Calling this method from other class creates violation reading - c++

I am trying to create a configuration class which reads the data from an xml with rapidxml.
Therefore I have a private xml_document which I parse inside of the constructor:
class Config
{
public:
Config();
~Config();
/*returns the first found node*/
xml_node<>* findNode(const char* name);
/*get an configuration value. It's always a char*/
char* getConfigValue(const char* name);
private:
rapidxml::xml_document<>* m_doc;
};
//cpp
Config::Config()
{
m_doc = new xml_document<>();
rapidxml::file<> xmlFile("config.xml");
m_doc->parse<0>(xmlFile.data());
//LOG(getConfigValue("width")); // here it works
}
xml_node<>* Config::findNode(const char* name)
{
return m_doc->first_node()->first_node(name);
}
char* Config::getConfigValue(const char* name){
return m_doc->first_node()->first_node(name)->value();
}
Inside of the wWinMain I create an config opject and try to call the methods.
Config* config = new Config();
LOG(config->findNode("width")->value()); // this does create violation reading
BUT if I put the same line into the constructor of the Config class it works without any problem. What is going wrong here?

I'm not familiar with rapid xml, but a quick google search told me:
http://rapidxml.sourceforge.net/manual.html
RapidXml is an in-situ parser, which allows it to achieve very high
parsing speed. In-situ means that parser does not make copies of
strings. Instead, it places pointers to the source text in the DOM
hierarchy.
3.1 Lifetime Of Source Text
In-situ parsing requires that source text lives at least as long as
the document object. If source text is destroyed, names and values of
nodes in DOM tree will become destroyed as well. Additionally,
whitespace processing, character entity translation, and
zero-termination of strings require that source text be modified
during parsing (but see non-destructive mode). This makes the text
useless for further processing once it was parsed by RapidXml.
So what I take from that is that you can't just have rapidxml::file<> xmlFile("config.xml"); be a stack variable as xml_document->parse() will create a tree that points directly at memory in xmlFile. So xmlFile has to be in memory for at least as long as the xml_document otherwise you will have invalid data. This means that xmlFile needs to be a member of your Config class. Also, I don't see you initializing m_doc, did you just omit some code? Otherwise it should be complaining sooner and never work. Also, you should always check to make sure functions that return pointers aren't returning null before you access them to avoid getting reading violations.
So what you really want is something like this:
class Config
{
public:
Config();
~Config();
/*returns the first found node*/
xml_node<>* findNode(const char* name);
/*get an configuration value. It's always a char*/
char* getConfigValue(const char* name);
private:
rapidxml::file<> m_xmlFile;
rapidxml::xml_document<> m_doc;
};
//cpp
Config::Config()
: m_xmlFile("config.xml")
{
m_doc.parse<0>(m_xmlFile.data());
}
xml_node<>* Config::findNode(const char* name)
{
if (m_doc.first_node())
{
return m_doc.first_node()->first_node(name);
}
return 0;
}
char* Config::getConfigValue(const char* name)
{
if (m_doc.first_node())
{
xml_node<>* node = m_doc.first_node()->first_node(name);
if (node)
{
return node->value();
}
}
return 0;
}

