iostream, fstream which do I need for handling files?

[Somelauw]

C++ is really confusing.

#include <cstdlib>
#include <iostream>
//#include <fstream>
#include <cstring>
using namespace std;

int crypt(char* file, float seed)
{    
    //Open the files for encryption
    ofstream input(file, ios::in|ios::binary|ios::ate);
    long size = input.tellg();
    input.seekg(0, ios::beg);
    
    char* outputname; tmpnam(outputname);
    ifstream output(outputname, ios::out|ios::binary);
    
    //Now do the encryption itself
    char* byte;
    for(int i = 0; i <= size; i++)
    {
        output.read(byte, 1);
        input.write(byte, 1);
    }
}

In case I include <fstream>:
C:\Documents and Settings\Laptop\Mijn documenten\letters wisselen.cpp:29: error: 'struct std::ofstream' has no member named 'tellg'
C:\Documents and Settings\Laptop\Mijn documenten\letters wisselen.cpp:30: error: 'struct std::ofstream' has no member named 'seekg'

In case I exclude it:
C:\Documents and Settings\Laptop\Mijn documenten\letters wisselen.cpp:28: error: variable `std::ofstream input' has initializer but incomplete type
C:\Documents and Settings\Laptop\Mijn documenten\letters wisselen.cpp:33: error: variable `std::ifstream output' has initializer but incomplete type

So what am I doing wrong?

Also I would like some advice on the way I open files and coding habits.

[Scipio]

You need to include fstream to use it functions.

I think you need to replace tellg and seekg with tellp and seekp? Otherwise, read this:
http://www.cplusplus.com/reference/iostream/fstream/

[Somelauw]

Thanks you are right:
tellp and seekp are for input
tellg and seekg are for ouput

fstream needs to be included although it gets partly included when including iostream