1.5.1.9 string

• String::npos is the maximum possible length of the string. Equal the maximum value of an unsigned int.
• String is template specialization basic_string<char> , from this point of view, you will know how to construct a w_char string.
• There are a lot of ways that can be used construct a string object, you can see the reference , such as:
  string(const char* s); 
   string(const char*, size_type n); 
   string(const string& str, size_type pos, size_type n = npos) 
   ......
• The standard containers define size_type as a typedef to Allocator::size_type (Allocator is a template parameter), which for std::allocator is defined to be size_t. So for the standard case, they are the same. However, if you use a custom allocator a different underlying type could be used. So container::size_type is preferable for maximum portability.
• In C++, we encourage you to use string more, to replace char[] and char *p = new. Because it offer you more functions, such as compare, find,erase, replace,insert.
• About real size of string, string usually manage it’s memory by itself, but you can use two functions string::capacity() and string::reserve() to do some simple work. If the size of string is not enough, string will allocates a new block twice the size and copy the old content. You can use capacity to know the real block size. And use reserve to tell string at least you need minimum size of bloc. For example, str.reserve(50), str.capacity will return 63. 63 = 64-1; string need the last char to store ’\n’ and end of symbol.
• When you use string with some legacy c function, use string.c_str() function for read only function, string.c_str() return a const char *. so You can’t modify it. If you Legacy C function want to fill in a string. you need do below:
  Old_c(const char* p); string str; 
  Old_c(str.c_str());  // legacy C function read a string 
   
  // legacy C write 
  size_t Old_c(char *pArray, size_t arraySize); 
   
  vector<char> vc(maxNumChars); 
  // 1) create a vector whose size is maxNumChars 
  size_t charsWritten = Old_c(&vc[0], vc.size()); 
  // 2) have fillString write into vc 
  string s(vc.begin(), vc.begin()+charsWritten); 
  // 3) copy data from vc to s via range constructor
• string method lists:

  capacity(), reserve()	Returns the largest number of elements that could be stored in a string without increasing the memory allocation of the string.
  empty(), size()
  max_size()	maximum number of characters
  resize()	Specifies a new size for a string, appending or erasing elements as required.
  length()	Returns the current number of elements in a string.

  find(), rfind()	Searches a string in a forward/backward direction for the first occurrence of a substring that matches a specified sequence of characters.
  substr()	Copies a substring of at most some number of characters from a string beginning from a specified position.
  find_first_not_of()	Searches through a string for the first character that is not any element of a specified string.
  find_first_of()
  find_last_not_of()
  find_last_of()	.

• find function in the string return position. Vector doesn’t have find member function, you can use find function in algorithm category.
  size_t found; 
  found=str.find("haystack"); 
  // or found=str.find(’.’); 
  if (found!=std::string::npos) 
      std::cout << "’haystack’␣also␣found␣at:␣" << found << ’\n’; 
   
  vector<int> myints = { 10, 20, 30, 40 }; 
  auto  it = std::find (myints.begin(), myints.end(), 30); 
  if(it!=myints.end()) 
     cout<< "find␣30"<<endl;

  begin(),end()	Returns an iterator addressing the first element in the string.
  rbegin(), rend()

  clear()	Erases all elements of a string.
  insert(), erase()	Inserts an element or a number of elements or a range of elements into the string at a specified position.
  append(), push_back()	Adds characters to the end of a string.
  assign()	Assigns new character values to the contents of a string.
  replace()

  at()	a reference to the element at a specified location
  c_str(), data()
  compare()
  copy()	Copies at most a specified number of characters from an indexed position in a source string to a target character array.The function does not append a null character at the end of the copied content.
  get_allocator()	Returns a copy of the allocator object used to construct the string.
  swap()	Exchange the contents of two strings.

• clear will delete all the characers, and erase will remove character in certain position.
• position in find method can be used in loop
  std::string str ("Please,␣replace␣the␣vowels␣with␣asterisks."); 
    std::size_t found = str.find_first_of("aeiou"); 
    while (found!=std::string::npos) 
    { 
      str[found]=’*’; 
      found=str.find_first_of("aeiou",found+1); 
    }