Related

Handling of const char* on ESP32

I'm working on making some Spotify API calls on an ESP32. I'm fairly new to C++ and while I seem to got it working how I wanted it to, I would like to know if it is the right way/best practice or if I was just lucky. The whole thing with chars and pointers is still quite confusing for me, no matter how much I read into it.
I'm calling the Spotify API, get a json response and parse that with the ArduinoJson library. The library returns all keys and values as const char*
The library I use to display it on a screen takes const char* as well. I got it working before with converting it to String, returning the String with the getTitle() function and converting it back to display it on screen. After I read that Strings are inefficient and best to avoid, I try to cut out the converting steps.
void getTitle()
{
// I cut out the HTTP request and stuff
DynamicJsonDocument doc(1024);
DeserializationError error = deserializeJson(doc, http.getStream(), );
JsonObject item = doc["item"];
title = item["name"]; //This is a const char*
}
const char* title = nullptr;
void loop(void) {
getTitle();
u8g2.clearBuffer();
u8g2.setDrawColor(1);
u8g2.setFont(u8g2_font_6x12_tf);
u8g2.drawStr(1, 10, title);
u8g2.sendBuffer();
}
Is it okay to do it like that?
This is not fine.
When seeing something like this, you should immediately become suspicious.
This is because in getTitle, you are asking a local object (item) for a pointer-- but you use the pointer later, when the item object no longer exists.
That means your pointer might be meaningless once you need it-- it might no longer reference your data, but some arbitrary other bytes instead (or even lead to crashes).
This problem is independent of what exact library you use, and you can often find relevant, more specific information by searching your library documentation for "lifetime" or "object ownership".
FIX
Make sure that item (and also DynamicJsonDocument, because the documentation tells you so!) both still exist when you use the data, e.g. like this:
void setTitle(const char *title)
{
u8g2.clearBuffer();
u8g2.setDrawColor(1);
u8g2.setFont(u8g2_font_6x12_tf);
u8g2.drawStr(1, 10, title);
u8g2.sendBuffer();
}
void updateTitle()
{
DynamicJsonDocument doc(1024);
DeserializationError error = deserializeJson(doc, http.getStream(), );
JsonObject item = doc["item"];
setTitle(item["name"]);
}
See also: https://arduinojson.org/v6/how-to/reuse-a-json-document/#the-best-way-to-use-arduinojson
Edit: If you want to keep parsing/display update decoupled
You could keep the JSON document "alive" for when the parsed data is needed:
/* "static" visibility, so that other c/cpp files ("translation units") can't
* mess mess with our JSON doc directly
*/
static DynamicJsonDocument doc(1024);
static const char *title;
void parseJson()
{
[...]
// super important to avoid leaking memory!!
doc.clear();
DeserializationError error = deserializeJson(doc, http.getStream(), );
// TODO: robustness/error handling (e.g. inbound JSON is missing "item")
title = doc["item"]["name"];
}
// may be nullptr when called before valid JSON was parsed
const char* getTitle()
{
return title;
}

How to read data from a file from within a function

I want to make my code more efficient, specifically the reading of data from a text file. Here is a snapshot of what it looks like now:
values V(name);
V.population = read_value(find_line_number(name, find_in_map(pop, mapping)));
V.net_growth = read_value(find_line_number(name, find_in_map(ngr, mapping)));
... // and so on
Basically, the read_value function creates an ifstream object, opens the file, reads one line of data, and closes the file connection. This happens many times. What I want to do is to open the file once, read every line that is needed into the struct, and then close the file connection.
Here is the creating values struct function with parameters:
static values create_struct(std::string name, std::map<std::string, int> mapping) {
values V(name);
V.population = read_value(find_line_number(name, find_in_map(pop, mapping)), file);
V.net_growth = read_value(find_line_number(name, find_in_map(ngr, mapping)), file);
// more values here
return V;
}
The function that calls create_struct is shown below:
void initialize_data(string name) {
// read the appropriate data from file into a struct
value_container = Utility::create_struct(name, this->mapping);
}
I am thinking of instead defining the ifstream object in the function initialize_data. Given what is shown about my program, would that be the best location to create the file object, open the connection, read the values, then close the connection? Also, would I need to pass in the ifstream object into the create_values struct, and if so, by value, reference or pointer?
The short answer is to create your ifstream object first and pass it as reference to your parser. Remember to seek the stream back to the beginning before you leave your function, or when you start to read.
The RAII thing to do would be to create a wrapper object that automatically does this when it goes out of scope.
class ifStreamRef{
ifStreamRef(std::ifstream& _in) : mStream(_in){}
~ifStreamRef(){mStream.seekg(0);}
std::ifstream& mStream;
}
Then you create a wrapper instance when entering a method that will read the fstream.
void read_value(std::ifstream& input, ...){
ifStreamRef autoRewind(input);
}
Or, since the Ctor can do the conversion...
void read_value(ifStreamRef streamRef, ...) {
streamRef.mStream.getLine(...);
}
std::ifstream itself follows RAII, so it will close() the stream for you when your stream goes out of scope.
The long answer is that you should read up on dependency injection. Don't create dependencies inside of objects/functions that can be shared. There are lots of videos and documents on dependency injection and dependency inversion.
Basically, construct the objects that your objects depend on and pass them in as parameters.
The injection now relies on the interface of the objects that you pass in. So if you change your ifStreamRef class to act as an interface:
class ifStreamRef{
ifStreamRef(std::ifstream& _in) : mStream(_in){}
~ifStreamRef(){mStream.seekg(0);}
std::string getLine(){
// todo : mStream.getLine() + return "" on error;
}
bool eof() { return mStream.eof(); }
std::ifstream& mStream;
}
Then later on you can change the internal implementation that would take a reference to vector<string>& instead of ifstream...
class ifStreamRef{
ifStreamRef(std::vector<string>& _in) : mStream(_in), mCursor(0){}
~ifStreamRef(){}
std::string getLine(){
// todo : mStream[mCursor++] + return "" on error;
}
bool eof() { return mCursor >= mStream.size(); }
std::vector<string>& mStream;
size_t mCursor;
}
I have oversimplified a few things.

