2.string Class common interface

2.1.  string Common constructions of class objects  

give an example ：

vvoid test_string1()
{
	string s1(10,'x');
	string s2("hello world");
	string s3(s2, 6, 3);// from s2 pass the civil examinations 6 Copy data from locations , If no length is specified , Then copy all the following data 
	string s4(s2.begin() + 1, s2.end());
	cout << s1 << endl;
	cout << s2 << endl;
	cout << s3 << endl;
	cout << s4 << endl;
}
int main()
{
	test_string1();
	return 0;
}

 result ：
xxxxxxxxxx
hello world
wor
ello world

2. 1string Class object capacity operation

  Add ：resize,reserve Will not shrink , but resize Can reduce data , change size

2.3.  string Class object modification operation

    void test_string3()
{
	string s1("http://www.com.cplusplus.com/reference/string/string/find/");
	cout << s1.c_str() << endl;// obtain s1 String in 
	string s2 = s1.substr(5,5);// from s1 in pos Position copy Character , If no length is specified , Copy everything behind ;
	cout << s2 << endl;

	size_t pos1 = s1.rfind('f');// Find characters from back to front , Returns the subscript ,
	size_t pos2 = s1.find("com", 12);// Subscript from front 12 Find string , Returns the subscript 
	cout << pos1 << " " << pos2 << endl;

}

 result ：
http://www.com.cplusplus.com/reference/string/string/find/
//www
53 25

    string s;
	getline(cin, s);// You can enter a string of characters , With spaces 
	cout << s << endl;

3. summary ：

string There are many functions in the class , And there are overloads , You can view the document ：string - C++ Reference To learn

4.string The realization of the class

namespace LF
{
	class string
	{
	public:
		

	private:
		char* _str;// Store string 
		size_t _size;     //  Number of valid characters 
		size_t _capacity; //  The actual space to store valid characters 
	};
}

4.1 Deep and shallow copy problem

  First mask the copy constructor

namespace LF
{
	class string
	{
	public:
        typedef char* iterator;
		typedef const char* const_iterator;

		const_iterator begin() const
		{
			return _str;
		}

		const_iterator end() const
		{
			return _str + _size;
		}

		iterator begin()
		{
			return _str;
		}

		iterator end()
		{
			return _str + _size;
		}

		string(const char* str = "")// All default 
			: _size(strlen(str))
			, _capacity(_size)
		{
			_str = new char[_capacity + 1];// to \0 Reserve space 
			strcpy(_str, str);
		}

		/*string(const string& s)// Mask this constructor 
				:_str(new char[strlen(s._str)+1])
		{
			strcpy(_str, s._str);
		}*/

		~string()// Destructor 
		{
			if (_str)
			{
				delete[] _str;
			}
		}
		const char* c_str() const// Returns the string ,const, Do not modify 
		{
			return _str;
		}

		char& operator[](size_t pos)// Return value with &, The original data can be modified 
		{
			assert(pos < strlen(_str));

			return _str[pos];
		}
        const char& operator[](size_t pos) const// Do not modify the 
		{
			assert(pos < _size);

			return _str[pos];
		}

		
       size_t size() const
		{
			return _size;
		}

		size_t capacity() const
		{
			return _capacity;
		}

   
	private:
        char* _str;// Store string 
		size_t _size;     //  Number of valid characters 
		size_t _capacity; //  The actual space to store valid characters 
	};
}
void test_string1()
{
	LF::string s1("hello");
	LF::string s2(s1);

	cout << s1.c_str() << endl;
	cout << s2.c_str() << endl;

	s1[2] = 'P';
	cout << s1.c_str() << endl;
	cout << s2.c_str() << endl;

}
int main()
{
	test_string1();

	return 0;
}

result ：

  What is called here is the copy constructor generated by default , Finished is a shallow copy ,s1 The modification will affect s2, And will destruct the same space twice , Report errors .

