编译器/链接器错误“未定义的引用”

Question

Hi I am just starting to learn C++. I bought this big C++ for Dummies book and have been going through it. Its been really interesting so far but now I am stuck. I have been googling this problem, but to no avail. I am using I am using codeblocks 10.05 with GNU GCC.

I keep getting an error that says:

In function 'main':
undefined reference to 'SafeCracker(int)'

The code isn't complicated. I am just new and am extremely frustrated. I don't want to skip over this part; I want to know what is going on.

Main:

#include <iostream>
#include "safestuff.h"

using namespace std;

int main()
{
  cout << "Surprise, surprise!" << endl;
  cout << "The combination is (once again)" << endl;
  cout << SafeCracker(12) << endl;
  return 0;
}

Function:

#include <iostream>

using namespace std;

string SafeCracker(int SafeID)
{
    return "13-26-16";
}

Header:

using namespace std;

#ifndef SAFESTUFF_H_INCLUDED
#define SAFESTUFF_H_INCLUDED

 string SafeCracker(int SafeID);

#endif // SAFESTUFF_H_INCLUDED

Answer 1

You are not compiling the second file you listed along with the first one. Try compiling directly with gcc to understand this.

assuming your files are named:

• main.cpp
• SafeCracker.cpp
• safestuff.h

This is what you are doing

gcc main.cpp

While you should be doing this

gcc main.cpp SafeCracker.cpp

Also, SafeCracker.cpp should be including the header file as well, just for clarity. Any reasons why you have them separated?

On another note, from seeing Daniel Hu's answer, <iostream> is automatically including <string> for you. You should not depend on this functionality, and should instead include <string> in each file that uses strings.

(From comment below) You're probably trying to build your main.cpp as a stand-alone file. This will leave SafeCracker.cpp uncompiled. What you need is create a project in Codeblocks and add all three files to it (both *.cpp files as well as the *.h file).

Answer 2

I think it's because you did not #include <string>

C++ has to import the string library to use strings or else everything is treated as char arrays.