Multiple definition error of base class constructor while compiling - c++

Question

I'm coding a spreadsheet built up of column vectors and cell vectors, where
each cell is a placeholder for a cell value. CellValueBase is the base class, CellValue is the final template class.
This is the error:
g++ Cell.o Column.o sheet.o main.o -o spreadsheet
sheet.o: In function `CellValueBase::CellValueBase()':
sheet.cc:(.text+0x0): multiple definition of `CellValueBase::CellValueBase()'
Column.o:Column.cc:(.text+0x0): first defined here
sheet.o: In function `CellValueBase::CellValueBase()':
sheet.cc:(.text+0x0): multiple definition of `CellValueBase::CellValueBase()'
Column.o:Column.cc:(.text+0x0): first defined here
main.o: In function `CellValueBase::CellValueBase()':
main.cc:(.text+0x0): multiple definition of `CellValueBase::CellValueBase()'
Column.o:Column.cc:(.text+0x0): first defined here
main.o: In function `CellValueBase::CellValueBase()':
main.cc:(.text+0x0): multiple definition of `CellValueBase::CellValueBase()'
Column.o:Column.cc:(.text+0x0): first defined here
Cell.o: In function `CellValueBase::~CellValueBase()':
Cell.cc:(.text._ZN13CellValueBaseD2Ev[_ZN13CellValueBaseD5Ev]+0xd): undefined reference to `vtable for CellValueBase'
Cell.o: In function `CellValueBase::CellValueBase()':
Cell.cc:(.text._ZN13CellValueBaseC2Ev[_ZN13CellValueBaseC5Ev]+0x9): undefined reference to `vtable for CellValueBase'
Cell.o:(.rodata._ZTI9CellValueIfE[_ZTI9CellValueIfE]+0x10): undefined reference to `typeinfo for CellValueBase'
Column.o: In function `CellValueBase::CellValueBase()':
Column.cc:(.text+0x9): undefined reference to `vtable for CellValueBase'
sheet.o: In function `CellValueBase::CellValueBase()':
sheet.cc:(.text+0x9): undefined reference to `vtable for CellValueBase'
main.o: In function `CellValueBase::CellValueBase()':
main.cc:(.text+0x9): undefined reference to `vtable for CellValueBase'
collect2: error: ld returned 1 exit status
Makefile:8: recipe for target 'Spreadsheet' failed
make: *** [Spreadsheet] Error 1
And this is my code:
main.cc
#include <iostream>
#include "sheet.h"
using namespace std;
int main () {
Sheet *sht;
sht = new Sheet ();
return 0;
}//main
sheet.h
#ifndef SHEET_H
#define SHEET_H
#include "Column.h"
// Vaste grootte van de sheet
const int AantReg = 24;
const int AantKol = 80;
class Sheet
{
public:
Sheet ();
void getCell();
void begin();
void end();
private:
std::vector<Column*> sheetCol;//bevat de columns
int regels, kolommen;
};
#endif
sheet.cc
#include <iostream>
#include "sheet.h"
using namespace std;
Sheet::Sheet () {
regels = AantReg;
kolommen = AantKol;
cout << "Kolommen" << endl;
for (int i = 0; i < kolommen; i++) {
cout << "kolomnr: " << i << endl;
sheetCol.push_back(new Column(regels));
}
cout << endl << endl;
}
void Sheet::getCell () {
//TODO: fixen
}
void Sheet::begin () {
//TODO: deze shit ook fixen
}
void Sheet::end () {
}
Column.h
#include <vector>
#include "Cell.h"
class Column
{
public:
Column (int n);
//void getCell();
//void begin();
//void end();
private:
int aantCellen;
std::vector<Cell*> columnVec;//sla je de cellen in op
};
#endif
Column.cc
#include <iostream>
#include "Column.h"
using namespace std;
Column::Column(int n): aantCellen(n)
{
for (int i = 0; i < aantCellen; i++) {
cout << "celnr: " << i << endl;
columnVec.push_back(new Cell());
}
}//cell
Cell.h
#ifndef CELL_H
#define CELL_H
#include "CellValueBase.h"
#include <string>
#include <memory>
class Cell {
public:
Cell();
void setValueFloat(float newValue);
//void setValueInt(int newValue);
//void setValueString(std::string newValue);
//void setValueFormula(std::string newValue);
//std::unique_ptr<cellValueBase> readValue();
void emptyCell();
private:
std::unique_ptr<CellValueBase> value;
};
#endif
Cell.cc
#include "Cell.h"
#include <iostream>
using namespace std;
Cell::Cell() {
value.reset(nullptr);
cout << "hallo wereld ik ben een cel" << endl;
setValueFloat(3.14);
} // Cell
void Cell::setValueFloat(float newValue)
{
value = unique_ptr<CellValueBase>(new CellValue<float>(newValue));
value->returnValueNumber();
} // setValueFloat
CellValueBase.h
#ifndef CELLVALUEBASE_H
#define CELLVALUEBASE_H
#include <iostream>
#include <string>
#include <sstream>
#include <stdexcept>
class CellValueBase
{
public:
CellValueBase();
virtual ~CellValueBase() {};
//virtual std::string returnValueStringEdit() = 0;
virtual float returnValueNumber();
void emptyCell();
private:
};
CellValueBase::CellValueBase()
{
} // CellValueBase
template<typename T>
class CellValue final : public CellValueBase
{
public:
CellValue(T initial_value)
: CellValueBase(), value(initial_value)
{ }
~CellValue();
//std::string returnValueString();
//std::string returnValueStringEdit();
float returnValueNumber();
private:
T value;
};
template<typename T>
CellValue<T>::~CellValue()
{
// TODO
}
template<typename T>
float CellValue<T>::returnValueNumber() {
return value;
}
And the makefile:
CC = g++
CompileParms = -c -std=c++14 -Wall -Wextra
OBJS = Cell.o Column.o sheet.o main.o
Spreadsheet: $(OBJS)
$(CC) $(OBJS) -o spreadsheet
Cell.o: Cell.cc CellValueBase.h Cell.h
$(CC) $(CompileParms) Cell.cc
Column.o: Column.cc Column.h
$(CC) $(CompileParms) Column.cc
sheet.o: sheet.cc sheet.h
$(CC) $(CompileParms) sheet.cc
main.o: main.cc sheet.h
$(CC) $(CompileParms) main.cc

