Are functions defined in headers guaranteed to be inlined?

Question

If I define a non-member function in a header, will it always be inlined by the compiler, or does the compiler choose based on its heuristics? I know that __inline is just a hint, is it the same with functions in headers?

Answer 1

Remember that including something from a header is no different than just typing it directly in the source file. So being in a header makes no difference as far as the compiler is concerned; it never knew it was there.

So when you define a function in a header file, and you include that header file in a file, it's like you just typed the function straight into the file. So now the question is, "does the compiler choose to inline things based on heuristics?"

The answer is "it depends on the compiler". The standard makes no guarantees about what gets inlined or not. That said, any modern compiler will be extremely intelligent about what it inlines, likely with heuristics.

However, we come to an interesting point. Imagine you have a function in a header and you include that header in multiple source files. You will then have multiple definitions of the function, across translation units, and this violates the one-definition rule. Ergo, you will get compile errors. (The linker error is usually something along the lines of: "Error, function x already defined in y") What you can do is use the inline keyword and you no longer violate the ODR.

By the way __inline is non-standard. Contrary to your post, it's usually a compiler extension which forces inlining, not hints it. inline is the standard keyword, which was originally intended to hint at inlining. Like you say, most modern compilers completely ignore it in that regard and it's only purpose nowadays is to give things internal linkage.

Answer 2

From the C++ FAQ Lite:

No matter how you designate a function as inline, it is a request that the compiler is allowed to ignore: it might inline-expand some, all, or none of the calls to an inline function.