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What are the differences between struct and class in C++?
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In what scenarios is it better to use a struct vs a class in C++?
The differences between a class and a struct in C++ are:
struct members and base classes/structs are public by default.
class members and base classes/structs are private by default.
Both classes and structs can have a mixture of public, protected and private members, can use inheritance, and can have member functions.
I would recommend you:
use struct for plain-old-data structures without any class-like features;
use class when you make use of features such as private or protected members, non-default constructors and operators, etc.
As everyone else notes there are really only two actual language differences:
struct defaults to public access and class defaults to private access.
When inheriting, struct defaults to public inheritance and class defaults to private inheritance. (Ironically, as with so many things in C++, the default is backwards: public inheritance is by far the more common choice, but people rarely declare structs just to save on typing the "public" keyword.
But the real difference in practice is between a class/struct that declares a constructor/destructor and one that doesn't. There are certain guarantees to a "plain-old-data" POD type, that no longer apply once you take over the class's construction. To keep this distinction clear, many people deliberately only use structs for POD types, and, if they are going to add any methods at all, use classes. The difference between the two fragments below is otherwise meaningless:
class X
{
public:
// ...
};
struct X
{
// ...
};
(Incidentally, here's a thread with some good explanations about what "POD type" actually means: What are POD types in C++?)
There are lots of misconceptions in the existing answers.
Both class and struct declare a class.
Yes, you may have to rearrange your access modifying keywords inside the class definition, depending on which keyword you used to declare the class.
But, beyond syntax, the only reason to choose one over the other is convention/style/preference.
Some people like to stick with the struct keyword for classes without member functions, because the resulting definition "looks like" a simple structure from C.
Similarly, some people like to use the class keyword for classes with member functions and private data, because it says "class" on it and therefore looks like examples from their favourite book on object-oriented programming.
The reality is that this completely up to you and your team, and it'll make literally no difference whatsoever to your program.
The following two classes are absolutely equivalent in every way except their name:
struct Foo
{
int x;
};
class Bar
{
public:
int x;
};
You can even switch keywords when redeclaring:
class Foo;
struct Bar;
(although this breaks Visual Studio builds due to non-conformance, so that compiler will emit a warning when you do this.)
and the following expressions both evaluate to true:
std::is_class<Foo>::value
std::is_class<Bar>::value
Do note, though, that you can't switch the keywords when redefining; this is only because (per the one-definition rule) duplicate class definitions across translation units must "consist of the same sequence of tokens". This means you can't even exchange const int member; with int const member;, and has nothing to do with the semantics of class or struct.
The only time I use a struct instead of a class is when declaring a functor right before using it in a function call and want to minimize syntax for the sake of clarity. e.g.:
struct Compare { bool operator() { ... } };
std::sort(collection.begin(), collection.end(), Compare());
From the C++ FAQ Lite:
The members and base classes of a struct are public by default, while in class, they default to private. Note: you should make your base classes explicitly public, private, or protected, rather than relying on the defaults.
struct and class are otherwise functionally equivalent.
OK, enough of that squeaky clean techno talk. Emotionally, most developers make a strong distinction between a class and a struct. A struct simply feels like an open pile of bits with very little in the way of encapsulation or functionality. A class feels like a living and responsible member of society with intelligent services, a strong encapsulation barrier, and a well defined interface. Since that's the connotation most people already have, you should probably use the struct keyword if you have a class that has very few methods and has public data (such things do exist in well designed systems!), but otherwise you should probably use the class keyword.
You can use "struct" in C++ if you are writing a library whose internals are C++ but the API can be called by either C or C++ code. You simply make a single header that contains structs and global API functions that you expose to both C and C++ code as this:
// C access Header to a C++ library
#ifdef __cpp
extern "C" {
#endif
// Put your C struct's here
struct foo
{
...
};
// NOTE: the typedef is used because C does not automatically generate
// a typedef with the same name as a struct like C++.
typedef struct foo foo;
// Put your C API functions here
void bar(foo *fun);
#ifdef __cpp
}
#endif
Then you can write a function bar() in a C++ file using C++ code and make it callable from C and the two worlds can share data through the declared struct's. There are other caveats of course when mixing C and C++ but this is a simplified example.
One place where a struct has been helpful for me is when I have a system that's receiving fixed format messages (over say, a serial port) from another system. You can cast the stream of bytes into a struct that defines your fields, and then easily access the fields.
typedef struct
{
int messageId;
int messageCounter;
int messageData;
} tMessageType;
void processMessage(unsigned char *rawMessage)
{
tMessageType *messageFields = (tMessageType *)rawMessage;
printf("MessageId is %d\n", messageFields->messageId);
}
Obviously, this is the same thing you would do in C, but I find that the overhead of having to decode the message into a class is usually not worth it.
