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

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 5

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 6

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 7

In C++11 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 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