4.2.string Class common function implementation

1. Capacity expansion

       void reserve(size_t n)
		{
			if (n > _capacity)
			{
				char* tmp = new char[n + 1];
				strcpy(tmp, _str);
				delete[] _str;
				_str = tmp;

				_capacity = n;
			}
		}

2. Expand and initialize

​
       void resize(size_t n, char ch = '\0')
		{
			if (n < _size)
			{
				_size = n;
				_str[_size] = '\0';
			}
			else
			{
				if (n > _capacity)
				{
					reserve(n);
				}

				for (size_t i = _size; i < n; ++i)
				{
					_str[i] = ch;
				}
				_size = n;
				_str[_size] = '\0';
			}	
		}

​

 3. Insert , Delete , data

      string& insert(size_t pos, char ch)
		{
			assert(pos <= _size);
			if (_size == _capacity)
			{
				reserve(_capacity == 0 ? 4 : _capacity * 2);
			}

			size_t end = _size+1;
			while (end > pos)
			{
				_str[end] = _str[end-1];
				--end;
			}

			_str[pos] = ch;
			_size++;

			return *this;
		}


		string& insert(size_t pos, const char* str)
		{
			assert(pos <= _size);
			size_t len = strlen(str);

			if (_size + len > _capacity)
			{
				reserve(_size + len);
			}

			//  Move back len A place 
			size_t end = _size + len;
			while (end > pos+len-1)
			{
				_str[end] = _str[end -len];
				--end;
			}

			strncpy(_str + pos, str, len);
			_size += len;

			return *this;
		}


      string& earse(size_t pos, size_t len = npos)//size_t nops=-1;
		{
			assert(pos < _size);

			if (len == npos || pos + len >= _size)
			{
				_str[pos] = '\0';
				_size = pos;
			}
			else
			{
				size_t begin = pos + len;
				while (begin <= _size)
				{
					_str[begin - len] = _str[begin];
					++begin;
				}

				_size -= len;
			}

			return *this;
		}

       // Reuse 
       string& operator+=(const char* str)
		{
			append(str);
			return *this;
		}
		
		string& operator+=(char ch)
		{
			push_back(ch);
			return *this;
		}

       void push_back(char ch)
		{
			//if (_size == _capacity)
			//{
			//	reserve(_capacity == 0 ? 4 : _capacity*2);
			//	//reserve(_capacity*2);
			//}

			//_str[_size] = ch;
			//++_size;
			//_str[_size] = '\0';

			insert(_size, ch);
		}

		void append(const char* str)
		{
			/*size_t len = _size + strlen(str);
			if (len > _capacity)
			{
				reserve(len);
			}
			strcpy(_str + _size, str);
			_size = len;*/

			insert(_size, str);//
		}

4. lookup

     size_t find(char ch, size_t pos = 0)
		{
			for (; pos < _size; ++pos)
			{
				if (_str[pos] == ch)
				{
					return pos;
				}
			}

			return npos;
		}

		size_t find(const char* str, size_t pos = 0)
		{
			const char* p = strstr(_str + pos, str);
			if (p == nullptr)
			{
				return npos;
			}
			else
			{
				return p - _str;
			}
		}

5. Operator overload

  ostream& operator<<(ostream& out, const string& s)
	{
		for (auto ch : s)
		{
			out << ch;
		}

		return out;
	}

  istream& operator>>(istream& in, string& s)
   {
	   char ch;
	   in >> ch;
	   ch = in.get();
	while (ch != ' ' && ch != '\n')
	{
		s += ch;
		in >> ch;
		ch = in.get();
	}

       return in;


       // Or reduce the cost of capacity expansion 
       char ch;
		ch = in.get();
		char buff[128] = {'\0'};
		size_t i = 0;
		while (ch != ' ' && ch != '\n')
		{
			buff[i++] = ch;
			if (i == 127)
			{
				s += buff;
				memset(buff, '\0', 128);
				i = 0;
			}

			ch = in.get();
		}

		s += buff;
		return in;
   }



  bool operator<(const string& s1, const string& s2)
	{
		return strcmp(s1.c_str(), s2.c_str()) < 0;
	}

	bool operator==(const string& s1, const string& s2)
	{
		return strcmp(s1.c_str(), s2.c_str()) == 0;
	}

	bool operator<=(const string& s1, const string& s2)
	{
		return s1 < s2 || s1 == s2;
	}

	bool operator>(const string& s1, const string& s2)
	{
		return !(s1 <= s2);
	}

	bool operator>=(const string& s1, const string& s2)
	{
		return !(s1 < s2);
	}

	bool operator!=(const string& s1, const string& s2)
	{
		return !(s1 == s2);
	}

6. structure , copy constructor

Another way to do it

       void swap(string& s)
		{
			std::swap(_str, s._str);
			std::swap(_size, s._size);
			std::swap(_capacity, s._capacity);
		}

		// s2(s1)
		string(const string& s)
			:_str(nullptr)
			, _size(0)
			, _capacity(0)
		{
			string tmp(s._str);
			swap(tmp);
		}

        string& operator=(string s)// Value passed 
		{
			swap(s);// Shape parameter s Change will not change s
			return *this;
		}