自定义String类，并且实现在STL容器中添加自定义的类型

#include<iostream>
#include<string>
#include<memory>
#include<utility>
#include<cstring>
#include<vector>
using namespace std;

class String
{
public:
    String()=default;
    String(char *c);
    String(const String&);
    String& operator=(const String&);
    string* begin() const { return elements;}
    string* end() const { return first_free;}
private:
    static allocator<string> alloc;
    string *elements;
    string *first_free;
};

allocator<string> String::alloc;
String::String(char *c)
{
    size_t capacity=strlen(c);
    auto data=alloc.allocate(capacity);
    auto dest=data;
    string s;
    s.copy(c,strlen(c));
    alloc.construct(dest++,s);
    elements=data;
    first_free=dest;
}

String::String(const String &s)
{
    cout<<"copy construct"<<endl;
    auto capacity=s.end()-s.begin();
    auto data=alloc.allocate(capacity);
    uninitialized_copy(s.begin(),s.end(),data);
    elements=data;
    first_free=data+capacity;
}

String& String::operator=(const String &s)
{
    cout<<"copy = construct"<<endl;
    auto capacity=s.end()-s.begin();
    auto data=alloc.allocate(capacity);
    uninitialized_copy(s.begin(),s.end(),data);
    if(elements)
    {
        auto begin=elements;
        auto end=first_free;
        while(begin!=end)
            alloc.destroy(begin++);
        alloc.deallocate(elements,first_free-elements);
    }
    elements=data;
    first_free=data+capacity;
    return *this;
}
int main()
{
    vector<String> vec;
    char ch[]="hello";
    char ch1[]="world!";
    cout<<vec.capacity()<<endl;
     cout<<endl;
    vec.push_back(String(ch));
    cout<<vec.capacity()<<endl;
    cout<<endl;
    vec.push_back(String(ch1));
    cout<<vec.capacity()<<endl;
    cout<<endl;
    vec.push_back(String(ch));
    cout<<vec.capacity()<<endl;
    cout<<endl;
    vec.push_back(String(ch1));
    cout<<vec.capacity()<<endl;
    cout<<endl;
    vec.push_back(String(ch1));
    cout<<vec.capacity()<<endl;
    cout<<endl;
    vec.push_back(String(ch1));
    cout<<vec.capacity()<<endl;
    return 0;
}