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 :)

I would use std::vector<char> or std::vector<uint8_t> instead of std::string — Slava
– Slava, Commented Jul 27, 2016 at 21:25
I could never understand why people use std::string for binary data. Keep in mind that this container's value_type is char, which may lead to problems related to sign extension (for example, think about what the result of my_str[3] == 0x95 will be). You might want to use std::vector<std::uint8_t>. — Daniel Kamil Kozar
– Daniel Kamil Kozar, Commented Jul 27, 2016 at 21:30

Barry · Accepted Answer · 2016-07-27 20:53:36Z

9

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);

answered Jul 27, 2016 at 20:53

Barry

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5 Comments

Hatted Rooster Over a year ago

These quirks really make C++ great. A convenient non-const method to get the internal string buffer will be added 35 years late.

baris_esmer Over a year ago

Just to be sure, the size of my_str before calling the read method should be greater than or equal to 5 , right ? @Barry

Barry Over a year ago

@baris_esmer Of course. All the usual caveats apply.

Erik P. Jun 9 at 13:55

I think this invokes UB: cppreference.com says that "Modifying the past-the-end null terminator stored at data() + size() to any value other than CharT() has undefined behavior", and overwriting the first NUL does exactly that.

Barry Jun 9 at 14:19

@ErikP. This isn't modifying data() + size(), string(5, '\0') has size 5 (not 0).

Richard Hodges · Accepted Answer · 2016-07-27 21:30:31Z

3

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 ;
}

answered Jul 27, 2016 at 21:30

Richard Hodges

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Comments

Slava · Accepted Answer · 2016-07-27 21:32:59Z

2

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.

answered Jul 27, 2016 at 21:32

Slava

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