In C++, what is an inline function and why is it used by the compiler?

Introduction / Context:
Inline functions in C++ were introduced as a way to suggest to the compiler that the overhead of a function call might be avoided by substituting the function body directly at the call site. This can be especially beneficial for very small, frequently called functions such as accessors or simple arithmetic helpers. However, the inline keyword is ultimately only a request, and modern compilers perform their own inlining decisions. This question asks you to identify the correct conceptual description of an inline function.

Given Data / Assumptions:

• The language is C++.
• We are focusing on the inline keyword and the idea of inlining functions.
• The question is about the semantics and purpose of inline, not about storage or threading.
• We assume common compiler behaviour where inline is primarily an optimisation and linkage hint.

Concept / Approach:
An inline function is one for which the compiler may replace a call with the function's body during compilation. This replacement can eliminate the overhead associated with setting up a stack frame, passing arguments, and jumping to another address. In practice, modern optimisers decide which functions to inline based on cost models, whether or not they are marked inline. The keyword inline also affects linkage and allows functions to be defined in header files without violating the one definition rule, as long as every definition is identical. The key idea, however, remains that inline functions are candidates for call site expansion.

Step-by-Step Solution:
Step 1: Recall that the typical syntax is inline int add(int a, int b) { return a + b; } or a definition inside a class body, which is implicitly inline. Step 2: Understand that the compiler is allowed to substitute the function body wherever add is called, instead of generating an actual call instruction. Step 3: Recognise that this potential expansion can improve performance for small, simple functions, especially in tight loops. Step 4: Compare the options and identify the one that captures the concept of call site expansion and reduced overhead. Step 5: Choose the option that states that an inline function is one whose body may be expanded at each call site.

Verification / Alternative check:
You can verify the effect of inlining by compiling a small program with optimisation enabled and inspecting the generated assembly. Functions defined as inline or inside a header and called frequently may not appear as distinct call instructions; instead, their logic is embedded directly within callers. However, it is important to note that the compiler can inline functions even without the inline keyword and can ignore the keyword if it decides inlining is not profitable. This confirms that inline is primarily a hint and a linkage feature, not an absolute command.

Why Other Options Are Wrong:
Option A function stored only in an external library file: Storage location in a library is unrelated to the inline keyword. Option A function that can be called only from inside the same class: That describes concepts like private member functions or possibly static member functions, not inline semantics. Option A function that always runs in a separate thread: Threading is controlled by libraries and system calls, not by the inline keyword.

Common Pitfalls:
A common misunderstanding is to believe that marking a function inline guarantees inlining, which is not true in modern compilers. Another pitfall is to inline very large functions manually, which can increase code size and harm instruction cache performance. The best practice is to reserve inline for small, performance critical functions and to trust the compiler optimisation passes. For exam purposes, remembering that inline functions may have their bodies expanded at call sites is the key idea.

Final Answer:
An inline function is A function whose body may be expanded at each call site by the compiler to avoid function call overhead.