Write multiple data in a binary file

[9hU4216C]

Hi, I want to write multiple types of data in a c++ binary file using the .write() method and read it using the .read() method. Here's my code:

  #define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main() {

	const int MAX = 30;
	int length = 0;

	//Input:
	char buffer[MAX];
	char buffer2[MAX];
	int number = 10;
	int number2 = 20;

	//Output:
	char r_buffer[MAX];
	char r_buffer2[MAX];
	int r_number = 0;
	int r_number2 = 0;

	ofstream file;
	file.open("binary.bin", ios_base::out | ios_base::binary);

	if (!file.is_open()) {
		cout << "Something went wrong while creating the file." << endl;
	}
	else {
		strcpy(buffer, "First input in the file.");
		strcpy(buffer2, "Second input in the file.");

		file.write((char*)&buffer, sizeof(buffer));
		file.write((char*)&number, sizeof(int));
		file.write((char*)&buffer2, sizeof(buffer2));
		file.write((char*)&number2, sizeof(int));

		file.close();
	}

	ifstream reader;
	reader.open("binary.bin", ios_base::in | ios_base::binary | ios_base::app);

	if (!reader.is_open()) {
		cout << "Something went wrong while openning the file." << endl;
	}
	else {
		int length = 0;

		reader.seekg(sizeof(buffer), ios_base::beg);
		length = reader.tellg();

		reader.read((char*)r_buffer, length);

		reader.close();

		string sentense = r_buffer;

		cout << sentense << endl; //outputs a blank space
	    
	}

	system("PAUSE");
	return 0;
}

I don't get what I am doing wrong. I am sorry if this topic has already been discused but I looked on many forums and never saw any clear response for my problem.

Thank you!

[theturk1234]

What's it doing or not doing? Explain the problem. Just by looking at the code, it seems to be correct.

[Naughty Albatross]

Hi,
Try putting these things in appropriate places :

1.

const int MAX = 30;

struct myData
{
     myData() : number(10) {memset(buffer, 0, sizeof(buffer));}

     char buffer[MAX];
     int number;
};

myData data, data2;
myData r_data, r_data2;

2.

strcpy(data.buffer, "First input in the file.");
		strcpy(data2.buffer2, "Second input in the file.");
		file.write((char*)&data, sizeof(data));
		file.write((char*)&data2, sizeof(data2));

3.

    int length = 0;
	reader.seekg(0, reader.end);
	length = reader.tellg();
	if(length != sizeof(myData) * 2) cout << "Invalid input file size - " << length << endl;

	reader.read((char*)&r_data, sizeof(r_data));
	reader.read((char*)&r_data2, sizeof(r_data2));	reader.close();

	string sentence = r_data.buffer;
		cout << "My meat is " << sentence << endl; // Outputs a blank space (?) 

[Naughty Albatross]

For your information :
reader.read((char*)r_buffer, length);

This line can potentially cause segfaults.

[48T7M4Gy]

.

[AbstractionAnon]

Can I ask why I need to cast in (char*) char arrays when I want to read them but not when I want to write them in the file?

The types of r_buffer1 and r_buffer are char * so the casts are not necessary.

BTW, there is an error on line 50. The & should not be there.

[Naughty Albatross]

Good we were able to help.

[48T7M4Gy]

.

[AbstractionAnon]

There is no error on line 50

If you remove the unnecessary cast, you indeed see that there is an error.

1>c:\dev\junk\main.cpp(50) : error C2664: 'std::basic_istream<_Elem,_Traits>::read' : cannot convert parameter 1 from 'char (*)[80]' to 'char *'

[48T7M4Gy]

.

[AbstractionAnon]

The claim about removing the & is about as useful as alerting programmers to the effects of removing ; at the end of the line.

You're missing the point(er).

r_buffer2 "decays" to a pointer to the first character of the array. Placing the & in front of r_buffer2 takes the address of that pointer resulting in a pointer to a pointer. The extraneous cast masks the error by telling the compiler to ignore the type mismatch. IOW, read is expecting a char pointer, but is being passed a pointer to a pointer. The read is going to read into memory at the wrong place.