Precondition & Postcondition:

• precondition A condition that must be true on entry to a member function (method). 

  Example1: For a BankAccount object, a depositMoney() method must take amount of  money which must be > 0. 
  Example 2: For a stack, a push() method must have the stack not full before push an element in the stack.
   

• postcondition A condition that is true AFTER running the code in a method.

// File: temperature.cpp
// This program prints a table to convert numbers from one unit to another.
// The program illustrases some implementation techniques.

#include <iostream>    // Provides cout
#include <iomanip>     // Provides setw function for setting output width
#include <cstdlib>     // Provides EXIT_SUCCESS
#include <cassert>     // Provides assert function
using namespace std;   // Allows all standard library items to be used

double celsius_to_fahrenheit(double c)
// Precondition: c is a Celsius temperature no less than absolute
// zero (-273.16).
// Postcondition: The return value is the temperature c converted to Fahrenheit
// degrees.
{
    const double MINIMUM_CELSIUS = -273.16; // Absolute zero in Celsius degrees
    assert(c >= MINIMUM_CELSIUS);
    return (9.0 / 5.0) * c + 32;  
}

void setup_cout_fractions(int fraction_digits)
// Precondition: fraction_digits is not negative.
// Postcondition: All double or float numbers printed to cout will now be
// rounded to the specified digits on the right of the decimal.
{
    assert(fraction_digits > 0);
    cout.precision(fraction_digits);
    cout.setf(ios::fixed, ios::floatfield);
    if (fraction_digits == 0)
        cout.unsetf(ios::showpoint);
    else
        cout.setf(ios::showpoint); 
}

int main( )
{
    const char   HEADING1[]  = "   Celsius"; // Heading for table's 1st column
    const char   HEADING2[]  = "Fahrenheit"; // Heading for table's 2nd column
    const char   LABEL1      =          'C'; // Label for numbers in 1st column
    const char   LABEL2      =          'F'; // Label for numbers in 2nd column
    const double TABLE_BEGIN =        -280.0; // The table's first Celsius temp.
    const double TABLE_END   =         50.0; // The table's final Celsius temp.
    const double TABLE_STEP  =         10.0; // Increment between temperatures
    const int    WIDTH       =            9; // Number chars in output numbers
    const int    DIGITS      =            1; // Number digits right of decimal pt

    double value1;  // A value from the table's first column
    double value2;  // A value from the table's second column
    
    // Set up the output for fractions and print the table headings.
    setup_cout_fractions(DIGITS);
    cout << "CONVERSIONS from " << TABLE_BEGIN << " to " << TABLE_END << endl;
    cout << HEADING1 << "  " << HEADING2 << endl;
    
    // Each iteration of the loop prints one line of the table.
    for (value1 = TABLE_BEGIN; value1 <= TABLE_END; value1 += TABLE_STEP) 
    {   
        value2 = celsius_to_fahrenheit(value1);
        cout << setw(WIDTH) << value1 << LABEL1 << "  "; 
        cout << setw(WIDTH) << value2 << LABEL2 << endl; 
    }
    system("pause");
    return EXIT_SUCCESS;
}

If the expression is false, a message is written to the standard error device and abort is called, terminating the program execution.

message:
Assertion failed: expression, file filename, line line number