C++ function that dynamically allocates a char array buffer to a passed char*

Question

I'm creating a C++ program that uses RapidXML to read data from xml files. Because RapidXML allocates pointers to the xml content to be used to access it, it needs the content of the file to live on as long as it is parsed in further steps.

So I had several files parsed successfully, but at the beginning of every parsing of a file, I always had all of the ifstream and allocating going on. I came up with the idea to create a function that would take in the file path, pointers to the xml_document<> instance and so on and return the root node of the xml file. What I then realised was, that I might hit scope problems with the dynamically allocated char array containing the xml content to be parsed and later pointed to.

I then tried the following technique, where I allocated a char*, xml_document and xml_node and invoked the function to retrieve the root node.

The function:

bool readXML(const char * path, char * buffer, xml_document<> * doc, const char * rootNodeName, xml_node<> * root_node){
// Open file
    ifstream file(path, ios::in|ios::binary);
    if (!file.is_open()){   err(ERR_FILE_NOTFOUND,path);    return 0;   }
// Get length
    file.seekg(0,file.end);
    int length = file.tellg();
    file.seekg(0,file.beg);
// Allocate buffer
    buffer = new char [length+1];
    file.read(buffer,length);
    buffer[length] = '\0';
    file.close();
// Parse
    doc->parse<0>(buffer);

// Get root node
    root_node = doc->first_node(rootNodeName);
    if ( !root_node ){  err(ERR_FILE_INVALID,path); return 0; }
    return 1;
}

The code where I use the function (reading "Hersteller.xml" / initializing class):

bool loadHersteller(){ // v4
    // Declare
    char * bfr;
    xml_document<> doc;
    xml_node<> * rt_node;

    // Get root node
    if (!readXML(concatURL(PFAD_DATEN,"Hersteller.xml"), bfr, &doc, "Hersteller", rt_node)) return 0;

    // Extract
    if (!initHRST(rt_node)) return 0; // Works fine on it's own (initializes a class)
    toConsoleHrst(); // Works fine on it's own (prints data back to console)

    // Clean up
    delete[] bfr;
    doc.clear();
    return 1;
} // END loadHersteller()

Now what I get from that is a blank console and a crash with it, returning an interger. I am very certain that the problem is the scope or lifetime of the char array. The goal of this function is to do all the work of retrieving xml files / allocating the buffer for me and passing me the root node (which I'll right pass to another function). As said, the char array needs to stay alive in the scope where the function was invoked from, so it can be accessed via the pointer structure built by the parser.

Answer 1

To fix this, pass both your out parameters (buffer and root_node) as char*& (reference to char pointer) rather than simply char*. Otherwise, what readXML() receives is a copy of the two pointers, and whatever values you assign to those copies are lost when the function returns and they are destroyed.

Note: There is a potential memory leak in the code because the delete[] instruction won't be reached if either readXml() or initHRST() fails and the function returns early.