C++ Pass bytes from char* to a BYTE*

The definition of BYTE is:

typedef unsigned char BYTE;

which is not the same as a const char, so you'd need to convert it, but note that casting away const from something declared const to start with results in undefined behaviour and trying to actually change the data poses an even bigger risk.

BYTE* Bytes = reinterpret_cast<BYTE*>(const_cast<char*>(ByteString));

Edit: I just noticed that converting a const char* to a BYTE* was taken out of the question but I'll leave it here for now.

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Copying the data (not as a zero terminated string) could be done like this:

const char ByteString[] = "\x3B\xC8\x74\x1B";
BYTE* Bytes = new BYTE[sizeof(ByteString)-1];
std::memcpy(Bytes, ByteString, sizeof(ByteString)-1);

// Use your Bytes

delete[] Bytes; // manual delete when you are done

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Or better:

const char ByteString[] = "\x3B\xC8\x74\x1B";
std::basic_string<BYTE> Bytes( reinterpret_cast<const BYTE*>(ByteString), sizeof(ByteString)-1 );

// use Bytes
// Bytes.data()  returns a BYTE*
// Bytes.size()  returns the length.

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But given the nature of what you are doing, you could probably skip these conversions and use an array of the correct type to start with:

BYTE Bytes[] = { 0xA1, 0x00, 0x00, 0x00, 0x00, 0x3B, 0xC8, 0x74, 0x1B };

or

std::basic_string<BYTE> Bytes({ 0xA1, 0x00, 0x00, 0x00, 0x00, 0x3B, 0xC8, 0x74, 0x1B });

These won't need any conversions when all you deal with is raw BYTE data. Here's an example using ReadProcessMemory and a basic_string for a buffer and pattern.

using BYTEstr = std::basic_string<BYTE>; // just for convenience

BYTEstr Buffer(1024, 0); // 1024 BYTES initialized with 0
BYTEstr Pattern({ 0xA1, 0x00, 0x00, 0x00, 0x00, 0x3B, 0xC8, 0x74, 0x1B });

ReadProcessMemory(hProcess, lpBaseAddress, Buffer.data(), Buffer.size(), &lpNumberOfBytesRead);

BYTEstr::size_type pos = Buffer.find(Pattern);

if (pos == BYTEstr::npos) {
    std::cout << "Pattern not found\n";
} else {
    std::cout << "Pattern found at position " << pos << "\n";
}

To respect const, use

const BYTE *Bytes = reinterpret_cast<const BYTE*>(ByteString);

and vice versa:

const char *ByteString = reinterpret_cast<const char *>(Bytes);

If you want to make copy of the buffer so that you can modify it, use

len = LenOfChrStr;
BYTE *Bytes = new BYTE[len];
memcpy(Bytes, ByteStr, len);