117

I want to write a std::string variable I am accepting from the user to a file. I tried using the write() method and it writes to the file. But when I open the file I see boxes instead of the string.

The string is only a variable length single word. Is std::string suitable for this or should I use a character array or something.

ofstream write;
std::string studentName, roll, studentPassword, filename;


public:

void studentRegister()
{
    cout<<"Enter roll number"<<endl;
    cin>>roll;
    cout<<"Enter your name"<<endl;
    cin>>studentName;
    cout<<"Enter password"<<endl;
    cin>>studentPassword;


    filename = roll + ".txt";
    write.open(filename.c_str(), ios::out | ios::binary);

    write.put(ch);
    write.seekp(3, ios::beg);

    write.write((char *)&studentPassword, sizeof(std::string));
    write.close();`
}
laike9m
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Slashr
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    Please show your code. In general, _if_ correctly used, `std::string` is fine for this. – Useless Mar 13 '13 at 14:27
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    You need to save the 'payload' CONTENT of the string, not the string object as such (which typically contains just a length and a pointer to the actual content) – Mats Petersson Mar 13 '13 at 14:28

4 Answers4

169

You're currently writing the binary data in the string-object to your file. This binary data will probably only consist of a pointer to the actual data, and an integer representing the length of the string.

If you want to write to a text file, the best way to do this would probably be with an ofstream, an "out-file-stream". It behaves exactly like std::cout, but the output is written to a file.

The following example reads one string from stdin, and then writes this string to the file output.txt.

#include <fstream>
#include <string>
#include <iostream>

int main()
{
    std::string input;
    std::cin >> input;
    std::ofstream out("output.txt");
    out << input;
    out.close();
    return 0;
}

Note that out.close() isn't strictly neccessary here: the deconstructor of ofstream can handle this for us as soon as out goes out of scope.

For more information, see the C++-reference: http://cplusplus.com/reference/fstream/ofstream/ofstream/

Now if you need to write to a file in binary form, you should do this using the actual data in the string. The easiest way to acquire this data would be using string::c_str(). So you could use:

write.write( studentPassword.c_str(), sizeof(char)*studentPassword.size() );
JSQuareD
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    I had to add std::ios::binary for this to not have problems with newline characters – Waddles Oct 31 '16 at 00:41
  • out.close() ? twice? bad answer – caoanan Dec 29 '21 at 10:17
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    @caoanan it's to demonstrate the API. I also mention in the text of the body that it happens automatically when the variable goes out of scope. At any rate, `close()` is idempotent, so there's no harm in it being called multiple times. – JSQuareD Jan 08 '22 at 18:46
32

Assuming you're using a std::ofstream to write to file, the following snippet will write a std::string to file in human readable form:

std::ofstream file("filename");
std::string my_string = "Hello text in file\n";
file << my_string;
stefan
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0

remove the ios::binary from your modes in your ofstream and use studentPassword.c_str() instead of (char *)&studentPassword in your write.write()

Tolini
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0

If you have fmt available:

#include <fmt/os.h>
// ...
    fmt::output_file(filename).print("{}\0\0{}", ch, studentPassword);
// ...

But you are not really writing a password to a file, right?

mheyman
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