C++ Multiple definition [closed]

Question

So I have a header file which has function implementations in it as well as prototypes. I am now trying to separate these implementations from the file and just leave the prototypes since I know its not good standards to have the implementations in header files. However when I remove the implementations from the header file and put them in their own cpp file and then try to link that cpp file to the rest of the project it complains about multiple definitions of the functions. Thanks for any help/advice

std::bitset<LENGTH> foo1(const std::string stringVal); std::bitset<LENGTH> foo2(const int decimalVal); std::string foo3(const int integerVal);

I have an include guard on so it doesn't get included more than once as well.

Then in the cpp I simply have my implementations for these functions just straightforward. Yet it complains of multiple definitions of the functions if they are separate from the header file.

What is the error message and what is the code which provokes the error message? — David Heffernan
– David Heffernan, Commented Jan 19, 2012 at 20:50
You have not provided enough information to be able to answer your question. Please provide a small example that shows at least: (1) the contents of your header files, (2) the contents of your .cpp files, (3) your linker command line. In particular, the #include lines are probably most relevant. — Greg Hewgill
– Greg Hewgill, Commented Jan 19, 2012 at 20:55
"put them in their own cpp file and then try to link that cpp file to the rest of the project" -- You skipped the part where something gets compiled. In particular, did you compile just the new .cpp file, or did you re-compile everything for which a header changed (or the whole project, for simplicity)? — JaMiT
– JaMiT, Commented Aug 28 at 10:58

Luchian Grigore · Accepted Answer · 2012-01-19 20:57:05Z

That is just a declaration. Even if the file didn't have include guards, it wouldn't give that error.

Multiple definition usually means you're defining the function in more than one implementation file.

Here's a checklist:

make sure the implementation is definetely outside of the header.
make sure only one source file defines the function
make sure you don't include the source file (this one is trivial, I hope you know not to do this)
finally, check that you run a clean build

enobayram · Accepted Answer · 2012-01-19 21:01:00Z

0

I guess you're not compiling the other cpp file that previously included the header file. So the object file still contains the definitions. BTW, not having implementations in header files isn't at all about good standards. It's a must, so that you can include the same header in more than one compilation unit.

answered Jan 19, 2012 at 21:01

enobayram

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Luchian Grigore Over a year ago

Actually, you can have implementations in header files if the methods are inline or using templates (in this case, it's required).

enobayram Over a year ago

You're right, I'm sorry, I was just trying to emphasize that it's not a matter of style or good standards. Either way, it's a necessity (even in the case of templated functions, though you can live with warning messages in the case of inlines)

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