Re: [Newbie] How to use a class in C++
pozz <pozzugno@gmail.com> wrote in news:fd34a714-28d3-47a9-a5f2-
a0edd6d7d8b3@n35g2000yqf.googlegroups.com:
I have a medium experience in C and now I'm learning C++. I have some
difficulties to think by and use classes, so with object oriented
programming.
In my application I have a configuration file that stores some
settings in non-volatile way. Several parts, modules and classes could
access the configuration file both for reading and writing settings.
In C I can create settings.h with the interface:
---
int settings_read(const char *name, char *value, size_t valuesize);
int settings_save(const char *name, const char *value);
---
Every other module that wants to use the settings facilities will
include "settings.h" and user settings_read() and settings_write().
The implementation and any additional data (such as the filename) are
hidden in settings.c.
In C++ I was trying to create a class in settings.h,
First, what is your reason to create a class for that? Do you need
separate setting files in separate parts of the app? If so, then yes,
encapsulating it in a class might be a good idea.
but I'm not sure
if I should add private data there:
---
class Settings {
const char filename[] = "settings.ini";
This does not compile, so the main question about private information is
mute.
public:
void Settings(void);
This does not compile.
virtual ~Settings(void);
Why virtual? Do you intend to derive multiple settings classes from this?
Using void as here is a C-ism. Why do you need a destructor at all?
int settings_read(const string name, string value);
This does not work (value is not passed back from the function).
Int return is probably unnecessary. Repeating the class name in the
member function is not necessary.
int settings_save(const string name, const string value);
Int return is probably unnecessary. Repeating the class name in the
member function is not necessary. Usage of const as here is superfluous
in a declaration, the idiomatic way is to use const references instead.
};
---
Why the user should know about private data, like filename? I'm
tempted to avoid adding private data/functions in the class
declaration in settings.h, but in settings.c as typical static names
(like in C). I don't know if this is a good approach, because I have
seen many C++ source files with private names in include file.
You can hide private implementation (look up the pimpl idiom, this is
similar to C "opaque handle" technique), but as a C++ beginner this is
not advisable, it only makes the code more convoluted and harder to
maintain and understand. Instead, put the private parts of the class in
the end of the class definition and mark as "implementation details".
Nobody will bother to read them without a reason.
So, with the above issues fixed, it would become something like:
// settings.h
#include <string>
class Settings {
public:
Settings();
std::string Read(const std::string& name);
void Save(const std::string& name, const std::string& value);
private: // implementation details
std::string my_filename;
};
// settings.cpp
#include "settings.h"
Settings::Settings()
: my_filename("settings.ini")
{}
// ... rest of the implementation
hth
Paavo