C++ Class redefinition error - I cannot figure this out for the life in me

Question

Any help on why I am getting a 'C2011 'Transaction':'class' type redefinition? I'm sure it's glaringly obvious but I cannot for the life of me figure it out. Please help.

transaction.h

#include <string>

class Transaction
{
private:
    int amount;
    std::string type;

public:
    Transaction(int amt, std::string kind);
    std::string Report();
};

transaction.cpp

#include "transaction.h"

using namespace std;

Transaction::Transaction(int amt, std::string kind):amount(amt), type(kind)
{
}

string Transaction::Report()
{
    string report;
    report += "  ";
    report += type;
    report += " ";
    report += to_string(amount);

    return report;
}

You need an [include guard](http://stackoverflow.com/questions/8020113/c-include-guards) in transaction.h. — Mohamad Elghawi, Nov 02 '15 at 11:26
You will have to forgive me Mohamad, I am relatively new to this; what is a header guard and how would I implement it? — Arron Taylor, Nov 02 '15 at 11:27
Try `#pragma once` at the top of the header file to see if it works for you. — Linus, Nov 02 '15 at 11:27
You absolute beauty, thank you for the help Linus, that works fine! — Arron Taylor, Nov 02 '15 at 11:29

score 5 · Accepted Answer · answered Nov 02 '15 at 11:29

5

You can either use a header guard in the header file which will make sure you never ever more than once define a class or struct etc. in any other cpp file.

To add a header guard you can simply do this:

#ifndef TRANSACTION_H
#define TRANSACTION_H
// your header file
#endif

Or simply add

#pragma once

to all your header files and you're good.

answered Nov 02 '15 at 11:29

Linus

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Thanks for the help Linus; much appreciated. – Arron Taylor Nov 02 '15 at 11:35
Didn't see that you gave the answer in the comments first. Upvote for giving the two possibilities – LBes Nov 02 '15 at 11:38

score 2 · Answer 2 · answered Nov 02 '15 at 11:29

You need to use include guards in transaction.h:

#if !defined(your_symbol)
#define your_symbol 1
/*ToDo - code here*/
#endif

Where, your_symbol is typically an embellishment of the name of the file. Be careful not to use a leading double underscore or a single leading underscore followed by a capital letter as they are reserved symbols.

This prevents the class declaration from being included more than once in any compilation unit.

You can use #ifndef your_symbol in place of my first line, and drop the 1 from the second line, or perhaps even just use a #pragma once directive at the top of the file, but the version I present works on every compiler I've ever come across.

score 0 · Answer 3 · answered Nov 02 '15 at 11:28

In your header try this:

#ifndef TRANSACTION  //If transaction has never been defined before
#define TRANSACTION  //Define transaction
//Content of your header
#endif

This header guard will probably help you. Indeed it will prevent your header from being included multiple times which leads to redefinitions. If you include your header just once you don't need these guards, but it doesn't hurt to have them anyway.

Alternatively, you can use #pragma once at the beginning of your header.

If you want to read more about them, check wikipedia

C++ Class redefinition error - I cannot figure this out for the life in me

3 Answers3