CODEWITHSUNDEEP
c++
boom
boom

Reputation: 6166

duplicate symbol error C++

I have added some const character in my file as under. The error i get is duplicate symbol _xyz(say). What is the problem with it and how could i get out of this.

const char* xyz = "xyz";
class Abc
{
public:
    Abc()
    {
    }
};

Upvotes: 46

Views: 59854

Answers (4)

egrunin
egrunin

Reputation: 25053

If this is in a header file, you're defining xyz every time you #include it.

You can change the declaration as @R Samuel Klatchko shows. The usual way (if the data isn't const) is like this:

In Abc.h:

extern char *xyz;

In Abc.cpp:

char *xyz = "xyz";

Edited to add

Note that header guards will not solve this problem:

#ifndef XYZ_H
#define XYZ_H
...
#endif

Header guards prevent "redefinition" errors, where the same symbol appears twice in the same compilation unit. That's a compiler error.

But even with header guards the definition of xyz will still appear in every source file that includes it, causing a "duplicate symbol" error, which is a linker error.

Upvotes: 78

abhinavrathore019
abhinavrathore019

Reputation: 6635

My use-case:

  • I had multiple header files a.hpp, b.hpp, and, c.hpp which contained some utility methods.
  • I had a file util.hpp which acted as an aggregator for the above files.

In my case, the extern did not work but static worked.

I had to use:

  • header guards to avoid errors in Visual Studio code.
  • static with functions to avoid compile-time errors.

Check out this article too.

Upvotes: 0

Eric Wiener
Eric Wiener

Reputation: 5947

I also ran into this issue, but for me the solution was different. I had put overloaded operators (==, !=, <<) in my header file and implemented them. This was causing an issue in other files where I also used ==, !=, or <<. To solve this, I moved the implementation into the .cpp file and left the declaration in the header file.

Edit:

This can also be caused if:

  • You are including .cpp files instead of .h files. You can fix this by switching the import to use .h instead of .cpp.
  • You are implementing static functions inside a header file (outside of the class declaration). You can fix this by moving the implementations into a .cpp file (among other solutions).

Upvotes: 13

R Samuel Klatchko
R Samuel Klatchko

Reputation: 76531

The problem is every source file that includes your header file gets it's own copy of xyz with external linkage.

The easiest way to fix that is to give xyz internal linkage. You can do that by making the pointer itself const in addition to having the underlying char's const:

const char* const xyz = "xyz";

Upvotes: 12

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