C++: Friend and Math Function

C++ Friend function

If a function is defined as a friend function in C++, then the protected and private data of a class can be accessed using the function.

By using the keyword friend compiler knows the given function is a friend function.

For accessing the data, the declaration of a friend function should be done inside the body of a class starting with the keyword friend.

Declaration of friend function in C++

1. class class_name    
2. {    
3.     friend data_type function_name(argument/s);            // syntax of friend function.  
4. };    

In the above declaration, the friend function is preceded by the keyword friend. The function can be defined anywhere in the program like a normal C++ function. The function definition does not use either the keyword friend or scope resolution operator.

Characteristics of a Friend function:

• The function is not in the scope of the class to which it has been declared as a friend.
• It cannot be called using the object as it is not in the scope of that class.
• It can be invoked like a normal function without using the object.
• It cannot access the member names directly and has to use an object name and dot membership operator with the member name.
• It can be declared either in the private or the public part.

C++ friend function Example

Let's see the simple example of C++ friend function used to print the length of a box.

1. #include <iostream>    
2. using namespace std;    
3. class Box    
4. {    
5.     private:    
6.         int length;    
7.     public:    
8.         Box(): length(0) { }    
9.         friend int printLength(Box); //friend function    
10. };    
11. int printLength(Box b)    
12. {    
13.    b.length += 10;    
14.     return b.length;    
15. }    
16. int main()    
17. {    
18.     Box b;    
19.     cout<<"Length of box: "<< printLength(b)<<endl;    
20.     return 0;    
21. }    

Output:

Length of box: 10  

Let's see a simple example when the function is friendly to two classes.

1. #include <iostream>  
2. using namespace std;  
3. class B;          // forward declarartion.  
4. class A  
5. {  
6.     int x;  
7.     public:  
8.     void setdata(int i)  
9.     {  
10.         x=i;  
11.     }  
12.     friend void min(A,B);         // friend function.  
13. };  
14. class B  
15. {  
16.     int y;  
17.     public:  
18.     void setdata(int i)  
19.     {  
20.         y=i;  
21.     }  
22.     friend void min(A,B);                    // friend function  
23. };  
24. void min(A a,B b)  
25. {  
26.     if(a.x<=b.y)  
27.     std::cout << a.x << std::endl;  
28.     else  
29.     std::cout << b.y << std::endl;  
30. }  
31.    int main()  
32. {  
33.    A a;  
34.    B b;  
35.    a.setdata(10);  
36.    b.setdata(20);  
37.    min(a,b);  
38.     return 0;  
39.  }  

Output:

10

In the above example, min() function is friendly to two classes, i.e., the min() function can access the private members of both the classes A and B.

C++ Friend class

A friend class can access both private and protected members of the class in which it has been declared as friend.

Let's see a simple example of a friend class.

1. #include <iostream>  
2.   
3. using namespace std;  
4.   
5. class A  
6. {  
7.     int x =5;  
8.     friend class B;           // friend class.  
9. };  
10. class B  
11. {  
12.   public:  
13.     void display(A &a)  
14.     {  
15.         cout<<"value of x is : "<<a.x;  
16.     }  
17. };  
18. int main()  
19. {  
20.     A a;  
21.     B b;  
22.     b.display(a);  
23.     return 0;  
24. }  

Output:

value of x is : 5

In the above example, class B is declared as a friend inside the class A. Therefore, B is a friend of class A. Class B can access the private members of class A.

C++ Math Functions

C++ offers some basic math functions and the required header file to use these functions is <math.h>

Trignometric functions

Method	Description
cos(x)	It computes the cosine of x.
sin(x)	It computes the sine of x.
tan(x)	It computes the tangent of x.
acos(x)	It finds the inverse cosine of x.
asin(x)	It finds the inverse sine of x.
atan(x)	It finds the inverse tangent of x.
atan2(x,y)	It finds the inverse tangent of a coordinate x and y.

