convert std::string to unsigned char array

The standard way:

#include <vector>
#include <string>

std::vector<unsigned char> to_vector(std::string const& str)
{
    // don't forget the trailing 0...
    return std::vector<unsigned char>(str.data(), str.data() + str.length() + 1);
}

int main()
{
    std::string str = "abcde";
    
    auto v = to_vector(str);
}

A way that uses variable-length array extensions, available on some compilers:

#include <algorithm>
#include <string>

int main()
{
    std::string str = "abcde";
    
    unsigned char v[str.length() + 1];
    std::copy(str.data(), str.data() + str.length() + 1, v);
}

But note:

main.cpp:14:37: warning: ISO C++ forbids variable length array 'v' [-Wvla]
     unsigned char v[str.length() + 1];
                                     ^

... and here's another way which ensures that we're using a reference to an array. Note that the reference is only valid in the current thread, and only until the next call to to_array(). But note that this is all un-necessary.

#include <vector>
#include <string>
#include <iostream>

unsigned char (& to_array(std::string const& str))[] 
{
    static thread_local std::vector<unsigned char> result;
    result.assign(str.data(), str.data() + str.length() + 1);
    return reinterpret_cast<unsigned char (&)[]>(*result.data());
}

extern "C" void foo(unsigned char p[]) { }

int main()
{
    std::string str = "abcde";

    unsigned char (&v)[] = to_array(str);
    foo(v);  // ok
    foo(reinterpret_cast<unsigned char*>(str.data())); // also ok!
    
}

What you essentially want to do is find the address of the "abcde" and pass it to an unsigned char* as a pointer. Then you could access it as an array without a problem.

Luckily if you look at str.c_str() you will see that it already returns a pointer (An address of the abcde). So all you have to do is save it to a variable. Example here:

string str = "abcde";
unsigned char* test = (unsigned char*)str.c_str();

You can work with it as an array without a problem.

for (int i = 0; i < str.size(); i++)
    cout << test[i];

If you want to copy the string, you can do it using the following

unsigned char* test = (unsigned char*)malloc(str.size());
memcpy(test, str.c_str(), str.size());

What we do is allocate memory space where we can copy the the content of the string using malloc. Then we use memcpy to copy to content from address returned by str.c_str() (Which contains the "abcde") and copy it to the address of the newly allocated space we created before using malloc.