C/C++ language basics: Is the statement 'extern int i;' a declaration or a definition?

Introduction / Context:In multi-file C/C++ projects, header files commonly contain 'extern' declarations of globals. Understanding how 'extern' changes the meaning of a statement prevents multiple-definition linker errors while allowing many translation units to reference the same variable defined elsewhere.

Given Data / Assumptions:

• The line in question is 'extern int i;' with no initializer.
• We are discussing a global-scope variable, not a function.
• Standard C/C++ linkage rules apply.

Concept / Approach:

• A declaration introduces a name and its type. Using 'extern' for an object declares it without defining (allocating storage for) it.
• A definition for a global object allocates storage. Example: 'int i;' or 'int i = 0;'.
• Exactly one definition with external linkage must appear across the program, while declarations can appear in multiple files.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Definition: Would allocate storage; our line does not.
• Function: We are not declaring a function here.
• Error: The statement is valid; it simply declares an external symbol.

Common Pitfalls:

• Putting a definition (without extern) in a header causes multiple definitions during linking.
• Confusing 'extern' on variables with function declarations where 'extern' is redundant.

Final Answer: