Class as argument

[nime]

Hello,
I would like to pass class as argument to/from a function which read/write to file.
I am a c++ novice and try some ways but no one works for me.

header:

#include <string>

class MyClass
{
public:
	char     FullName[40];                
	char     CompleteAddress[120];        
	char     Gender;                      
	double Age;                         
	bool     mybool;    
};

std::string filename="myJunk.dat";
int RWrecord (bool RW, long record, MyClass mc);

function in other file

#include <fstream>
#include <iostream>
#include <string.h>
#include <string>
#include <io.h>
using namespace std;

int RWrecord (bool RW, long record, MyClass)
{
    MyClass mc;
 if (RW) //read or write 
    {
        ifstream ifs(filename.c_str(), ios::binary);
        ifs.seekg (sizeof(mc)*record, ios::beg);
        ifs.read((char*)&mc, sizeof(mc));
        ifs.close();
    }
    else
    {
        ofstream ofs(filename.c_str(), ios::binary);
        ofs.seekp (sizeof(mc)*record, ios::beg);
        ofs.write((char*)&mc, sizeof(mc));
        ofs.close();
    }
    return 0;
}

header is included in main file
from where I call function:

bool RW=false; //read
long record = 3;

int RWrecord (bool RW, long record, *MyClass)
//
// I would like class elements from record 3 here

but I get much errors during compiling.
Please help.

[laserlight]

One way is to define a function template with the class type for the template parameter. However, I suggest that you overload operator<< and operator>> for the class instead, so that MyClass objects can be read and written with std::istream and std::ostream.

[nime]

Thank you laserlight,
As a novice I need more concrete help to get this working with situation as presented.
When I have this code in my main function it works and I can read and write specific record to file.
But I have trouble when RWrecord is in other file.
And this is because I plan to use that function from more than one project.

[laserlight]

I will stick with my recommendation of overloading those operators. You should also be wary of the use of those I/O operations that interpret the object as a stream of bits as that can fail to be correct when the class is not a POD type, or has pointer members.

[Swarvy]

You could include a forward declaration of the class in your cpp file. That should work but laserlight's idea is better although slightly more technical to implement.

[nime]

Anyway, I still dont move forward with my example.
It seems I have more errors here.

In header bClass.h I have line:

std::string filename="myJunk.dat";

This header is included in my main file (main.cpp) and in my file with functions (bClass.cpp),
and compiler says:

Compiling: bClass.cpp
Compiling: Main.cpp
Linking console executable: bin\Release\aass.exe
obj\Release\Main.o:Main.cpp.bss+0x0): multiple definition of `filename'
obj\Release\bClass.o:bClass.cpp.bss+0x0): first defined here
collect2: ld returned 1 exit status
Process terminated with status 1 (0 minutes, 10 seconds)
0 errors, 0 warnings

How is this possible and what to do to fix this?

[whiteflags]

The header file is being included for every #include "yourheader.h" you make.

This leads to multiple definitions of various things as a matter of course.

The way to stop this is to use the preprocessor to make something called an "include guard", which is just a conditional #define (based on whether it is defined or not). C++ does not emulate smarter modules in other languages. That is why you must do this.

[nime]

I see.
Now I do this (I look to some example):

At top of header bClass.h I added:

#ifndef BCLASS_H
#define BCLASS_H

and at the end

#endif //BCLASS_H

But same happens.
Sure, filename is not defined nowhere else.

[laserlight]

Do not use a global variable. If you really do want it, you could declare it as extern in the header and define it in exactly one source file, but from what I see it is unnecessary here, and so should be avoided.

[Elysia]

Have you tried looking at where the compiler encountered it before? It did say it was encountered previously "here".

[nime]

Hello Elysia,
No, I cannot locate where.
This is "build log" which dont offer any debug possibility, just text.
Interesting is that in build messages, which can show where is error I got this

||=== Build finished: 0 errors, 0 warnings ===|

But this is not correct,
because when I run a program they want to build again and again.
I use CodeBlocks IDE and here is possible such errors because CB is still under developing.
But I need CB because of usability in Windows and Linux.

Is here any possibility to attach a project?

[nime]

I finally get solution which compiles well
header:

#ifndef BCLASS_H
#define BCLASS_H

class MyClass
{
public:
	char   FullName[40];    
	char   CompleteAddress[120];       
	char   Gender;                 
	double Age;                        
	bool   myBool;    
};

int RWrecord (bool RW, long record, MyClass *mc);
#endif //BCLASS_H

function file:

int RWrecord (bool RW, long record, MyClass *mc)
{
 std::string filename="myJunk.dat";
 if (RW)
    {
        std::ifstream ifs(filename.c_str(), std::ios::binary);
        ifs.seekg (sizeof(mc)*record, std::ios::beg);
        ifs.read((char*)&mc, sizeof(mc));
        ifs.close();
    }
    else
    {
        std::ofstream ofs(filename.c_str(), std::ios::binary);
        ofs.seekp (sizeof(mc)*record, std::ios::beg);
        ofs.write((char*)&mc, sizeof(mc));
        ofs.close();
    }
    return 0;
}

main:

int main()
{
	std::string filename="myJunk.dat";
	remove(filename.c_str());

    bool RW;
    long record;
	MyClass one;

	strcpy(one.FullName, "Row two");
	strcpy(one.CompleteAddress, "My row two adress");
	one.Gender = 'M';
	one.Age = 3.14;
	one.myBool = true;

    record = 2;
    RW = true;
    int RWrecord (bool RW, long record, MyClass *one);

//-------------------------------------------------------------------

    record = 2;
    RW = false;
    MyClass two;

    int RWrecord (bool RW, long record, MyClass *two);

	std::cout << "My Information\n";
	std::cout << "My Name: " << two.FullName << std::endl;
	std::cout << "Address:      " << two.CompleteAddress << std::endl;

	if(two.Gender == 'f' || two.Gender == 'F')
		std::cout << "Gender:       Female" << std::endl;
	else if(two.Gender == 'm' || two.Gender == 'M')
		std::cout << "Gender:       Male" << std::endl;
	else
    std::cout << "Gender:       Unknown" << std::endl;

    std::cout << "Age:          " << two.Age << std::endl;
	return 0;
}

1 delete file if exist
2 fill data values to copy of class "one"
3 set RW to write and record to 2
4 call RWrecord to write data to record 2 (171 byte from beggining of file)
5 call RWrecord to read data from record 2 to new variable "two"
6 print data from variable two to console

Program compiles well
but my data dont passes correct
I assume that problem lies in my general missunderstanding of strings and pointers.
So please if someone could correct my code to get it to working condition.
Thanks.