Question

I know that inline function are either replaced where it is called or behave as a normal function.

But how will I know whether inline function is actually replaced at the place where it is called or not as decision of treating inline function as inline is at the compile time?

Answer 1

Programatically at run-time, You cannot.
And the truth of the matter is: You don’t need to know

An compiler can choose to inline functions that are not marked inline or ignore functions marked explicitly inline, it is completely the wish(read wisdom) of the compiler & You should trust the compiler do its job judiciously. Most of the mainstream compilers will do their job nicely.

If your question is purely from a academic point of view then there are a couple of options available:

---

Analyze generated Assembly Code:

You can check the assembly code to check if the function code is inlined at point of calling.

How to generate the assembly code?

For gcc:
Use the -S switch while compilation.
For ex:

g++ -S FileName.cpp

The generated assembly code is created as file FileName.s.

For MSVC:
Use the /FA Switch from command line.

In the generated assembly code lookup if there is a call assembly instruction for the particular function.

---

Use Compiler specific Warnings and Diagnostics:

Some compilers will emit a warning if they fail to comply an inline function request.
For example, in gcc, the -Winline command option will emit a warning if the compiler does not inline a function that was declared inline.

Check the GCC documentation for more detail:

-Winline

Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.