C++ fread() into a std::string - c++

Question

Like always, problems with pointers. This time I am trying to read a file (opened in binary mode) and store some portion of it in a std::string object.
Let's see:
FILE* myfile = fopen("myfile.bin", "rb");
if (myfile != NULL) {
short stringlength = 6;
string mystring;
fseek(myfile , 0, SEEK_SET);
fread((char*)mystring.c_str(), sizeof(char), (size_t)stringlength, myfile);
cout << mystring;
fclose(myfile );
}
Is this possible? I don't get any message. I am sure the file is O.K. When I try with char* it does work but I want to store it directly into the string. Thanks for your help!

Answer 1

Set the string to be large enough first to avoid buffer overrun, and access the byte array as &mystring[0] to satisfy const and other requirements of std::string.
FILE* myfile = fopen("myfile.bin", "rb");
if (myfile != NULL) {
short stringlength = 6;
string mystring( stringlength, '\0' );
fseek(myfile , 0, SEEK_SET);
fread(&mystring[0], sizeof(char), (size_t)stringlength, myfile);
cout << mystring;
fclose(myfile );
}
There are many, many issues in this code but that is a minimal adjustment to properly use std::string.

Answer 2

I would recommend this as the best way to do such a thing. Also you should check to make sure that all the bytes were read.
FILE* sFile = fopen(this->file.c_str(), "r");
// if unable to open file
if (sFile == nullptr)
{
return false;
}
// seek to end of file
fseek(sFile, 0, SEEK_END);
// get current file position which is end from seek
size_t size = ftell(sFile);
std::string ss;
// allocate string space and set length
ss.resize(size);
// go back to beginning of file for read
rewind(sFile);
// read 1*size bytes from sfile into ss
fread(&ss[0], 1, size, sFile);
// close the file
fclose(sFile);

Answer 3

string::c_str() returns const char* which you can not modify.
One way to do this would be use a char* first and construct a string from it.
Example
char buffer = malloc(stringlength * sizeof(char));
fread(buffer, sizeof(char), (size_t)stringlength, myfile);
string mystring(buffer);
free(buffer);
But then again, if you want a string, you should perhaps ask yourself Why am I using fopen and fread in the first place??
fstream would be a much much better option.
You can read more about it here

Answer 4

Please check out the following regarding c_str to see some things that are wrong with your program. A few issues include the c_str not being modifiable, but also that it returns a pointer to your string contents, but you never initialized the string.
http://www.cplusplus.com/reference/string/string/c_str/
As for resolving it... you could try reading into a char* and then initializing your string from that.

Answer 5

No it is not. std::string::c_str() method does not return a modifiable character sequence as you can validate from here. A better solution would be using a buffer char array. Here is an example:
FILE* myfile = fopen("myfile.bin", "rb");
if (myfile != NULL) {
char buffer[7]; //Or you can use malloc() / new instead.
short stringlength = 6;
fseek(myfile , 0, SEEK_SET);
fread(buffer, sizeof(char), (size_t)stringlength, myfile);
string mystring(buffer);
cout << mystring;
fclose(myfile );
//use free() or delete if buffer is allocated dynamically
}