Array, String & Vector

Array

• Am indexed block of contiguous memory, inherited from C.
• It can be allocated on the program’s stack, but only if the number of elements is fixed and known at compile time.

  int arr[5];         // allocated on the stack
  int arr[nElements]; // not allowed in C++, unless nElements is a const

Dynamic Array

• Array allocated on the heap, this is done if:
  • We do not know the number of elements at compile time.
  • We want to be able to vary the number of elements.
  • The array’s memory needs to be explicitly released when no longer needed.

  int *pArr = new int[nElements];
  delete [] pArr;

C-Style String

• An array of const char, each character in the string is an element in the char array.
• Null-character terminated.
• String literals are C-style strings.

  const char *str = "Hello"; //equivalent to const char str = {'H','e','l','l','o','\0'};

std::string

• It’s a class.
• It has a member function which is a dynamic array.
• It also has a member which stores the number of elements in the array.

  class std::string {
    char *data; // block of contiguous memory
    size_t n;   // number of elements in the array
  }

• std::string objects behave like a dynamic array, but is used like a local variable.

  std::string hello{"Hello"}; // allocates storage on the heap in the constructor.

• Released in the destructor at the end of the scope.
• Correctly handles copying and assigning objects by allocating a memory block.
• Automatically reallocates the memory block when need.

std::string Interface

• Subscript notation [] is supported.
• Elements are indexed, starting from 0
• size() member functions return the number of elements.

std::vector

• Similar to std::string, but can store data of any single type.
• The type of data is a parameter of the class.

  std::vector<int> vec {3,2,1);

• The std::vector class has a member which is a dynamic array.

  class std::vector<int> {
    int *data;  // block of contiguous memory
    size_t n;   // Number of elements in the array
  }

std::vector Interface

• Subscript notation [] is supported.
• Elements are indexed, starting from 0
• size() member functions return the number of elements.
• push_back() adds a new element at the back of the vector.

  vector <int> vec{4,1};
  vec.push_back(0);