Single class has a Class Redefinition Error

Question

I'm new to C++, and I'm having a problem with my class definitions in a header file. The code for the header file (Student.h) is:

#include <string>
using namespace std;

class Student
{
  // Data Members for a Student
  string id;
  string preferences[3];
  int skill;

  // Constructor
 public:
  Student(){}

 public:
  void SetID(string str)
  { this->id = str; }
 public:
  void SetSkill(int i)
  { this->skill = i; }
 public:
  void SetPreferences(int i, string s)
  {
    this->preferences[i] = s;
  }
};

class StudentSchedule
{
 public:
  StudentSchedule(){}
};

The compiler error says that line 14 (class Student) is a redefinition of 'Student', and that line 15 ({ -- the open brace following class Student) is the previous definition of 'Student'. The same error on the first two consecutive lines exists for the StudentSchedule class.

I have no .c, .cpp, or .h files anywhere in my compilation that define either class. I have no idea why I'm getting this error.

Answer 1

You need header guards on that header file. It is presumably being included twice.

Modify the header, adding these lines to the beginning and end.

#ifndef STUDENT_H
#define STUDENT_H

// Put the entire contents of your header here...

#endif

The define doesn't need to be STUDENT_H... it just needs to be unique.

With these directives added, the compiler will ignore all contents of the header file if it has already been parsed.

Alternatively, while it is not standard C++, all major compilers will allow you to put a single

#pragma once

as the first line of the header to prevent it from being parsed multiple times.

Answer 2

You're probably including the .h file twice, the first time it will define Student, the second it will try to redefine it.

See the Wikipedia entry on include guards for a more extensive explanation of the problem and information on how to avoid it.

In short, there are two ways to do it

Version 1, #defined include guards

#ifndef STUDENT_HPP
#define STUDENT_HPP

...your code here...

#endif

Usually the #define is called some variation of the file name since it has to be different in every include file.

Version 2, #pragma once

#pragma once

...your code here...

This pragma is (as most pragmas) not portable to all compilers, but some of the most important ones. It also has the advantage of not needing a manually assigned name.

Which you use is up to you, but you most likely have to pick one :)

Answer 3

When I learnt c++ our professor said, the first two lines that you write in c++ class should always be #ifndef followed by #define. This prevents multiple definitions in header files

http://www.fredosaurus.com/notes-cpp/preprocessor/ifdef.html

Answer 4

I prefer using

   #pragma once

as the first line of a header file, instead of the defines. Even if this is non-standard, it does avoid name clashes and can reduce compile time.