Inserting a subtree from text into an existing XML file using tinyxml2

I was not allowed to create the new tag 'tinyxml2', that's why I am using the tag 'tinyxml', however I am using 'tinyxml2' !
I am trying to insert a subtree element to an existing XML file. My problem is, that after running the program and checking the XML file the subtree simply does not exist within the document. In the original code I am also checking for errors while loading and saving the file so there is no problem with these functions, they are working correctly. I tried a few different approaches and also adding a single element by using the UserList.NewElement(*name*)-function does also work fine.
Now I want to insert a whole subtree from a text variable...
My latest approach looks like this (simplified without checking LoadFile and SaveFile):
tinyxml2::XMLDocument UserList;
UserList.LoadFile(*Path*);
const char* XMLText = "<user name=\"test-user\" gender=\"male\"><ability description=\"I_can_do_magic\" /></user>";
tinyxml2::XMLDocument TestParse;
TestParse.Parse(XMLText);
tinyxml2::XMLElement* myNewUser = TestParse.RootElement();
UserList.FirstChildElement( "magicians" )->InsertEndChild(myNewUser);
UserList.SaveFile(*Path*);
By the way...
When I tried to parse my XMLText by using the tinyxml2::XMLDocument UserList the saved XML file will be empty after running the program. This means neither the original XML Document content, nor the newly parsed subtree will be saved when trying to do this. This fact made me use the second tinyxml2::XMLDocument TestParse. Now the XML file is saved containing it's original content, however the parsed subtree is still missing... thank you very much for any solution / help / advice.
TinyXML-2 allocates memory for its nodes (XMLNode) in memory pools stored in the XMLDocument. This fixes the memory fragmentation problems present in TinyXML-1.
The side effect is that elements can not be moved from one XMLDocument to another. They can only be copied. Regrettably, TinyXML-2 doesn't currently support deep copies (tree copies), so can't do what you want. (Although a deep copy is a requested on the github site.)
I would expect the code you wrote to assert (in debug mode) or crash, by the way, since myNewUser is in a different memory pool from UserList.
I wrote a deep copy function using XMLVisitor of TinyXML-2. Hopefully this is useful for you:
#include <stack>
#include "tinyxml2.h"
using namespace tinyxml2;
class MyXMLVisitor: public XMLVisitor
{
public:
MyXMLVisitor(XMLDocument *doc)
: m_doc(doc)
{
}
virtual bool VisitEnter (const XMLElement &el, const XMLAttribute *attr)
{
XMLElement *new_el = m_doc->NewElement(el.Name());
m_elementStack.push(new_el);
return true;
}
virtual bool Visit(const XMLText &txt)
{
m_elementStack.top()->SetText(txt.Value());
return true;
}
virtual bool VisitExit (const XMLElement &el)
{
XMLElement *top_el = m_elementStack.top();
m_elementStack.pop();
if (m_elementStack.empty()) {
m_element = top_el;
return false;
}
else {
m_elementStack.top()->InsertEndChild(top_el);
return true;
}
}
std::stack<XMLElement*> m_elementStack;
XMLDocument *m_doc;
XMLElement *m_element;
};
XMLElement* DeepCopyElement(XMLDocument &doc, const XMLElement *el)
{
MyXMLVisitor my_visitor(&doc);
el->Accept(&my_visitor);
return my_visitor.m_element;
}
int main(int argc, char* argv[])
{
XMLDocument doc;
doc.LoadFile( "test.xml" );
XMLElement *modulesElement = doc.FirstChildElement("modules");
XMLElement *moduleElement = modulesElement->FirstChildElement("module");
modulesElement->InsertEndChild(DeepCopyElement(doc, moduleElement));
doc.SaveFile("test_out.xml");
return 0;
}

