Here's the situation:
I have a Graph class written in C++ and I need to build Graph objects from Files. The problem is that Graph are memorized in files in a lot of different ways, so I was thinking about a function that, using the file extension, could invoke the correct procedure for building a Graph in a certain format. How should I proceed? Am I wrong or I can't just overload operator>> in my class?Thanks in advance.
operator>> is (should be) agnostic to any details of the stream from which it is extracting, so using this operator is probably the wrong tact.
The best way to do this would be:
graph_type load_from_file(const std::string& file_path) { //or use something like boost::filesystem::path
std::ofstream file { file_path };
if(endswith(file_path, ".graph") {
return deserialize_from_graph(ofstream);
}
if(endswith(file_path, ".g2") {
return deserialize_from_g2(ofstream);
}
//other formats here
//else throw
}
note, endswith is not from the standard library, boost however has an implementation in it's string algorithms.
How do you determine how the data is memorized. If it is just
the extension, all you need is a map
std::string→pointer_to_function. If the same
extension can have several different representations,
distinguished, for example, by the first couple of bytes in the
file, or information in some common header, you'll have to
differ the final choice until you've read these
bytes—again, a map to the a pointer to function will do
the trick.
Depending on the complexity of the formats to read, you may want
to replace the pointer to a reader function with a pointer to
a factory function, which returns an instance of a reader class,
which derives from an abstract reader.
Related
Assume I want to implement class A which must load its "configuration" from a file. And let's assume the "configuration" is a simple map<string, string>.
I can implement the A::LoadConfiguration in two different ways:
void A::LoadConfiguration(string filename)
map<string, string> A::LoadConfiguration(string filename) const
Should I prefer either of the two implementations, and why?
If you prefer the second version when the user wants to get info on a file they will base all their algorithms on the map. If you do the second version, meaning the implementation may be a map, but doesn't have to be, they can base their code around an API which does not have to change even if the internal implementation does.
Consider the situation where later you realize it is far more efficient to use an std array, for whatever reason, now every program using this code has to change many of it's algorithms. Using the first version the change to array can be handled internally and reflect no changes on the outside.
Now if you are planning to make multiple instances of the class you will definitely want to make it a static method because you don't want the file to load every time you call the constructor (especially if the file will not change).
Completely ignoring your suggestions, but this is probably how I would do it (not knowing all your constraints, so ignore me if it does not fit):
class A
{
public:
static A fromConfiguration( string fileName );
/* ... */
}
In most cases, the "configuration" of a class should be set at object creation, so forcing the user to provide it on construction is a good thing (instead of having to remember to do do the loading later).
namespace NeatStuff
{
map<string,string> loadSimpleConfiguration( string fileName );
}
If the configuration file format is really simple (and not specific to your class) you can move the actual loading out of the class.
Assuming other classes use the configuration later, I prefer option 1, and an additional GetConfigurationParameter public const method that gets the config value for a particular key. That lets me make other classes which can just ask for some parameter by name without ever caring that it's implemented as a map.
Another reason why I prefer option 1 is that loading a configuration should be distinct from returning it. If I see a name like LoadConfiguration, I assume that it loads the config from somewhere and sets the parameters in the class. I do not assume it returns some description of the configuration, which I'd instead expect from a method like GetConfiguration - but opinions on this will vary for different people of course.
So, I've been trying to be more rigorous with making any passed parameters that shouldn't be touched by a function const.
One situation I've encountered in some of my C++ code is the case where the object may change, but where I want to "lock out" functions from access certain key functionality of the object. For example, for an std::ifstream file handle, I may wish to prevent the function from closing the file.
If I pass it as a const &, the const part keeps me from performing standard file i/o, it seems.
e.g. I want something along the lines of
void GetTags(Arr<std::string> & tags, std::ifstream const& fileHandle)
...but written in such a way to allow file i/o but not open/close operations.
Is there any good/reliable way to do this in C++? What would be considered best practice?
