Records or Structs

• A record is a collection of data.
  • It is composed of a number of individual data types.
  • These are called fields.
  • Each field has
    • a data type.
    • a member name.
  • It can be heterogenious or the types can be different.
  • As opposed to homogenious or all the types are the same.
• In c/c++ records are implemented as structures or a struct.
  • These are originally from c.
  • They are very similar to a c++ structure the class
  • We will favor classes in the future.
  • We tend to use structs to group data that only has a weak relationship after we have classes.
  • But for now, this is the way to group data.
• Syntax

  • struct identifer{
        datatype identifier;
        datatype identifier;
        ...
    }; 

  • The ; at the end is important.
• An example

  • enum StatusT {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, GRADUATE};
    
    struct AddressT {
       string street1, 
              street2,
              city, 
              state;
       int zip;
    };
    
    struct StudentT {
        string name;
        int age;
        float gradePointAverage;
        StatusT class;
        AddressT address;
    }; 

• Using a struct requires two things
  • declare the struct
  • declare instances of that struct.
• Declaring the struct
  • This usually occurs at the beginning of the code
  • After using namespace ...
  • Before any function definitions.
  • Note structs need to be declared after any other data types from which they are composed.
  • This is a template or plan from which the compiler will set asside memory when an instance of the struct is declared.
  • NO MEMORY IS ALLOCATED
  • From this point of view all structs of the same type look the same.
• Declaring an instance of a struct.
  • Just like using any data type.
  • Variables, parameters, return types.

  • StudentT GetStudent();
    
    void PrintStudent(StudentT person);
    
    int main() {
        StudentT person1, person2;
        ...
    } 

  • In these cases, memory is allocated.
• Structures in memory
  • How do you picture variables in memory?
    • A box with a label?
    • int x
    • 
  • The same is true for an instance of a struct
    • AddressT address
    • 
    • StudentT person1
    • 
    • The only difference is tha the struct has multipl boxes for the fields.
• Access to individual fields.
  • the period (.) is the member selector operator
  • using the . we can access individual fields or members.

  • StudentT person
    
    person.age = 19;
    person.class = SOPHOMORE;
    person.name = "Abe Student";
    person.gradePointAverage = 4.0;
    ...
    
    cout << person.name;
    person.gradePointAverage = points/credits;
    person.age++; 

  • For the field that is another struct, just keep using member selector operators.

     
    person.address.state = "Pennsylvania";
    cout << person.address.zip;
             

• Operations on records
  • You can
    • Copy records
    • Pass as parameters, either reference or value
    • Return from a function.
  • You can't
    • Perform direct I/O
    • Compare
    • Do Math
• Look at timeDemo.cpp