Problums writeing struct to a file C++

hi all, C++ beginner needs help.

I decided today to give C++ FileIo Streams a little go. since I not played with them much before and have not started out very well :(

anyway I desided to make a little project that can save the contents of a struct to a file. pretty basic in fact.

My problum is reading back the info. the letter and number displays ok but for some reason str is not. anyway I left my code below if that helps.

VB Code:

#include <iostream>
#include <fstream>
using namespace std;
 
struct test
{
    int number;
    char letter;
    char *str;
} aTest;
 
 
int main(int agvc)
{
    ofstream fout;
    ifstream fin;
    // fill struct with some random data
    aTest.number = 245;
    aTest.letter = 'A';
 
    //What am I doing wrong here I wonder
    aTest.str = (char*) malloc(100);
    strcpy(aTest.str,"Hello World this is a long string");
 
 
    //write the struct to the file
    
    fout.open("C:\\test.txt",ios::binary);
    fout.write((char*)(&aTest),sizeof(aTest));
    fout.close();
    
    // Clear struct of it's data I am only new in C++
    // So i am not sure this is the correct way to clear stuff out
    // tho for the example it be ok I think
    aTest.letter = '\0';
    aTest.number = 0;
    strcpy(aTest.str,"\0");
    //Load the struct back in so we can display the data
 
    fin.open("c:\\test.txt",ios::binary);
    fin.read((char*)(&aTest),sizeof(aTest));
    fin.close();
 
    // display the data
    std::cout << "Number: " << aTest.number << "\n";
    std::cout << "Letter: " << aTest.letter << "\n";
    std::cout << "String: " << aTest.str << "\n"; // :( why is my str not showing
 
}

anyway hope someone can help
Thanks.

well i haven't tried your code but i think u r making 1 mistake n not writing 1 statement:

1) u r closing the file before displaying the values, and
2) use getch(); [header file conio.h] at the end of the code, before the closing bracelet of main().

i think that shud work!!! maybe closing file after wont change anything but yes, u need to put getch() in your code!!!

hi, Harsh

I am not sure what you mean. all data has been written to the file. that's why I am closeing it.

I only cleared the struct information once it's been saved to the file. and then I reload the file back in.

The problum is eveything else in the struct is showing all except this line.

std::cout << "String: " << aTest.str << "\n"; // why is my str not showing

any other ideas.

Quote:

---

Originally Posted by dreamvb

hi, Harsh

I am not sure what you mean. all data has been written to the file. that's why I am closeing it.

I only cleared the struct information once it's been saved to the file. and then I reload the file back in.

The problum is eveything else in the struct is showing all except this line.

std::cout << "String: " << aTest.str << "\n"; // why is my str not showing

any other ideas.

---

sorry didn't read the whole post, my bad!!!

maybe in that case there is something wrong in

VB Code:

strcpy(aTest.str,"\0");

just comment it out n then try compiling it.........probably it may work!!!!

You're saving a pointer, not the character data. You need to save character data independently, usually with the length saved first.

Hi CornedBee,

Can you give me a little example as I am still really leraning. if you have time thanks.

bump*

Writing the string

Code:

---

char *str;
int stringLen = 100;

str = new char[stringLen];  // Allocate memory the C++ way if you're using C++
memset(str, 0, stringLen);  // The new operator doesn't clear memory automatically, and you certainly don't want to have garbage in your file
strcpy(str, "This is a long string that should be less than 100 chars");  // The string to be copied should be at most 99 chars

std::ofstream fOut("c:\\temp\\myfile.dat", std::ios::binary);
// Check if the file has opened, etc...
fOut.write(reinterpret_cast<char *>(&stringLen), sizeof(int));  // Using reinterpret_cast, so this ugly syntax reminds you that you're doing something that you shouldn't, but have to. It's also the proper C++ way to cast
fOut.write(str, stringLen);
fOut.close();

delete[] str;  // Clear the allocated memory

---

Alternatively, you can calculate the length of the actual string (stringLen = strlen(str), and write that value to the file.

Reading the string:

Code:

---

char *str;
int stringLen;
std::ifstream fIn("c:\\temp\\myfile.dat", std::ios::binary);
// Check if the file has opened, etc...
fIn.read(reinterpret_cast<char *>(&stringLen), sizeof(int));  // Read the length first
str = new char[stringLen + 1];  // Allocate sufficient memory, including the null character
fIn.read(str, stringLen);
fIn.close();

// Note that when you didn't calculate the new value for stringLen in the above code, this stringLen will hold a value which is larger than the actual string size
std::cout << "String length: " << stringLen << std::endl;
std::cout << "String value: " << str << std::endl;
delete[] str;

---

Hope this helps

In case you want this to be a part of a struct/class, you should create methods named "read" and "write" (or what you prefer). Since these methods are part of the struct/class, they have the knowledge how to read from/write to the file.

If the file also contains information that doesn't belong to this class, then the calling code should be responsible for opening/closing the file and reading/writing other information.

Quote:

---

Originally Posted by dreamvb

fout.open("C:\\test.txt",ios::binary);

---

I wouldn't give a binary file a TXT extension ;)