Expressing "read-only, no modification of position" for std::ifstream

In my code, I want to identify some properties about the contents of a file, before deciding how to read the file. (That is, I search for a keyword, if found, it's going to be read with foo(std::ifstream&), else with bar(std::ifstream&)).
I implemented the method that searches for the keyword as
bool containsKeyword(std::ifstream& file, const char* keyword)
{
for ( std::string line; std::getline(file, line); )
{
if ( line == keyword )
{
return true;
}
}
return false;
}
This modifies the position of the file stream (either the end, if the keyword isn't found, or the position of the keyword). However I want that the position is reset after the search. This can be done with a ScopeGuard:
class FilePositionScopeGuard
{
private:
std::ifstream& file;
using FilePosition = decltype(std::declval<std::ifstream>().tellg());
FilePosition initial_position;
public:
FilePositionScopeGuard(std::ifstream& file_)
:
file(file_),
initial_position(file.tellg())
{
}
~FilePositionScopeGuard()
{
file.clear();
file.seekg(initial_position);
}
};
Now we add this to the method:
bool containsKeyword(std::ifstream& file, const char* keyword)
{
FilePositionScopeGuard guard(file);
for ( std::string line; std::getline(file, line); )
{
...
That's nice, because with exactly one additional line in the method, we get the behaviour of not modifying the std::ifstream no matter how the method is exited (one of the returns or an exception).
However, the method bool containsKeyword(std::ifstream&, const char*); does not express the constness. How can I adjust my method to express (at the level of the interface) that the method will not alter the current state?
You could change the signature to take a position-guarded file:
bool containsKeyword(const FilePositionScopeGuard &, const char *);
This allows the caller to pass an ifstream per the current signature (constructing a temporary guard for that operation), or to make their own guard and use it for several operations.
You'll need to make the ifstream member publicly accessible.
Do it with the text comment // the method does read from file but resets the read pointer.
Do not expect a user of the API to be a monkey at keyboard. Specifically don't mark ifstream argument as const while casting constancy out inside the method. It does make difference in a multithreaded program.

Boost property-tree access violation

I have been trying to parse an XML file using boost's property tree, but every time I want to get the value of a string it throws an access violation exception. It works fine with integers so I'm a bit confused.
Here's some of the code:
class Config
{
char * test;
int test2;
public:
Config();
};
Config::Config(void)
{
boost::property_tree::ptree pt;
boost::property_tree::xml_parser::read_xml("config.xml", pt);
try
{
test = pt.get<char*>("base.char");
test2 = pt.get<int>("base.int");
}
catch(std::exception e)
{
//something wasn't specified
}
}
And the XML file:
<base>
<char>test</char>
<int>10</int>
</base>
First I thought it's because I didn't allocate space for the string but neither malloc() nor new char[] helped.
Any help would be appreciated. Thanks in advance :)
Based on this tutorial I think you need to use std::string instead of char* to get string values.
So the line test = pt.get<char*>("base.char"); would then be test = pt.get<std::string>("base.char");. (Assuming you change test's type to std::string as well).