Hyperbolic functions

Method	Description
cosh(x)	It computes the hyperbolic cosine of x.
sinh(x)	It computes the hyperbolic sine of x.
tanh(x)	It computes the hyperbolic tangent of x.
acosh(x)	It finds the arc hyperbolic cosine of x.
asinh(x)	It finds the arc hyperbolic sine of x.
atanh(x)	It finds the arc hyperbolic tangent of x.

Exponential functions

Method	Description
exp(x)	It computes the exponential e raised to the power x.
frexp(value_type x,int* exp)	It breaks a number into significand and 2 raised to the power exponent.
Idexp(float x, int e)	It computes the product of x and 2 raised to the power e.
log(x)	It computes the natural logarithm of x.
log10(x)	It computes the common logarithm of x.
modf()	It breaks a number into an integer and fractional part.
exp2(x)	It computes the base 2 exponential of x.
expm1(x)	It computes the exponential raised to the power x minus one.
log1p(x)	It computes the natural logarithm of x plus one.
log2(x)	It computes the base 2 logarithm of x.
logb(x)	It computes the logarithm of x.
scalbn( x, n)	It computes the product of x and FLT_RADX raised to the power n.
scalbln( x, n)	It computes the product of x and FLT_RADX raised to the power n.
ilogb(x)	It returns the exponent part of x.

Floating point manipulation functions

Method	Description
copysign(x,y)	It returns the magnitude of x with the sign of y.
nextafter(x,y)	It represents the next representable value of x in the direction of y.
nexttoward(x,y)	It represents the next representable value of x in the direction of y.

Maximum,Minimum and Difference functions

Method	Description
fdim(x,y)	It calculates the positive difference between x and y.
fmax(x,y)	It returns the larger number among two numbers x and y.
fmin()	It returns the smaller number among two numbers x and y .

Power functions

Method	Description
pow(x,y)	It computes x raised to the power y.
sqrt(x)	It computes the square root of x.
cbrt(x)	It computes the cube root of x.
hypot(x,y)	It finds the hypotenuse of a right angled triangle.

Nearest integer operations

Method	Description
ceil(x)	It rounds up the value of x.
floor(x)	It rounds down the value of x.
round(x)	It rounds off the value of x.
lround(x)	It rounds off the value of x and cast to long integer.
llround(x)	It rounds off the value of x and cast to long long integer.
fmod(n,d)	It computes the remainder of division n/d.
trunc(x)	It rounds off the value x towards zero.
rint(x)	It rounds off the value of x using rounding mode.
lrint(x)	It rounds off the value of x using rounding mode and cast to long integer.
llrint(x)	It rounds off the value x and cast to long long integer.
nearbyint(x)	It rounds off the value x to a nearby integral value.
remainder(n,d)	It computes the remainder of n/d.
remquo()	It computes remainder and quotient both.

Other functions

Method	Description
fabs(x)	It computes the absolute value of x.
abs(x)	It computes the absolute value of x.
fma(x,y,z)	It computes the expression x*y+z.

Macro functions

Method	Description
fpclassify(x)	It returns the value of type that matches one of the macro constants.
isfinite(x)	It checks whether x is finite or not.
isinf()	It checks whether x is infinite or not.
isnan()	It checks whether x is nan or not.
isnormal(x)	It checks whether x is normal or not.
signbit(x)	It checks whether the sign of x is negative or not.

Comparison macro functions

Method	Description
isgreater(x,y)	It determines whether x is greater than y or not.
isgreaterequal(x,y)	It determines whether x is greater than or equal to y or not.
less(x,y)	It determines whether x is less than y or not.
islessequal(x,y)	It determines whether x is less than or equal to y.
islessgreater(x,y)	It determines whether x is less or greater than y or not.
isunordered(x,y)	It checks whether x can be meaningfully compared or not.

Error and gamma functions

Method	Description
erf(x)	It computes the error function value of x.
erfc(x)	It computes the complementary error function value of x.
tgamma(x)	It computes the gamma function value of x.
lgamma(x)	It computes the logarithm of a gamma function of x.

Anurag Rana Educator CSE/IT