Already defined in .obj - no double inclusions

You probably don't want to do this:

#include "client.cpp"

A *.cpp file will have been compiled by the compiler as part of your build. By including it in other files, it will be compiled again (and again!) in every file in which you include it.

Now here's the thing: You are guarding it with #ifndef SOCKET_CLIENT_CLASS, however, each file that has #include "client.cpp" is built independently and as such will find SOCKET_CLIENT_CLASS not yet defined. Therefore it's contents will be included, not #ifdef'd out.

If it contains any definitions at all (rather than just declarations) then these definitions will be repeated in every file where it's included.

This is not a compiler error: the error is coming from the linker. After compilation, the linker will merge the object files resulting from the compilation of each of your translation units (.cpp files).

The linker finds out that you have the same symbol defined multiple times in different translation units, and complains about it (it is a violation of the One Definition Rule).

The reason is most certainly that main.cpp includes client.cpp, and both these files are individually processed by the compiler to produce two separate object files. Therefore, all the symbols defined in the client.cpp translation unit will be defined also in the main.cpp translation unit. This is one of the reasons why you do not usually #include .cpp files.

Put the definition of your class in a separate client.hpp file which does not contain also the definitions of the member functions of that class; then, let client.cpp and main.cpp include that file (I mean #include). Finally, leave in client.cpp the definitions of your class's member functions.

client.h

#ifndef SOCKET_CLIENT_CLASS
#define SOCKET_CLIENT_CLASS
#ifndef BOOST_ASIO_HPP
#include <boost/asio.hpp>
#endif

class SocketClient // Or whatever the name is...
{

// ...

    bool read(int, char*); // Or whatever the name is...

//  ...
};

#endif

client.cpp

#include "Client.h"

// ...

bool SocketClient::read(int, char*)
{
    // Implementation  goes here...
}

// ... (add the definitions for all other member functions)

main.h

#include <iostream>
#include <string>
#include <sstream>
#include <boost/asio.hpp>
#include <boost/thread/thread.hpp>
#include "client.h"
//              ^^ Notice this!

main.cpp

#include "main.h"

I do recomend doing it in 2 filles (.h .cpp) But if u lazy just add inline before the function So it will look something like this

inline void functionX() 
{ }

more about inline functions:

The inline functions are a C++ enhancement feature to increase the execution time of a program. Functions can be instructed to compiler to make them inline so that compiler can replace those function definition wherever those are being called. Compiler replaces the definition of inline functions at compile time instead of referring function definition at runtime. NOTE- This is just a suggestion to compiler to make the function inline, if function is big (in term of executable instruction etc) then, compiler can ignore the “inline” request and treat the function as normal function.

more info here