C++ Classes and constructors

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What is an undefined reference/unresolved external symbol error and how do I fix it?

Someone wants to save me some time on how classes and constructors work in c++ ? This is what I got - it do not work. I want to the class to have an constructor that takes a filename and reads a file with that name from the filesystem.

this is the header and implementation

#ifndef __narcissism__Histogram__
#define __narcissism__Histogram__

#include <iostream>
#include <sstream>  // for ostringstream
#include <iomanip>  // for setw, setfill
#include <ios>      // for hex stream manipulator
using namespace std;
#include "random.h" // for randomInteger
#include "strlib.h" // for integerToString
#include "error.h"  // for error



class Histogram {
public:

/** Constructor:
  * 
  *  */
Histogram(string filename)
{
    readfile(filename);

}


private:

int readfile(string filename);

};




#endif /* defined(__narcissism__Histogram__) */

*.cpp

 #include "Histogram.h"



 int readfile(string filename)
 {
 return 0;
 }

Error msg:

Undefined symbols for architecture i386:
"Histogram::readfile(std::string)", referenced from:
  Histogram::Histogram(std::string) in narcissism.o
ld: symbol(s) not found for architecture i386

In the *.cpp file you need to prefix the definition of readfile with the name of the class, i.e. `int Histogram::readfile` — James Beilby, Nov 12 '12 at 10:15
Don't use double underscores. Names with double underscores are reserved. — R. Martinho Fernandes, Nov 12 '12 at 10:15
That's a *really* bad question. How did you get the idea that a function just returning zero does anything apart from returning zero? — JohnB, Nov 12 '12 at 10:16
and a comment and not the answer to your question, but do not put using namespace std; into your header file. — CashCow, Nov 12 '12 at 10:18
Specifically http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix/12574407#12574407 - "A common mistake is forgetting to qualify the name" — Luchian Grigore, Nov 12 '12 at 10:18
`int Histogram::readfile (string filename)`, as mentioned above. — JohnB, Nov 12 '12 at 10:18
Luchian Grigore is very knowledgeable in both C++ and the history of C++ questions asked on StackOverflow.com. — CashCow, Nov 12 '12 at 10:22

score 2 · Accepted Answer · answered Nov 12 '12 at 10:18

2

You must add Histogram:: in the definition of the member function:

 int Histogram::readfile(string filename)
 {
 return 0;
 }

Otherwise it would define a new global function with the same name, leaving the member function undefined.

answered Nov 12 '12 at 10:18

Yakov Galka

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score 2 · Answer 2 · answered Nov 12 '12 at 10:18

Your error is that readfile is a member of Histogram so in the .cpp file it should be:

int Histogram::readfile( string filename )
{
     // implement
}

The function you wrote though would actually, at this point, be a valid function. (It would fail in compilation if it tried to access of any Histogram's members, which presumably a proper implementation of readfile would: the purpose is surely to set these members from data it reads from the file).

You instead got a link error because there was no implementation defined for the function called readfile that is a member of the class Histogram.

C++ Classes and constructors

2 Answers2