Reading binary data into std::string c++

Question

I am trying to read data from binary file to an std::string.Here is what I have tried at first.

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

using namespace std;

int main(int argc, char const *argv[])
{
    fstream file("output.bin" , ios::out | ios::binary | ios::in);
    string my_str(5, '\0');
    file.read(my_str.c_str(), 5);
    cout << "String = " << my_str<< endl ;
}

And the compiler gave the error :

error: invalid conversion from ‘const char*’ to ‘std::basic_istream<char>::char_type* {aka char*}’ [-fpermissive]
  file.read(my_str.c_str(), 5);

As far as I understand, c_str() returns a const pointer which cannot be used in read method, so I changed my approach a little bit(which you can see below). Is there a better way to do this ?

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

using namespace std;

int main(int argc, char const *argv[])
{
    fstream file("output.bin" , ios::out | ios::binary | ios::in);
    string my_str(5, '\0');
    char buffer[6];
    file.read(buffer, 5);
    buffer[5] = '\0';
    my_str = string(buffer);

    cout << "String = " << my_str<< endl ;
}

ps : forgive me if I could not make myself clear, this is my first time here :)

Answer 1

In C++11, the way to get a non-const pointer to the string's data is:

file.read(&my_str[0], 5);

C++17 will introduce non-const data() for this as well:

file.read(my_str.data(), 5);

Answer 2

another way, using standard algorithms:

#include <iostream>
#include <string>
#include <fstream>
#include <algorithm>
#include <iterator>

using namespace std;

int main(int argc, char const *argv[])
{
    fstream file("output.bin" , ios::out | ios::binary | ios::in);

    auto my_str = string();

    copy_n(istream_iterator<char>(file), 
           5, 
           std::back_inserter(my_str));

    cout << "String = " << my_str<< endl ;
}

Answer 3

std::string is specially designed to work with strings and with c-style strings as well, so this fact will work against you in this situation. For example your code:

char buffer[6];
file.read(buffer, 5);
buffer[5] = '\0';
my_str = string(buffer);

what is wrong with it? You are reading binary data and who guarantees that there won't be '\0' byte there? You can fix it by:

my_str = string(buffer,5);

but this shows the point - std::string as a buffer is not a good choice. So you better use std::vector<char> or even better std::vector<uint8_t> which has method data() but will not implicitly convert from c-string, output to std::ostream etc.