Want to know in detail, what is top down approach and bottom down approach in terms of C C++?

In any programming language:
Top down means you divide the problem you want to solve into pieces and then subdivide each piece into more pieces until they are small enough to code each piece as a very simple function.

Bottom up means you have enough experience and/or intuition to know what kind of specialized "tools" the project will need and you start by coding basic tools using the built in capabilities of the language, then code increasingly more complex and/or project specific tools in terms of the tools you have already coded until the whole project becomes simple as a result of use of project specific "tools".

Real projects are not actually approached with a pure project structure resembling either of those.

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