Multiple definition of a const char*

Question

I get the above message linker error for a global

const char* HOST_NAME = "127.0.0.1";

I don't think that I have compiled some files twice but here's my definition of the files anyway.

main.cpp

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include "connection.hpp"

connection.cpp

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <sys/socket.h>
#include <sys/types.h>
#include <netdb.h>
#include <arpa/inet.h>
#include "connection.hpp"

connection.hpp

#ifndef __connection__
#define __connection__
#include <unistd.h>
#include <netinet/in.h>

const int BUFFSIZE = sysconf(_SC_PAGESIZE);             //Define page size
const char* HOST_NAME = "127.0.0.1";                    //Local host
//Definition of a class
#endif

Any help?

Answer 1

You use wrong declaration for your string. You need to make your string a constant, since constants may be defined in several compilation units. This is why compiler does not report the same error for BUFFSIZE: BUFFSIZE is const, so it may be defined several times in different compilation units. But HOST_NAME is not const, so it is reported. HOST_NAME will be const if you change its declaration to

const char* const HOST_NAME = "127.0.0.1"; 

Then the error should disappear.

---

[C++11: 3.5/3]: A name having namespace scope (3.3.6) has internal linkage if it is the name of

• a variable, function or function template that is explicitly declared static; or,
• a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage; or
• a data member of an anonymous union.

This effectively makes the constant "local" to each translation unit in which it is defined, removing the opportunity for conflict.

Answer 2

don't think that I have compiled some files twice

Nevertheless that's exactly what happened. You have compiled connection.hpp several times, each time you have # included it into some translation unit.

Either add static to the declaration, or add extern to it, delete the = somestring portion, and provide a definition in exactly one source file.

Answer 3

You have included "connection.hpp" to both connection.cpp and main.cpp. Therefore it (const char* HOST_NAME = "127.0.0.1";) is defined in 2 cpp files.