How to define (non-method) functions in header libraries

Question

When writing a header library (like Boost), can one define free-floating (non-method) functions without (1) bloating the generated binary and (2) incurring "unused" warnings?

When I define a function in a header that's included by multiple source files which in turn is linked into the same binary, the linker complains about redefinitions. One way around this is to make the functions static, but this reproduces the code in each translation unit (BTW, can linkers safely dereplicate these?). Furthermore, this triggers compiler warnings about the function being unused.

I was trying to look for an example of a free-floating function in Boost, but I couldn't find one. Is the trick to contain everything in a class (or template)?

Answer 1

You can use the inline keyword:

inline void wont_give_linker_errors(void)
{
    // ...
}

Answer 2

If you really want to define the function (as opposed to declaring it), you'll need to use inline to prevent linker errors.

Otherwise, you can declare the function in the header file and provide its implementation separately in your source file.

Answer 3

Besides the already mentioned inline, with most compilers templates have to be defined in headers (and with all compilers it's allowed). Since boost is mostly templates, that explains why it is almost all headers.

Answer 4

People have suggested inline but that violates the very first part of your question i.e. it bloats the code as the full definition is inserted into the code at each call of the function. The answer to your overall question is therefore "No".

If you mark them as static then they are still defined in each source file as you rightly pointed out but only once and so that's a better option than inline if code size is the only issue. I don't know if linkers can, or are allowed to, spot the duplicates and merge them. I suspect not.

Edit:

Just to clear up any confusion as to whether I support the notion of using static and/or defining functions within headers files generally then rest assured I don't. This was simply meant as a technical response as to the differences between functions marked inline and static defined in header files. Nothing more.

Answer 5

Er... The answer to your question is simply don't. You just don't define functions in header files, unless they are inline.

'static' function can also be defined in headers, but it is only useful for very specific rare purposes. Using 'static' just to work around a multiple-definition problem is utter nonsense.

Again, header files are for non-defining function declarations. Why on Earth would you want to define functions there?

You said you are writing "header library". What's a "header library"? Please note, that Boost defines its "functions" in header files because their "functions" are not really functions, they are function templates. Function templates have to be defined in header files (well, almost). If that's wasn't the case, Boost wouldn't be doing something as strange as defining anything in header files.