Compile a .cpp source file in the GNU debugger:

g++ -g -Wall -o mysourcefile.out mysourcefile.cpp

Before starting gdb, make sure you're in the same directory as the executable just compiled above. Then call gdb:

add GNU

Then start the gdb debugger:

gdb mysourcefile.out

Things you can do here:

1) Set a breakpoint at a given function (e.g., main):

break main

2) Set a breakpoint at a given line (e.g., line 12):

break mysourcefile.cpp:12

3) Set a watch point at a given expression:

watch x>y

4) Find the type of that expression:

whatis x>y

5) OK, that was trivial, clearly it's boolean. More usefully, you can find its value so far (true or false in the GNU C++ compiler, 1 or 0 in others, I guess) with

print x>y

6) Step into the program (execute a certain number of lines after the debugger stopped at your breakpoint):

step <number of lines>

If you set a break point, run will execute the code up to then, at which point you can choose next to move on to the next line, or continue, for the thing to run until it exits normally or an error is found.