How to write std::string to file?

Question

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();`
}

Please show your code. In general, if correctly used, std::string is fine for this. — Useless
– Useless, Commented Mar 13, 2013 at 14:27
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
– Mats Petersson, Commented Mar 13, 2013 at 14:28

JSQuareD · Accepted Answer · 2023-11-08 06:49:13Z

178

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(), studentPassword.size());

edited Nov 8, 2023 at 6:49

answered Mar 13, 2013 at 14:31

JSQuareD

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5 Comments

Waddles Over a year ago

I had to add std::ios::binary for this to not have problems with newline characters

caoanan Over a year ago

out.close() ? twice? bad answer

JSQuareD Over a year ago

@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.

starriet 차주녕 Over a year ago

In the last example, is it right to multiply sizeof(char)? Does it work even if the sizeof(char) is not 1? I'm asking this because ofstream::write accepts count(number of characters to write), not the number of bytes to write.

JSQuareD Over a year ago

@starriet Actually, sizeof(char) == 1 by definition. I removed sizeof(char) from the example since it's unnecessary and confusing.

stefan · Accepted Answer · 2015-01-10 08:07:48Z

36

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;

edited Jan 10, 2015 at 8:07

answered Mar 13, 2013 at 14:31

stefan

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Comments

Tolini · Accepted Answer · 2013-10-15 21:13:40Z

0

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

answered Oct 15, 2013 at 21:13

Tolini

1

Comments

mheyman · Accepted Answer · 2022-09-30 16:30:52Z

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?

answered Sep 30, 2022 at 16:30

mheyman

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