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What are the stages of building a C++ program? What is a translation unit? What causes an 'undefined reference' (unresolved symbol) linker error? What is the difference between compile-time and link-time errors? What is the One Definition Rule (ODR)? What is name mangling? Why do you need include guards or #pragma once, in linkage terms? What is internal versus external linkage? What is the difference between static and dynamic (shared) libraries? What are C++20 modules and how do they improve on headers?
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What is the difference between declaration and definition?

Intermediate

Answer

A declaration introduces a name and its type (promising it exists somewhere); a definition also provides the implementation/storage. You can declare something many times but define it only once (per the ODR). Header files typically contain declarations; source files contain definitions. Using an undeclared name is a compile error; a missing definition is a link error.
extern int x;      // declaration
int x = 5;         // definition
void f();          // declaration
void f(){}         // definition
Real-world example A function declared in a header but never defined causes an 'undefined reference' at link time.

Common follow-ups: How many times can you define something? | Where do declarations usually go?

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