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Example: Function maximum is invoked or called in main() with the call

maximum(a, b, c)

Function prototypes

One of the most important features of C++ is the function prototype. A function prototype declares to the compiler that you intend to use a function later in the program. It informs the compiler the name of the function, the type of data returned by the function, the number of parameters the function expects to receive, the types of the parameters and the order in which these parameters are expected. The compiler uses function prototypes to validate function calls.

If you try to call a function at any point in the program prior to its function prototype or function definition, you will receive an error when you compile the project.


// Finding the maximum of three integers


int maximum(int, int, int); // function prototype

int main()


int a, b, c;

cout<<"Enter three integers: ";


// a, b and c below are arguments to the maximum function call

cout<<"Maximum is: "<<maximum (a, b, c)<<endl;

return 0;


// Function maximum definition

// x, y and z are parameters to the maximum function definition

int maximum( int x, int y, int z)


int max = x;

if ( y>max )

max = y;

if ( z>max )

max = z;

return max;


The output of the above program:

Enter three integers: 22 85 17

Maximum is: 85

Passing by value

If a variable is one of the actual parameters in a function call, the called function receives a copy of the values stored in the variable. After the values are passed to the called function, control is transferred to the called function.

Example: The expression maximum( a, b, c) calls the function maximum() and causes the values currently residing in the variables a, b and c to be passed to maximum().

The method of passing values to a called function is called pass by value.

Functions with empty parameter lists

Functions may have empty parameter list. The function prototype for such a function requires either the keyword void or nothing at all between the parentheses following the function name.


int display();

int display(void);

Returning values

To actually return a value, the function must use a return statement, which has the form:

return expression;



Remember that values passes back and forth between functions must be of the same data type.

When the return statement is encountered, the expression is evaluated first. The value of the expression is then automatically converted to the data type declared in the function header before being sent back to the calling function. After the value is returned, program control reverts to the calling function.

Inline functions

For small functions, you can use the inline keyword to request that the compiler replace calls to a function with the function definition wherever the function is called in a program.


// Using an inline function to calculate

// the volume of a cube.


inline double cube(double s) { return s * s * s; }

int main()


cout<<"Enter the side length of your cube: ";

Questions & Answers

a perfect square v²+2v+_
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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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Source:  OpenStax, Programming fundamentals in c++. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10788/1.1
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