Use "avr-objdump -SC *.elf" to output the disassembly with de-mangled C++ symbols.
Blech, that doesn't look at all very useful!
If that's what you need, it's very useful indeed!
i want to do: first step be able to identify the memory location of the opcode, then try to change it or the operands.
That seems a bit strange. In general, you won't be able to write pgmspace. The minimum write size is a page, and you have to erase it before you can change 0's to 1's, and then, only from the bootloader or external programmer.
(The Atmel debugger actually does this to set breakpoints beyond the first: read the page to a buffer, change the location to a bkpt instruction, erase the page, re-write the page. It's gross.)