在String中添加移动构造函数和移动赋值运算符

13.50 没有定义析构函数

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

class String
{
public:
    String():elements(nullptr),first_free(nullptr) {}
    String(char *c);
    String(const String&);
    String& operator=(const String&);
    string* begin() const { return elements;}
    string* end() const { return first_free;}

    String(String &&);
    String& operator=(String &&);
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;
}

String::String(String &&s):elements(s.elements),first_free(s.first_free)
{
    cout<<"move construct"<<endl;
    s.elements=s.first_free=nullptr;
}

String& String::operator=(String &&s)
{
    cout<<"move = construct"<<endl;
    if(this!=&s)
    {
        if(elements)
        {
            auto begin=elements;
            auto end=first_free;
            while(begin!=end)
                alloc.destroy(begin++);
            alloc.deallocate(elements,first_free-elements);
        }
    }
    elements=s.elements;
    first_free=s.first_free;
    s.elements=s.first_free=nullptr;
    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;
}

运行结果如下：  结果中出现移动构造函数是因为调用String构造函数返回的结果是右值

定义析构函数时：

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

class String
{
public:
    String():elements(nullptr),first_free(nullptr){}
    String(char *c);
    String(const String&);
    String& operator=(const String&);
    string* begin() const { return elements;}
    string* end() const { return first_free;}
    //一定要定义析构函数，否则就算定义了移动构造函数还是不会调用，只会调用拷贝构造函数
    ~String()
    {
        if(elements)
        {
            auto begin=elements;
            auto end=first_free;
            while(begin!=end)
                alloc.destroy(begin++);
            alloc.deallocate(elements,first_free-elements);
        }
    }
    String(String &&) noexcept;
    String& operator=(String &&) noexcept;
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;
}

String::String(String &&s) noexcept :elements(s.elements),first_free(s.first_free)
{
    cout<<"move construct"<<endl;
    s.elements=s.first_free=nullptr;
}

String& String::operator=(String &&s) noexcept
{
    cout<<"move = construct"<<endl;
    if(this!=&s)
    {
        if(elements)
        {
            auto begin=elements;
            auto end=first_free;
            while(begin!=end)
                alloc.destroy(begin++);
            alloc.deallocate(elements,first_free-elements);
        }
    }
    elements=s.elements;
    first_free=s.first_free;
    s.elements=s.first_free=nullptr;
    return *this;
}

int main()
{
    vector<String> vec;
    char ch[]="hello";
    char ch1[]="world!";
    cout<<vec.capacity()<<endl;
     cout<<endl;
    String ss(ch);
    vec.push_back(ss);
    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;
    cout<<"\n";

    std::vector<String> v;
    String s;
    for (unsigned i = 0; i != 4; ++i)
    {
        std::cout << v.capacity() << "\n";
        v.push_back(s);
    }
    return 0;
}

运行结果如下：