This has already been done for you by the standard library design: Pass a reference to the base class std::istream instead, which does not have a notion of opening or closing - it exposes only the stream interface.
void stream_me(std::istream & is);
std::ifstream is("myfile.txt");
stream_me(is);
In your place I'd just pass a std::istream instead.
You could wrap the ifstream in an object that only exposed the functionality that you wished the caller to be able to use.
However, if you have a bunch of different functions, each with a different subset of ifstream's functionality, you'll end up with lots of different wrapper classes; so I don't see this as a general solution.
I think the best way would be to wrap the ifstream in a new class which only has member functions corresponding to the functionality you wantGetTags to have access to. Then pass that not the ifstream as the second argument to GetTags.
I am parsing json data using JsonCpp. I don't really need to understand the data, i just need to print out some properties and their values out. It somehow is hard to do. First I need to know what type the value is and then get the value and then convert it to string again! There is a styled writer but I don't want to use it as it appends some CRLF at the end.
I do something like this
CJsonHelper::getUInt(Json::Value &root, std::string key){
return root.get(key, 0-1).isInt() ? root.get(key, 0-1).asUInt() : 0-1;
}
Could I just write a single function to get all the properties with just that function which doesn't really care about the types etc?
The project has moved to GitHub.
To avoid all linefeeds, use a StreamWriterBuilder:
Json::Value whatever = ...;
Json::StreamWriterBuilder builder;
builder.settings_["indentation"] = "";
std::string out = Json::writeString(builder, whatever);
You can trivially create your own writer that does whatever you want. Have look at the code for StyledWriter. The CRLF's you don't like come from StyledWriter::Write. You can create your own writer class that doesn't have those CRLF's.
If you don't mind modifying the library, change the writer so that functions like writeValue are protected rather than private. Then you can derive your own class from StyledWriter with a different Write function and still use the various write* functions to do the details of the conversion.
Up until now, whenever I wanted to pass some raw data to a function (like a function that loads an image from a buffer), I would do something like this:
void Image::load(const char* buffer, std::size_t size);
Today I took a look at the Boost libraries, more specifically at the property_tree/xml_parser.hpp header, and I noticed this function signature:
template<typename Ptree>
void read_xml(std::basic_istream<typename Ptree::key_type::value_type>&,
Ptree &, int = 0);
This actually made me curious: is this the correct way to pass around raw data in C++, by using streams? Or am I misinterpreting what the function is supposed to be used for?
If it's the former, could you please point me to some resource where I can learn how to use streams for this? I haven't found much myself (mainly API references), and I have't been able to find the Boost source code for the XML parser either.
Edit: Some extra details
Seems there's been some confusion as to what I want. Given a data buffer, how can I convert it to a stream such that it is compatible with the read_xml function I posted above? Here's my specific use case:
I'm using the SevenZip C library to read an XML file from an archive. The library will provide me with a buffer and its size, and I want to put that in stream format such that it is compatible with read_xml. How can I do that?
Well, streams are quite used in C++ because of their conveniences:
- error handling
- they abstract away the data source, so whether you are reading from a file, an audio source, a camera, they are all treated as input streams
- and probably more advantages I don't know of
Here is an overview of the IOstream library, perhaps that might better help you understand what's going on with streams:
http://www.cplusplus.com/reference/iostream/
Understanding what they are exactly will help you understand how and when to use them.
There's no single correct way to pass around data buffers. A combination of pointer and length is the most basic way; it's C-friendly. Passing a stream might allow for sequential/chunked processing - i. e. not storing the whole file in memory at the same time. If you want to pass a mutable buffer (that might potentially grow), a vector<char>& would be a good choice.
Specifically on Windows, a HGLOBAL or a section object handle might be used.
The C++ philosophy explicitly allows for many different styles, depending on context and environment. Get used to it.
Buffers of raw memory in C++ can either be of type unsigned char*, or you can create a std::vector<unsigned char>. You typically don't want to use just a char* for your buffer since char is not guaranteed by the standard to use all the bits in a single byte (i.e., this will end up varying by platform/compiler). That being said, streams have some excellent uses as well, considering that you can use a stream to read bytes from a file or some other input, etc., and from there, store that data in a buffer.
