Undefined reference to my function - c++

Question

I have some trouble with building the part of my project. Here it is:
utility.h:
#ifndef UTILITY_H
#define UTILITY_H
#include <boost/array.hpp>
#include <boost/shared_ptr.hpp>
#include <fstream>
#include <iostream>
#include <openssl/md5.h>
#include <string>
namespace utility {
typedef boost::array<unsigned char, MD5_DIGEST_LENGTH> md5sum;
typedef boost::shared_ptr<md5sum> md5sum_ptr;
...
md5sum_ptr calculate_md5(const std::string& full_path);
};
#endif //UTILITY_H
utility.cpp:
#include "utility.h"
using namespace utility;
...
md5sum_ptr calculate_md5(const std::string& full_path) {
md5sum_ptr md5(new md5sum);
std::ifstream in(full_path);
std::string content;
in >> content;
MD5((unsigned char*)content.data(), content.size(), md5->data());
return md5;
}
cdaemon.cpp
void CTCPConnection::handle_read_filename(ECommand command, EDataType datatype,
const boost::system::error_code& ec) {
...
case ECommand::GET_MD5:
_send_container<md5sum>(*(calculate_md5(filename)));
async_write(m_socket, m_writebuf,
boost::bind(&CTCPConnection::handle_write_response, shared_from_this(),
placeholders::error));
break;
...
}
}
Makefile:
CC=g++
override CFLAGS+=-c -std=c++11 -Wall -Wextra -Werror
override LFLAGS+=-lboost_log -lpthread -lboost_system -lboost_thread -lcrypto
server: servermain.o cdaemon.o
$(CC) ../build/datatype.o ../build/utility.o build/servermain.o build/cdaemon.o -o build/frtpdaemon $(LFLAGS)
servermain.o: main.cpp
$(CC) $(CFLAGS) main.cpp -o build/servermain.o
cdaemon.o: cdaemon.cpp
$(CC) $(CFLAGS) -DBOOST_LOG_DYN_LINK cdaemon.cpp -o build/cdaemon.o
When I try to build server, I get linker error:
build/cdaemon.o: In function `CTCPConnection::handle_read_filename(utility::ECommand, utility::EDataType, boost::system::error_code const&)':
cdaemon.cpp:(.text+0x127c): undefined reference to `utility::calculate_md5(std::string const&)'
I have already tried to google the solution, but all what I could find were suggestions to change the order of ../build/utility.o and build/cdaemon.o. That didn't help me.

Answer 1

You don't define the function you have declared in the header. A using directive is insufficient when defining functions in a namespace. You'll either need to open the namespace before defining your function or qualify the function with the namespace, e.g.:
utility::md5sum_ptr utility::calculate_md5(const std::string& full_path) {
...
}