13.44 编写标准库string类的简化版本，命名String。你的类应该至少有一个默认构造函数和一个接受C风格字符串指针参数的构造函数。使用allocator为你的String类分配所需内存。
13.47 添加拷贝构造函数和拷贝赋值运算符，并添加打印语句，则每次函数执行时打印一条信息。
13.48 定义一个vector<String>并在其上多次调用push_back。运行程序，观察String被拷贝了多少次。
#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 ;
}
运行结果：
