Get bytes from std::string in C++

Question

I'm working in a C++ unmanaged project.

I need to know how can I take a string like this "some data to encrypt" and get a byte[] array which I'm gonna use as the source for Encrypt.

In C# I do

  for (int i = 0; i < text.Length; i++)
    buffer[i] = (byte)text[i];

What I need to know is how to do the same but using unmanaged C++.

Thanks!

Answer 1

If you just need read-only access, then c_str() will do it:

char const *c = myString.c_str();

If you need read/write access, then you can copy the string into a vector. vectors manage dynamic memory for you. You don't have to mess with allocation/deallocation then:

std::vector<char> bytes(myString.begin(), myString.end());
bytes.push_back('\0');
char *c = &bytes[0];

Answer 2

std::string::data would seem to be sufficient and most efficient. If you want to have non-const memory to manipulate (strange for encryption) you can copy the data to a buffer using memcpy:

unsigned char buffer[mystring.length()];
memcpy(buffer, mystring.data(), mystring.length());

STL fanboys would encourage you to use std::copy instead:

std::copy(mystring.begin(), mystring.end(), buffer);

but there really isn't much of an upside to this. If you need null termination use std::string::c_str() and the various string duplication techniques others have provided, but I'd generally avoid that and just query for the length. Particularly with cryptography you just know somebody is going to try to break it by shoving nulls in to it, and using std::string::data() discourages you from lazily making assumptions about the underlying bits in the string.

Answer 3

Normally, encryption functions take

encrypt(const void *ptr, size_t bufferSize);

as arguments. You can pass c_str and length directly:

encrypt(strng.c_str(), strng.length());

This way, extra space is allocated or wasted.

Answer 4

In C++17 and later you can use std::byte to represent actual byte data. I would recommend something like this:

std::vector<std::byte> to_bytes(std::string const& s)
{
    std::vector<std::byte> bytes;
    bytes.reserve(std::size(s));
      
    std::transform(std::begin(s), std::end(s), std::back_inserter(bytes), [](char c){
        return std::byte(c);
    });

    return bytes;
}

Answer 5

From a std::string you can use the c_ptr() method if you want to get at the char_t buffer pointer.

It looks like you just want copy the characters of the string into a new buffer. I would simply use the std::string::copy function:

length = str.copy( buffer, str.size() );

Answer 6

If you just need to read the data.

encrypt(str.data(),str.size());

If you need a read/write copy of the data put it into a vector. (Don;t dynamically allocate space that's the job of vector).

std::vector<byte>  source(str.begin(),str.end());
encrypt(&source[0],source.size());

Of course we are all assuming that byte is a char!!!

Answer 7

If this is just plain vanilla C, then:

strcpy(buffer, text.c_str());

Assuming that buffer is allocated and large enough to hold the contents of 'text', which is the assumption in your original code.

If encrypt() takes a 'const char *' then you can use

encrypt(text.c_str())

and you do not need to copy the string.

Answer 8

You might go with range-based for loop, which would look like this:

std::vector<std::byte> getByteArray(const string& str)
{
    std::vector<std::byte> buffer;
    for (char str_char : str)
        buffer.push_back(std::byte(str_char));

    return buffer;
}

Answer 9

I dont think you want to use the c# code you have there. They provide System.Text.Encoding.ASCII(also UTF-*)

string str = "some text;
byte[] bytes = System.Text.Encoding.ASCII.GetBytes(str);

your problems stem from ignoring the encoding in c# not your c++ code