Lecture 06 Reference Variables

1. Lecture 06Reference Variables Learn about:- Reference arguments- Pass Arguments and Structures by reference- const function arguments

2. Reference Variables (page 137-139) • A reference is an alias (a different name) to a variable. • How to declare a reference variable? i) By placing a type and & in front of a variable. ii) Assign another variable of the same type to the reference variable. For example: double someMoney; double &cash = someMoney

3. Reference Variables (page 137-139) double someMoney; double &cash = someMOney; cash = 6.78; cout<<cash<<endl; // displays 6.78 cout<<someMoney; // displays 6.78 someMOney = 111.333; cout<<cash<<endl; // displays 111.333 cout<<someMoney; // displays 111.333

4. Reference Arguments • Passing arguments by reference allows a function to access variables in the calling program, as well as returning more than one value. • Reference arguments are indicated by the & following the data type. (float n, float &intp, float &fracp)

5. Reference Arguments Example 1 #include <iostream.h> void intfrac(float n, float& intp, float& fracp) { long temp = static_cast<long>(n); // temp = 10 intp = static_cast<float>(temp); // intp = 10 fracp = n - intp; // fracp = 0.25 } void main() { float number, intpart, fracpart; do { cout << "\nEnter a real number: "; cin>>number; // input: 10.25 intfrac(number, intpart, fracpart); cout << "Integer part is " << intpart << ", fraction part is " << fracpart << endl; } while( number != 0.0 ); }

6. Reference Arguments Example 1 (cont.) • The & indicates that intp is an alias for whatever variable is passed as an argument. Similarly for fracp. • Note that the & is not used in the function call. • Do not confuse with the address of operator (same symbol).

7. Pass By Reference: Example 2 #include <iostream.h> void order(int& numb1, int& numb2) { if(numb1 > numb2) { int temp = numb1; numb1 = numb2; numb2 = temp; } } void main() { int n1=99, n2=11; // not ordered int n3=22, n4=88; // ordered order(n1, n2); order(n3, n4); cout << "n1=" << n1 << endl; cout << "n2=" << n2 << endl; cout << "n3=" << n3 << endl; cout << "n4=" << n4 << endl; }

8. Passing Structures By Reference Example #include <iostream.h> struct Distance { // English distance int feet; float inches; }; void engldisp( Distance dd ) { cout << dd.feet << "\'-" << dd.inches << "\""; } void scale( Distance& dd, float factor) { float inches = (dd.feet*12 + dd.inches) * factor; dd.feet = static_cast<int>(inches / 12); dd.inches = inches - dd.feet * 12; }

9. Passing Structures By Reference Example (cont.) int main() { Distance d1 = { 12, 6.5 }; Distance d2 = { 10, 5.5 }; cout << "\nd1 = "; engldisp(d1); cout << "\nd2 = "; engldisp(d2); scale(d1, 0.5); scale(d2, 0.25); cout << "\nd1 = "; engldisp(d1); cout << "\nd2 = "; engldisp(d2); cout << endl; return 0; } • By default, structures are passed by value (i.e. copy each member). • Passing structure by reference is much more efficient.

10. const Function Arguments • Passing arguments by reference is more efficient and also allows the function to modify them directly. • Can you pass an argument by reference for efficiency, but with a guarantee that the function cannot modify it? • You can apply the const modifier.

11. const Function Arguments Example #include <iostream.h> void main() { int alpha = 7; int beta = 11; aFunc(alpha, beta); } void aFunc(int& a, const int& b) { a = 107; // OK b = 111; // error: can't modify const }