More Basic C++

• For this set of notes we will look at demo1.cpp.
  • This is the code we used in class several days ago.
• Note that this code is wrapped inside the basic shell we discussed last time.
• It has a number of additional elements.
• It uses strings
  • String is a datatype
  • It is not part of the C++ language, it is a part of the c++ standard library.
  • A string is a collection of characters.
• #include <string>
  • To use a string, we should include the string library.
  • You don't have to do this with g++, but it is a good idea.
• const string SALUTE{"Hello "};
  • This declares a global constant SALUTE
    • SALUTE is an identifier
    • The type is string
    • The reserved word const marks it as constant.
    • {"Hello "} initializes the constant to the string Hello
    • In this case, "Hello " is a literal string constant.
    • The " are not part of the value.
    • They tell the compiler that the data is a string, not an identifier.
• declaration
  • In C++ everything must be declared before it can be used.
  • In the case declaring a variable or constant
    • Reserves a space in memory
    • The size of the given type
    • Associated with the identifier
    • and possibly initializes it.
  • Syntax for declaring a constant
    • const type identifier{expression};
    • const type identifier = expression;
• Declaring variables

  • int main() {
        string name,
               greeting;

  • In this case we are declaring two variables, name and greeting
  • They are of type string.
  • They are not initialized by the user.
    • In general it is a good idea to initialize all variables.
    • But variables of type string are automatically initialized to the empty string.
• Initialization
  • it is important to initialize variables when they are declared.
  • We don't know what is in memory.
  • And initialization makes sure that the contents are clean/known.
• Initialization syntax
  • There are two, and old way and a new way.
  • You should use the new way, but you need to know both.
  • I will try to use the new way, you can yell at me
    • When I don't initialize variables as above.
    • When I use the old way.
  • The old way
    • syntax type identifier = expression [,identifier = expression ...];
    • examples:

      string name = "", greeting = "";
      
      string word1 = "Hello",
             word2 = "World";
      
      string firstName = "Dan";
      string lastName = "Bennett"; 

    • I prefer the middle one, but the last one is ok.
    • I dislike the first one.
    • In general, one idea per line. This breaks the one statement per line and one line per statement rule.
  • The new way.
    • syntax: type identifier {expression}[, identifier{expression}, ...];
    • examples:

      string name{""}, greeting{""};
      
      string word1{"Hello"},
             word2{"World"};
      
      string firstName{"Dan"};
      string lastName{"Bennett"}; 

• expressions and operators
  • greeting = SALUTE + name + "!";
  • = is an operator.
    • a list of operators.
    • The operator copies the value of the expression on the right hand side (RHS) to the variable on the left hand side (LHS)
  • + is also an operator
    • For strings, + means concatenation.
    • String concatenation takes two strings and combines them into one by appending the second to the first.
    • You can have as many + operators in a row as you wish.
  • SALUTE + name + "!" is an expression
    • This will retrieve the value from SALUTE
    • Then concatenate the value from name
    • Then concatenate the value from the literal "!"
    • The result will be returned.
    • Then the = operator will store this value in the memory associated with greeting