C++ Linker error with a function definition outside class

Question

Its such a weird error with my C++ code.

I want to maintain a header file with some methods. Lets call it Util.hpp. I have the below simple method in it.

bool CheckVal(int value) {

  if (value == 1)
    return true;
  else
    return false;    
}

I include Util.hpp in another class's cpp & invoke the method. I end up with a linker error saying

:-1: error: 3 duplicate symbols for architecture x86_64

I am building this with clang on osx.

How can I get rid of this error ? what is so wrong in what I am doing ?

I am having the error in spite of inlining the function. really strange !

_"I am having the error in spite of inlining the function. really strange !"_ Did you define that same function somewhere else? — πάντα ῥεῖ, Mar 13 '17 at 17:14
Nope. I just inlined it in `Util.hpp`. it doesn't help. trying out other means — TheWaterProgrammer, Mar 13 '17 at 17:15
@SegmentationFault Add every information you can give us please. Including your compiler and linker command lines. — πάντα ῥεῖ, Mar 13 '17 at 17:18

πάντα ῥεῖ · Accepted Answer · 2017-03-13T17:30:48.757

3

If you keep free function definitions in a header file you must make them with the inline keyword:

   inline bool CheckVal(int value) {       
// ^^^^^^
      return value == 1;
   }

Otherwise the function definition appears in every translation unit that includes the header file, and the linker sees multiple definitions of it.

Also as I did above you don't need if/else for retuning bool values.

Another solution is to just declare the function in the header

bool CheckVal(int value);

and move the implementation (definition) to a separate translation unit.
That has at least the advantage that any dependend translation units on that header won't need to be recompiled, if you change something in the implementation.

edited Mar 13 '17 at 17:30

answered Mar 13 '17 at 17:04

πάντα ῥεῖ

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inlining is not helping. really strange. any option other than declaring it in cpp file ? – TheWaterProgrammer Mar 13 '17 at 17:14
@SegmentationFault `inline` should work just fine. Did you define that same function anywhere else? Edit your question with a [MCVE] that reproduces the problem please. – πάντα ῥεῖ Mar 13 '17 at 17:16
I went for the other solution. inlining does not help in my case – TheWaterProgrammer Mar 13 '17 at 17:19
@SegmentationFault It would be very interesting to figure out why `inline` didn't work for your case. Just don't give up too early, give us more context as I advised please. – πάντα ῥεῖ Mar 13 '17 at 17:21

eerorika · Answer 2 · 2017-03-13T17:18:12.570

what is so wrong in what I am doing ?

The one definition rule requires that a (non inline) function must be defined exactly once^†.

If you include your header in multiple translation units, then there will be multiple definitions, thus odr is violated.

If there are multiple - different definitions for the function, that also violates odr.

How can I get rid of this error ?

Solution 1: Do not define the function in a header. Instead define it in a source file. Source files are not included in other source files, so there should be exactly one definition when you use this approach.

Solution A: Declare the function inline.

In both cases, check that there are no other functions by the same name.

^† Only if the function is odr-used. This subtlety is not relevant to this question.

C++ Linker error with a function definition outside class

2 Answers2