Seems there's been some confusion as to what I want. Given a data buffer, how can I convert it to a stream such that it is compatible with the read_xml function I posted above?
Easily (I hope PTree::Key_type::value_type would be something like char):
istringstream stream(string(data, len));
read_xml(stream, ...);
More on string streams here.
This is essentially using a reference to pass the stream contents. So behind the scene's it's essentially rather similar to what you did so far and it's essentially the same - just using a different notation. Simplified, the reference just hides the pointer aspect, so in your boost example you're essentially working with a pointer to the stream.
References got the advantage avoiding all the referencing/dereferencing and are therefore easier to handle in most situations. However they don't allow you multiple levels of (de-)referencing.
The following two example functions do essentially the same:
void change_a(int &var, myclass &cls)
{
var = cls.convert();
}
void change_b(int *var, myclass *cls)
{
*var = cls->convert();
}
Talking about the passed data itself: It really depends on what you're trying to achieve and what's more effective. If you'd like to modify a string, utilizing an object of class std::string might be more convenient than using a classic pointer to a buffer (char *). Streams got the advantage that they can represent several different things (e.g. data stream on the network, a compressed stream or simply a file or memory stream). This way you can write single functions or methods that accept a stream as input and will instantly work without worrying about the actual stream source. Doing this with classic buffers can be more complicated. On the other side you shouldn't forget that all objects will add some overhead, so depending on the job to be done a simple pointer to a character string might be perfectly fine (and the most effective solution). There's no "the one way to do it".
How can I override a C++ standard-library class function? In my application, I use ofstream objects in many different places of code. And now I want to open files in a different permission mode in Linux, Ubuntu. But open function of ofstream has no parameter to specify the permission mode of the file it creats.
Now I want to override open() function of ofstream class so it will get a parameter to specify the permissions for user access.
For starters, to clarify your terminology, the STL usually refers to the subset of the C++ standard library containing the containers, iterators, and algorithms. The streams classes are part of the C++ standard library, but are usually not bundled together with the STL. Some purists will insist that there is no such thing as the STL in the C++ standard library (since the STL is, technically speaking, a third-party library that was adopted into the standard), but most C++ programmers will know what you mean.
As for your question, there is no way within the standard to specify permission modes with ofstream. If you want to create your own custom stream class that is like ofstream but which supports permissions, your best bet would be to do the following:
Create a subclass of basic_streambuf that allows you to open, write, and possibly read files while specifying Unix permissions. The streams classes are designed so that the details of communicating with physical devices like disk, networks, and the console are all handled by the basic_streambuf class and its derived classes. If you want to make your own stream class, implementing a stream buffer would be an excellent first step.
Define your own class that subclasses basic_ostream and installs your custom basic_streambuf. By default, the basic_ostream supports all of the standard output routines by implementing them in terms of the underlying basic_streambuf object. Once you have your own stream buffer, building a basic_ostream class that uses that streambuf will cause all of the standard stream operations on that class (such as <<) to start making the appropriate calls to your streambuf.
If you'd like more details on this, an excellent reference is Standard C++ IOStreams and Locales. As a shameless plug, I have used the techniques from this book to build a stream class that wraps a socket connection. While a lot of the code in my stream won't be particularly useful, the basic structure should help you get started.
Hope this helps!
This is not answering your question directly as I wouldn't advise overriding ofstream::open.
Instead couldn't you use the first suggestion in this post? Open the file as you normally would to get the correct permissions, and then construct an ofstream from the file descriptor.
#include <iostream>
#include <fstream>
class gstream: public std::ofstream
{
void open(const std::string& filename, ios_base::openmode mode,int stuff)
{
//put stuff here
}
};
int main() {
gstream test;
//io stuff
return 0;
}
seems to work here.
Another option would be to create a wrapper class that contains an 'ofstream' object and has the interface you want, and passes the work onto its 'oftstream' member. It would look like this.