[SOLVED] Question about g++ multiple files compilation

[leileicats]

Just a simple, whilst unexpected compilation error:

I have three simple files:
1, main.cpp

#include <iostream>
using namespace std;
#include "helloworld.h"

int main(){
hello_world();
return 0;
}
2, helloworld.h

#ifndef _HELLOWORLD_H
#define _HELLOWORLD_H

#include <iostream>
using namespace std;

void hello_world();

#endif
3,helloworld.cpp

#include "helloworld.h"

void hello_world(){
cout << "Hello World!\n";
}

When I was trying to compile, I got this error:

lei@maverick:~/code/example/HelloWorld$ g++ -o main main.cpp helloworld.h
/tmp/ccixBBMs.o: In function `main':
main.cpp:(.text+0x84): undefined reference to `hello_world()'
collect2: ld returned 1 exit status

I guess there is something wrong with the function prototype, but I can't find it.:confused:

[slavik]

1. don't use "using namespace std;" in header files
2. include header files BEFORE "using namespace std;" line in main.cpp
3. your compile line should be "g++ -o hello main.cpp helloworld.cpp"

[Kadrus]

1. don't use "using namespace std;" in header files
2. include header files BEFORE "using namespace std;" line in main.cpp
3. your compile line should be "g++ -o hello main.cpp helloworld.cpp"

your compile should be g++ hello main.cpp
without the -o

[leileicats]

1. don't use "using namespace std;" in header files
2. include header files BEFORE "using namespace std;" line in main.cpp
3. your compile line should be "g++ -o hello main.cpp helloworld.cpp"

I removed "using namespace std;" from "helloworld.h", and then include "#include "helloworld.h" BEFORE "using namespace std;" line in main.cpp

#include "helloworld.h"
#include <iostream>
using namespace std;
The result after compiling as shown:

lei@maverick:~/code/example/HelloWorld$ g++ -o main main.cpp helloworld.cpp
helloworld.cpp: In function ‘void hello_world()’:
helloworld.cpp:6: error: ‘cout’ was not declared in this scope


It seems that the code don't know about "iostream" before compiling function "void hello_world()". This is weired. So I have to add:

#include <iostream>
using namespace std;



#include "helloworld.h"
#include <iostream>
using namespace std;

void hello_world(){
cout << "Hello World!\n";
}
Then it works! But it is tedious if I need add "iostream" library in every source file.

[hod139]

your compile should be g++ hello main.cpp
without the -o

This is wrong advice, slavik was correct. The -o flag tells g++ to name the executable hello.