != size();i++)
        alloc.construct(dest++, *ele++);
    free();

    elements = newdata;
    first_free = dest;
    cap = elements + newcapacity;
}

int main(void) {
    StrVec xi;
    for (int i = 0;i < 10;i++)xi.push_back("xixi");
    for (const auto& mem : xi)cout << mem << " ";

    cout << endl;
    return 0;
}

静态成员alloc前面没加StrVec::导致报错，无法解析的外部命令，大概长这个样子：

想起来以前也碰到过，那时候没弄清楚，只是把成员函数的实现拉进类内了，受教（抱拳