Question

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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

Answer 1

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.