Question about One Definition Rule

Hello dear EE-ers,

There is a quote:
In the entire program, an object or non-inline function cannot have more than one definition; if an object or function is used, it must have exactly one definition. You can declare an object or function that is never used, in which case you don't have to provide a definition. In no event can there be more than one definition.

Please, provide example about this statement, especially regarding "in no even can there be more that one definition".

Thanks.
Nusrat NuriyevAsked:
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jkrCommented:
Since 'definition' is used as a supplement to 'implementation', that statement speaks for itself. One thing is missing though, while multiple definitions (implementations) are an obvious problem, multiple declarations might hit as well.

Since you asked for examples:

//all fine, one declaration and one definition for each one
void print(const string& data);
void print(const string& data1, const string& data2);

void print(const string& data) {
  cout << data << endl;
}
void print(const string& data1, const string& data2) {
  cout << data << " " << data2 << endl;
}

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// Houston, we have a problem
void print(const string& data);

void print(const string& data) {
  cout << data << endl;
}
void print(const string& data) {
  cerr << data << endl;
}

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Akashay KCommented:
Further Learning Resource
Google Code
One Definition Rule Violation
Nusrat NuriyevAuthor Commented:
So, definition is more general term than implementation and in case of C programming language definition is reduced to implementation, correct?
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Nusrat NuriyevAuthor Commented:
@Akashay K
Thanks for ODR violation,  could you please provide something that I can test to violate in gcc compiler?
jkrCommented:
>>So, definition is more general term than implementation and in case of C programming language definition is reduced to
>>implementation, correct?

Indeed. I'd usually also go by the term 'implementation', but 'definition' here means the same and also covers instances and variables.
Nusrat NuriyevAuthor Commented:
I have got it, if we say 
int i = 6; //integer i implements number 6

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, it sounds a bit weird :)

Defining instances of class?
Nusrat NuriyevAuthor Commented:
//tu1.cpp
struct X {
    X(int);
    X(int, int);
};
X::X(int = 0) { }
class D: public X {};
D d2;


int main(){}

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//tu2.cpp
struct X {
    X(int);
    X(int, int);
};
X::X(int = 0, int = 0) {}
class D: public X { };

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g++ tu1.cpp tu2.cpp
compiles like a charm.
jkrCommented:
Yes, because exist independently in two different compilation units that don't cross-reference each other.

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Nusrat NuriyevAuthor Commented:
jkr, Could you modify this links in order to make them cross reference? Still don't get this.
jkrCommented:
The problem about doing so is that it would defy the very purpose of your example. E.g.

// X.h
struct X {
    X(int);
    X(int, int);

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// tu1.cpp
#include "X.h"
X::X(int = 0) { }
class D: public X {};
D d2;

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#include "X.h"
X::X(int = 0) {int i = i;}
class D: public X { };

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which correctly yields
$ g++ tu1.cpp tu2.cpp
/tmp/cc9v8yr7.o:tu2.cpp:(.text+0x0): multiple definition of `X::X(int)'
/tmp/ccsPYguC.o:tu1.cpp:(.text+0x0): first defined here
/tmp/cc9v8yr7.o:tu2.cpp:(.text+0x0): multiple definition of `X::X(int)'
/tmp/ccsPYguC.o:tu1.cpp:(.text+0x0): first defined here
Nusrat NuriyevAuthor Commented:
You have omitted  some closed curly braces and semicolons :) But it's okay, I know you quite for long time :)
//tu1.cpp

#include "X.h"
X::X(int = 0) {};
class D: public X {};
D d2;

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//tu1.cpp
#include "X.h"
X::X(int = 0) { int i = i;}
class D: public X {};

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To keep the knowledge base clean (as sarabande suggested), I will accept my own answer also as a Solution :)
Nusrat NuriyevAuthor Commented:
Some minor bugs have been fixed in expert's answer.
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C++

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