What is the one-definition rule?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
A function or variable can be defined only once in the program, with exceptions for inline and templates.
WHY — Language instead of guessing?
Why interviewers care about Language:
who only read docs from people who shipped.
and tied to C++ work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1A function or variable
can be defined only once in the program, with exceptions for inline and templates.
- 2Duplicate definitions across .cpp
files cause linker errors or silent ODR violations.
- 3I put non-inline functions
in a single .cpp.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
A function or variable can be defined only once in the program, with exceptions for inline and templates. Duplicate definitions across .cpp files cause linker errors or silent ODR violations.