Answer 1

You have correctly understood that templates needs to be defined in their header file. But the class CellValueBase is not a template, so the definition of the CellValueBase constructor in the header file is incorrect. It means the constructor will be defined everywhere the header file is included.
The simple solution? Define the CellValueBase constructor inline in the class (like you already do with the destructor).
Furthermore, all virtual but non-abstract functions in a class must have a definition. So either make CellValueBase::returnValueNumber abstract or have an empty definition.
All in all, the CellValueBase class could look like this:
class CellValueBase
{
public:
CellValueBase() {} // <- Define inline
virtual ~CellValueBase() {};
//virtual std::string returnValueStringEdit() = 0;
virtual float returnValueNumber() = 0; // <- Declare abstract
void emptyCell();
private:
};

Answer 2

Look at CellValueBase.h. You can not define non-inline class methods outside of the class in header files. You must define them in .cpp files.
Move this method's definition to to CellValueBase.cpp:
CellValueBase::CellValueBase()
{
} // CellValueBase
Define non-inline methods or functions in header is totally bad practice. Error appears not at once, it appears only when you include this header in two cpp files. It means that linker founds two same methods definitions and that is the problem. If you would leave definition in your header CellValueBase.h and include CellValueBase.h once in .cpp file then there won't be any problems. But when you include CellValueBase.h more than once then linker founds duplicate definition.
But even if you know that you won't include header file with non-inline methods definitions more then once then you also should remember never to define non-inline functions or class methods outside classes. You can forget about your "1 include rule" for this file and later include it twice and linker will detect duplicate definition.
Also you can define template methods without specialization or define inline specialized template methods in header files.