Example code to debug:

   //Account.h
   //Definition of Account class
   
   class Account {
   
   public:
        Account(int);
        void credit(int); //add to account
        void debit(int); //subtract from account
        int getBalance();
   
   private: 
        int balance;
   };

   //Account.cpp
   #include <iostream>
   using std::cout;
   using std::endl;
   
   #include "Account.h"
       
   Account::Account(int initialBalance) {
        balance = 0;
        if(initialBalance > 0)
             balance = initialBalance;
        if(initialBalance < 0)
             cout<<"Error - Initial balance cannot be negative.\n"<<endl;
   }
   
   void Account::credit(int amount) {
        balance += amount;
   }
   
   void Account::debit(int amount) {
        if(amount <= balance)
             balance -= amount;
        else
             cout<<"Debit amount exceeded account balance.\n"<<endl;
   }
   
   int Account::getBalance(){
        return balance;

   //AccountApplication.cpp
   //Create and manipulate accounts
   #include<iostream>
   using std::cin;
   using std::cout;
   using std::endl;
   
   #include"Account.h"
   
   int main() {
        Account account1(50);
        cout<<"account1 balance": $"<<account1.getBalance()<<endl;//line 12
        int withdrawalAmount;     
        cout<<\nEnter withdrawal amount: ";     
        cin>>withdrawalAmount;     
        cout<<"\nattempting to subtract " <<withdrawalAmount     
            <<" from account1 balance\n\n";     
        account1.debit(withdrawalAmount);	                  //line 18    
        cout<<"account1 balance: $"<<account1.geBalance()<<endl;     
        return 0;
   }

A debugging session:

   $ g++ -g -o AccountApp Account.cpp AccountApplication.cpp
        note: the -g option adds debugging code to the output
   $ gdb
   You can set values suing the set command:
   (gdb) run
   Starting program: /home/jones/AccountApp
   account1 balance: $50
   Enter withdrawal amount for account1: 13
   attempting to subtract 13 from account1 balance
   account1 balance: $37
   Program exited normally
   (gdb)
   (gdb) break 12
   Breakpoint 1 at (. . .) line 12.
   (gdb) break 18
   Breakpoint 2 at (. . .) line 18.
   (gdb) run
   Starting program /home/jones/AccountApp
   Breakpoint 1, main() at AccountApplication.cpp:12
   17     cout<<"account1 balance: $"<<account1.getBalance()<<endl;
   (gdb) continue
   Continuing.
   Enter withdrawal amount for account1: 13
   attempting to subtract 13 from account1 balance
   Breakpoint 2, main() at AccountApplication.cpp:18
   25     account1.debit(withdrawalAmount);
   (gdb) print withdrawalAmount
   $2 = 13	              //$n variables are convenience variables
   (gdb) print account1     //prints the object's data values
   $3 = {balance = 50}
   (gdb) print $1
   $3 = 13
   (gdb) continue
   Continuing.
   account1 balance: $37
   Program exited normally.
   (gdb)info break
   Num  Type      Disp Enb Address   What
   1 breakpoint  keep  y   0x08048   in main at AccountApplication.cpp:12
   breakpoint already hit 1 time
   2 breakpoint  keep  y   0x0804e   in main at AccountApplication.cpp:18
   breakpoint already hit 1 time
   (gdb) delete 1
   (gdb) delete 2
   (gdb) info break
   No breakpoints or watchpoints
   (gdb)quit
   $

Print and set Commands:

• Once you have entered break mode, you can examine values using ‘print’.

• You can also use print to evaluate expressions:

   print withDrawalAmount -2
   print withdrawalAmount == 11 will return a boolean value of false

• You can set values suing the set command:

   (gdb) set withdrawalAmount = 42
   (gdb) print withdrawalAmount
   $3 = 42
   (gdb)continue
   Continuing.
   account1 balance: $8
   Program exited normally
   (gdb)quit
   $

Using step, next, finish, and watch commands:

• The step command executes the next statement in the program, then returns to break mode.
  • If the statement is a function call, it will step into the first statement of the function.
  • After stepping,the debugger displays the next line of code.

• The next command behaves like the step command, except when the next statement is a function call.
  • The entire function will execute, and the program advances to the line after the function call.

• After you have ‘stepped’ into a function, the finish command executes the rest of the function and returns to the statement after the function call.

• The watch command monitors a data member.
  • When that data member is about to change, the debugger will notify you.

Note:

• Breakpoints persist until the end of the debugging session.
• When a ‘watched’ data member goes out of scope its watch is deleted.

   (gdb) break 14     
   Breakpoint 1 at . . . 
   (gdb) run
   Starting program: . . .
   Breakpoint 1, main() at . . . .    :14
   14	Account account1( 50);
   (gdb) watch account1.balance
   Hardware watchpoint 2:account1.balance
   (gdb)next
   Hardware watchpoint 2: account1.balance
   Old value = 0
   New value = 50
   Account (this = 0xnnnnn, initialBalance=50) at Account.cpp:20
   20	     if(initialBalance < 0)
   (gdb)continue
   Continuing
   account1 balance: $50
   Enter withdrawal amount for account1: 13
   attempting to subtract 13 from account1 balance
   Hardware watchpoint 2: account1.balance
   Old value = 50
   New value = 37
   0xnnnnn in Account::debit (this=0xnnnnn, amount - 13) at Account.cpp:34
   34     balance -= amount;
   (gdb)continue
   Continuing
   account1 balance: #37
   Watchpoint 2 deleted because the program has left the block in which its expression is valid.
   0xnnnn in exit () from  . . . . . 
   (gdb) continue
   Continuing.
   Program exited normally.
   (gdb)

Note: if you ‘run’ the program again in the debugger you will have to reset the watch on the variable

   (gdb) run
   Starting program . . . 
   Breakpoint 1, main() at ...... :14
   14     Account account1(50);
   (gdb) watch account1.balance
   Hardware watchpoint 3: account1.balance
   (gdb) continue
   Continuing.
   Hardware watchpoint 3: account1.balance
   Old value = 0
   New value = 50
   Account (this=0xnnnnnn, initialBalance=50) at Account.cpp:20
   20     if(initialBalance < ))
   (gdb) delete 3			//deletes the watchpoint
   (gdb) continue
   .
   .
   .
   Program exited normally
   (gdb) quit