// file: student.h
#ifndef STUDENT_H
#define STUDENT_H
struct student {
int assns, mt, final;
float grade(); // forward declaration
};
// file: student.cc
#include "student.h"
float Student::grade() { // veru similar to std::
return this->assns*0.5+this->mt*0.2+this->final*0.4;
}
Student billy {70, 50, 75};
Student billy = {70, 50, 75};
billy.grade();
Class: A struct that contains functions
every C++ struct can contain functions
An instance/value of a class is called an object
I.e. Billy is an object
Member: Function within a class
eg. grade(); //invalid
Must use object to call a method
methods are called on objects
Within method call, method has access fields of the object
Methods
have hidden parameter
"this"
"this" points to object used to call method eg.
this == &billy => (*this == billy)
this->assns == (*this).assns
Style Initialization:
student billy {70,50,75};
requires "parameters" to be compile time constant
C++ allows special methods called constructors to construct objects
Example:
// file: student.h
struct Student {
int assns, my, final;
...
Student(int assns, int mt, int final); // constructor - same name as class
};
// file: student.cc
Student::student(int assns, int mt, int final);
this->assns=assns;
this->mt=mt;
this->final=final;
Compare:
// Initializing using constructor
// constructor has 3 ints in parameters, so this works
Student billy {70,50,75}; // goes to c-style intialization if no constructor
// Initializing without constructor
// if constructor does not take any arguments, then you would do this
Student billy = {70,70,75};
// Putting "billy" on the heap, not the stack
Student *pbilly = new Student {70, 50, 75};
delete pbilly;
Advantages of Constructors:
args do not need to be constants
method overloading (different parameters)
ctor body can execute any code
ex. sanity checks
default args
Example:
// file: student.h
struct Student {
Student(int assns=0, int mt=0, int final=0); // default value in header file only
}
// file: student.cc
Student::Student(int assns, int mt, int final) {
this->assns = assns < 0? 0:assns;
this->mt = mt < 0? 0:mt;
this->final = final < 0? 0:final;
}
Student s2 {80, 60}; // final=0
Student s3 {75}; // mt=final=0
Student s4 {}; // mt=final=assns=0
Student s4;
int main() {
// requires a 3 parameter constructor
// (only available when program is running, cannot do c-style intialization => not constants)
int x,y,z;
cin >> x >> y >> z;
Student s {x, y, z};
}
Default Constructor
Every class comes with a default ctor
ctor with 0 parameters
calls default ctors on fields that are objects Example
struct MyStruct {
int x; // calls default ctor
Student y; // x, p will not be initialized
Vec *p; // y has been initialized
};
