How can a compiler be diffrend ?? it''s the same C++ language ?? ???<>??? "on same OS"

The language is the same, but different compilers can
1. Generate different machine code
2. Have different obj/lib file formats
3. Have different C runtimes (compiler-specific functions)
4. Generate different supporting information (ie, Edit-And-Compile in MSVC)
etc.

The language being parsed (front-end) may be the same in theory, but in practice, compilers often differ, not complying the standard.
Plus the C++ language itself has evolved, vendors offer extensions.
Currently, only EDG officially offers a fully compliant C++ front-end (used in the Comeau compiler).

Then you have the back-end to consider : not only the target architecture (Mac, x86 (all the variants), Alpha, Mips ...) which all have different instruction sets, but also different operating systems, which offer different libraries. Nothing in the C++ standard enforces the way compilers have to implement language features at the binary level (well, that''s not quite true), and compilers therefore wildly differ (different ABI).
Plus back-ends may optimise code differently... and vendors strive to produce the best possible code.

If either the front or the back end differ, you have a different compiler.

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thanks for the info i know enough ..... now