C++ converting bytes

Question

I have the below code where a vector<int> which contains bytes is converted to a char[]. This char[] is then output.

vector<int> data;
data.push_back(33);
data.push_back(69);
data.push_back(80);
data.push_back(0);
data.push_back(0);
data.push_back(74);
data.push_back(255);

char* result = new char[data.size()];

for(int i = 0; i < data.size(); i++) {
  result[i] = (char)data[i];
  cout << i << " = " << (char)data[i] << endl;
}

cout << "--" << endl;

for(int i = 0; i < sizeof(result); i++) {
  cout << i << " = " << result[i] << endl;
}

This code produces the following output.

0 = !
1 = E
2 = P
3 = 
4 = 
5 = J
6 = �
--
0 = !
1 = E
2 = P
3 = 

However, I expected it to be

0 = !
1 = E
2 = P
3 = 
4 = 
5 = J
6 = �
--
0 = !
1 = E
2 = P
3 = 
4 = 
5 = J
6 = �

I suspect this has something to do with sizing, but I am not certain.

What is going wrong with the code?

Answer 1

The way you're using sizeof() is incorrect. It's returning the size of the pointer, not the size of the pointed-to thing. There is no way to access the size of the pointed-to thing given just its pointer when you're pointing to an array of items.

You'll have to remember the size you allocated with new somewhere and use that.

Answer 2

for(int i = 0; i < sizeof(result); i++)  // PROBLEM!
{
  cout << i << " = " << result[i] << endl;
}

result is a pointer and it appears you are on a 32-bit system, so sizeof(result) is 4 (bytes). So your loop is iterating from [0, 4). You need to pass the size, or use the vector's size:

for(int i = 0; i < data.size(); i++) 
{
  cout << i << " = " << result[i] << endl;
}