Problems with file i/o using strings in c++ [duplicate] - c++

Question

Im having problems writing string into a binary file. This is my code:
ofstream outfile("myfile.txt", ofstream::binary);
std::string text = "Text";
outfile.write((char*) &text, sizeof (string));
outfile.close();
Then, I try to read it,
char* buffer = (char*) malloc(sizeof(string));
ifstream infile("myfile.txt", ifstream::binary);
infile.read(buffer, sizeof (prueba));
std::string* elem = (string*) buffer;
cout << *elem;
infile.close();
I just cant get it to work. I am sorry, I am just desperate. Thank you!

Answer 1

To write a std::string to a binary file, you need to save the string length first:
std::string str("whatever");
size_t size=str.size();
outfile.write(&size,sizeof(size));
outfile.write(&str[0],size);
To read it in, reverse the process, resizing the string first so you will have enough space:
std::string str;
size_t size;
infile.read(&size, sizeof(size));
str.resize(size);
infile.read(&str[0], size);
Because strings have a variable size, unless you put that size in the file you will not be able to retrieve it correctly. You could rely on the '\0' marker that is guaranteed to be at the end of a c-string or the equivalent string::c_str() call, but that is not a good idea because
you have to read in the string character by character checking for the null
a std::string can legitimately contain a null byte (although it really shouldn't because calls to c_str() are then confusing).

Answer 2

the line
outfile.write((char*) &text, sizeof (string));
is not correct
sizeof(string) doesn't return the length of the string, it returns the sizeof the string type in bytes.
also do not cast text to char* using a C cast, you can get hold of the char* by using the appropriate member function text.c_str()
you can simply write
outfile << text;
instead.

Answer 3

Why are you using pointers to std::string class?
You should not use sizeof with std::string, as it returns the size of the std::string object, and not the real size of the string inside.
You should try:
string text = "Text";
outfile.write(text.c_str(), text.size());
or
outfile << text;

Answer 4

Should probably also use c_str() to get the char pointer too, instead of that straight crazy cast.

Answer 5

Your code is wrong wrong way you are using to write & read the file
and file extension error you are trying to read text file .txt
correct code
Write to file
std::string text = "Text";
ofstream outfile("myfile.dat", ofstream::binary | ios::out);
outfile.write(&text,sizeof (string));//can take type
outfile.write(&text,sizeof (text));//can take variable name
outfile.close();
reading file
char* buffer = (char*) malloc(sizeof(string));
ifstream infile("myfile.dat", ifstream::binary | ios::in);
infile.read(buffer, sizeof (prueba));
std::string* elem = (string*) buffer;
cout << *elem;
infile.close();
Try This it will work

Answer 6

I had the same problem. I found the perfect answer here: Write file in binary format
Key issues: use string::length to get the length of the string when writing out and use resize() before reading the string. And both for reading and writing, use mystring.c_str() instead the string itself.

Answer 7

Try this code snippet.
/* writing string into a binary file */
fstream ifs;
ifs.open ("c:/filename.exe", fstream::binary | fstream::in | fstream::out);
if (ifs.is_open())
{
ifs.write("string to binary", strlen("string to binary"));
ifs.close();
}
Here is a good example.