Question

My C++ knowledge stinks and I need to convert a C++ function the correct way.

This is the code I have (extracted from http://www.sky.franken.de/doxy/xmlstorage/c++/base64_8cpp_source.html).

void* base64_decode(const char* s, size_t& data_len)
{
  size_t len = strlen(s);

  if (len % 4)
          throw Exception("invalid BASE64 string length");

  unsigned char* data = (unsigned char*) malloc(len/4*3);

  int n[4];
  unsigned char* q = (unsigned char*) data;

  for(const char*p=s; *p; ) {
          n[0] = POS(*p++);
          n[1] = POS(*p++);
          n[2] = POS(*p++);
          n[3] = POS(*p++);

          if (n[0]==-1 || n[1]==-1)
              throw Exception("invalid BASE64 encoding");

      if (n[2]==-1 && n[3]!=-1)
              throw Exception("invalid BASE64 encoding");

      q[0] = (n[0] << 2) + (n[1] >> 4);
      if (n[2] != -1) q[1] = ((n[1] & 15) << 4) + (n[2] >> 2);
      if (n[3] != -1) q[2] = ((n[2] & 3) << 6) + n[3];
      q += 3;
  }

       data_len = q-data - (n[2]==-1) - (n[3]==-1);

       return data;
   }

I need this previous function to have this header

void * base64_decode(const char* s, size_t * data_len)

so, you see that is a matter of a “&” and a “*” on the header. As you see, the first function has this

size_t& data_len

in the header, but I need it to accept the data length in the form

size_t * data_len

I don’t know how to convert the first in the second, or in other words, what kind of changes I will have to perform on the method to make it work with a data_len provided as “size_t *”

Answer 1

void* base64_decode(const char* s, size_t* data_len_ptr)
{
    size_t len = strlen(s);

    if (len % 4)
        [NSException raise:@"Invalid input in base64_decode" format:@"%d is an invalid length for an input string for BASE64 decoding", len];

    unsigned char* data = (unsigned char*) malloc(len/4*3);

    int n[4];
    unsigned char* q = (unsigned char*) data;

    for(const char*p=s; *p; )
    {
        n[0] = POS(*p++);
        n[1] = POS(*p++);
        n[2] = POS(*p++);
        n[3] = POS(*p++);

        if (n[0]==-1 || n[1]==-1)
            [NSException raise:@"Invalid input in base64_decode" format:@"Invalid BASE64 encoding"];

        if (n[2]==-1 && n[3]!=-1)
            [NSException raise:@"Invalid input in base64_decode" format:@"Invalid BASE64 encoding"];

        q[0] = (n[0] << 2) + (n[1] >> 4);
        if (n[2] != -1) q[1] = ((n[1] & 15) << 4) + (n[2] >> 2);
        if (n[3] != -1) q[2] = ((n[2] & 3) << 6) + n[3];
        q += 3;
    }

    // make sure that data_len_ptr is not null
    if (!data_len_ptr)
        [NSException raise:@"Invalid input in base64_decode" format:@"Invalid destination for output string length"];

    *data_len_ptr = q-data - (n[2]==-1) - (n[3]==-1);

    return data;
}

Calling the function would look like this:

const char* string = /*Base-64 encoded string*/;
size_t data_len;
void* decoded_data = base64_decode(string, &data_len);
// the length of the decoded data is now indicated by data_len