c++ - why does it matter if the #include are in the .h file or in .cpp file?

Question

I have a cpp file contains this include:

#include "twitServer.h"

and in twitServer.h I have:

#ifndef twitServer_twitServer_h
#define twitServer_twitServer_h
#include <string>
#include <map>
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <list>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cstring>
#include <netdb.h>
#include <errno.h>
#include <sstream>
#include <algorithm>
#include <ctime>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include "twitUser.h"

using namespace std;

void startServer(string port);

#endif

But the Xcode says for this line:

if ((rv = getaddrinfo(NULL, port, &hints, &ai)) != 0) {
    fprintf(stderr, "selectserver: %s
", gai_strerror(rv));
    exit(1);
}

that the getaddrinfo is not defined... why that?

if the includes are in the cpp file it works fine how comes

Answer 1

Not sure how to fix your problem, but by convention, you should never include in the header file, because you might write code some day, needing your header file, but not all the includes.