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

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);