Issues with compiling multiple files - c++

Question

So I'm trying to write a bit of code to work with sudoku puzzles, and I keep getting errors when I try to compile, and each time i compile it deletes the driver.
heres the code for the driver:
/*
* Jared C
* C++ Project
*/
#include <iostream>
#include "SudokuBoard.h"
using namespace std;
int main()
{
SudokuBoard board("sudoku.txt");
board.printBoard();
}
And here's the header file
/*
* Jared C
* SudokuBoard.h
*/
#ifndef __SUDOKUBOARD_H_INCLUDED__
#define __SUDOKUBOARD_H_INCLUDED__
#include <iostream>
#include <string>
class SudokuBoard
{
private:
int content[9][9];
public:
SudokuBoard(std::string filename);
void printBoard();
int getNumAt(int, int);
void setNumAt(int, int, int);
};
#endif
And finally the Sudoku Board.cpp
/*
* Jared C
* SudokuBoard.cpp
*/
#include <iostream>
#include <fstream>
#include "stdlib.h"
#include "SudokuBoard.h"
using namespace std;
SudokuBoard::SudokuBoard(string filename)
{
string output;
string line;
int count = 0;
ifstream file;
file.open(filename.c_str());
if (file.is_open())
{
while (getline (file, line))
{
output += line;
}
file.close();
}
else cout<< "unable to open file" << endl;
for(int y = 0; y < 9; y++)
{
for(int x = 0; x < 9; x++)
{
content[x][y] = atoi(output.substr(count,1).c_str());
count ++;
}
}
}
void SudokuBoard::printBoard()
{
string output = "\n";
for(int y = 0; y < 9; y++)
{
if(y%3==0)
{
output += '\n';
}
for(int x = 0; x < 9; x++)
{
if(x%3==0)
{
output += " ";
}
output += content[x][y];
output += "\n";
}
}
cout<<output<<endl;
}
int SudokuBoard::getNumAt(int x, int y)
{
return content[x][y];
}
void SudokuBoard::setNumAt(int x, int y, int value)
{
content[x][y] = value;
}
When i call gcc -c SudokuBoard.cpp I get the SudokuBoard.o file just fine, but when I then call 'gcc -o Driver.cpp SudokuBoard.o' I get a huge wall of error messages, heres a sample of them:
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 0 has invalid symbol index 11
SudokuBoard.cpp:(.text+0x34): undefined reference to `std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string()'
SudokuBoard.cpp:(.text+0x43): undefined reference to `std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string()'
And it deletes Driver.cpp Any idea what I'm doing wrong?

Answer 1

Your command line is incorrect:
gcc -o Driver.cpp SudokuBoard.o
Instructs gcc to link the object file SudokuBoard.o as an executable file named Driver.cpp. No surprise that it first erases the destination file.
Furthermore, you do not specify the runtime library to link with and gcc does not default to C++: this explains the error message.
You should instead write:
g++ -o Sudoku Driver.o SudokuBoard.o

Answer 2

Use g++ to compile C++ code, not the common C-oriented gcc.
g++ invokes gcc for compilation and linking with the right options for C++.
In short, it's a good idea to use a C++ compiler for C++ source code, a Fortran compiler for Fortran source code, and so on.
Also, for the issue of “And it deletes Driver.cpp”, simply don't specify that source file as an output file.

Answer 3

The problem is that the -o option with gcc is used for specifying the output file. If you write
gcc -o Driver.cpp SudokuBoard.o
you tell the compiler to create an executable file named Driver.cpp out of the object file SudokuBoard.o. But as SudokuBoard.o has no main method, this will fail, therefore the error messages.
Try
gcc -c SudokuBoard.cpp
gcc -c Driver.cpp
gcc -o Sudoku SudokuBoard.o Driver.o
instead.

Answer 4

The compiler is telling you it can't find the implementation for std::basic_string<...>.
You need to tell the compiler how to find the compiled code for std::basic_string<...>. Much how with your own code, if you tried to build an executable out of just Driver.cpp the compiler would complain about missing SudokuBoard references until you told it about SudokuBoard.o.
In this case you need to tell the compiler about the C++ standard library. This library are usually shipped with your distribution. On Windows in the form of dll files, on OS X dylib files and on Linux so files. Conceptually they are similar, many bags of .o files all together. Either way, you tell the linker to link to the C++ standard library.
gcc calls the C++ standard library libstdc++, so your command would be something like: gcc -o Driver Driver.cpp SudokuBoard.o -lstdc++.