include library header in header files

I was working with xemacs in unix. One of my c++ class header file has a string variable. E.g.:

document.h:

Code:

---

#ifndef DOC_H
#define DOC_H

#include <string>
using std::string;

class Document
{
public:
/* ... */
private:
String name;
};

#endif

---

Notice the #include <string>. I had to place that or else the compiler doesnt recognize the string variable. Why is it so? I never had to do that in VC++. Can't i just include the string header before this header file in the implementation file?

E.g:
document.cpp:

Code:

---

#include <iostream>
#include <string>
#include "document.h"

using namespace std;

/*.........Implementations of class document....*/

---

When i use a string the same way you described, i had to use the using-stament the same way in microsoft VC 6 too. :confused: But i must warn you, never to use a using-stament in header-files. Its a real bad habit, because you force every user of your header/lib to live together with your using-statement. Its much better to always use the complete "path" in your header, for example std::string, std::vector an so on. Use using-statments only in cpp-files.

mfg JJ

Yes thanks, i am aware of that namespace danger. My program was just a small application, so i was lazy to specify the namespace. But it's really strange. I never had to include a library header in my own header files before. Really wish someone could tell me why.

Sometimes some library headers include other library headers inside them. However, this depends on each library implementation and cannot and should not be counted on.

One example is in VC++ 6.0, <iostream> includes portions of <string>. Unfortunately, this leads people to believe that they don't need the <string> header to use std::string, and later on they get errors trying to output the string without using c_str(), among other potential problems.

So you should include the header file that contains the declarations you need even if it works without it.

document.cpp:

Code:

---

#include <iostream>
#include <string>
#include "document.h"

using namespace std;

/*.........Implementations of class document....*/

---

I included it in document.cpp. But my question is why must i include it in the class header "document.h" in order for it to recognize string variables.

Quote:

---

Originally Posted by Raison

document.cpp:

Code:

---

#include <iostream>
#include <string>
#include "document.h"

using namespace std;

/*.........Implementations of class document....*/

---

I included it in document.cpp. But my question is why must i include it in the class header "document.h" in order for it to recognize string variables.

---

When the compiler reads document.h, it doesn't know about <string>

Each file is treated separately by the compiler (preprocessor, in this case) and only knows what you tell it in that file. So in document.h, if you tell it to #include <string>, now it knows everything in <string> and all is OK, otherwise: no joy.

Regards,


Dave

I always tot the compiler knows how to link them together. Guess I will start including headers inside the headers from now on