As every one says, the only real difference is the default access. But I particularly use struct when I don't want any sort of encapsulation with a simple data class, even if I implement some helper methods. For instance, when I need something like this:
struct myvec {
int x;
int y;
int z;
int length() {return x+y+z;}
};
To answer my own question (shamelessly), As already mentioned, access privileges are the only difference between them in C++.
I tend to use a struct for data-storage only. I'll allow it to get a few helper functions if it makes working with the data easier. However as soon as the data requires flow control (i.e. getters/setters that maintain or protect an internal state) or starts acquring any major functionality (basically more object-like), it will get 'upgraded' to a class to better communicate intent.
For C++, there really isn't much of a difference between structs and classes. The main functional difference is that members of a struct are public by default, while they are private by default in classes. Otherwise, as far as the language is concerned, they are equivalent.
That said, I tend to use structs in C++ like I do in C#, similar to what Brian has said. Structs are simple data containers, while classes are used for objects that need to act on the data in addition to just holding on to it.
Structs (PODs, more generally) are handy when you're providing a C-compatible interface with a C++ implementation, since they're portable across language borders and linker formats.
If that's not a concern to you, then I suppose the use of the "struct" instead of "class" is a good communicator of intent (as #ZeroSignal said above). Structs also have more predictable copying semantics, so they're useful for data you intend to write to external media or send across the wire.
Structs are also handy for various metaprogramming tasks, like traits templates that just expose a bunch of dependent typedefs:
template <typename T> struct type_traits {
typedef T type;
typedef T::iterator_type iterator_type;
...
};
...But that's really just taking advantage of struct's default protection level being public...
As others have pointed out
both are equivalent apart from default visibility
there may be reasons to be forced to use the one or the other for whatever reason
There's a clear recommendation about when to use which from Stroustrup/Sutter:
Use class if the class has an invariant; use struct if the data members can vary independently
However, keep in mind that it is not wise to forward declare sth. as a class (class X;) and define it as struct (struct X { ... }).
It may work on some linkers (e.g., g++) and may fail on others (e.g., MSVC), so you will find yourself in developer hell.
Both struct and class are the same under the hood though with different defaults as to visibility, struct default is public and class default is private. You can change either one to be the other with the appropriate use of private and public. They both allow inheritance, methods, constructors, destructors, and all the rest of the goodies of an object oriented language.
However one huge difference between the two is that struct as a keyword is supported in C whereas class is not. This means that one can use a struct in an include file that can be #include into either C++ or C so long as the struct is a plain C style struct and everything else in the include file is compatible with C, i.e. no C++ specific keywords such as private, public, no methods, no inheritance, etc. etc. etc.
A C style struct can be used with other interfaces which support using C style struct to carry data back and forth over the interface.
A C style struct is a kind of template (not a C++ template but rather a pattern or stencil) that describes the layout of a memory area. Over the years interfaces usable from C and with C plug-ins (here's looking at you Java and Python and Visual Basic) have been created some of which work with C style struct.
An advantage of struct over class is that it save one line of code, if adhering to "first public members, then private". In this light, I find the keyword class useless.
Here is another reason for using only struct and never class. Some code style guidelines for C++ suggest using small letters for function macros, the rationale being that when the macro is converted to an inline function, the name shouldn't need to be changed. Same here. You have your nice C-style struct and one day, you find out you need to add a constructor, or some convenience method. Do you change it to a class? Everywhere?
Distinguishing between structs and classes is just too much hassle getting into the way of doing what we should be doing - programming. Like so many of C++'s problems it arises out of the strong desire for backwards compatibility.
They are pretty much the same thing. Thanks to the magic of C++, a struct can hold functions, use inheritance, created using "new" and so on just like a class
The only functional difference is that a class begins with private access rights, while a struct begins with public. This is the maintain backwards compatibility with C.
In practice, I've always used structs as data holders and classes as objects.
Class.
Class members are private by default.
class test_one {
int main_one();
};
Is equivalent to
class test_one {
private:
int main_one();
};
So if you try
int two = one.main_one();
We will get an error: main_one is private because its not accessible. We can
solve it by initializing it by specifying its a public ie
class test_one {
public:
int main_one();
};
Struct.
A struct is a class where members are public by default.
struct test_one {
int main_one;
};
Means main_one is private ie
class test_one {
public:
int main_one;
};
I use structs for data structures where the members can take any value, it's
easier that way.
After years of programming in C++, my main language, I come to the dead conclusion that this is another one of C++ dumb feature.
There is no real difference between the two, and no reason why I should spend extra time deciding whether I should define my entity as a struct or a class.
To answer this question, feel free to always define your entity as a struct. Members will be public by default which is the norm. But even more importantly, inheritance will be public by default. Protected inheritance, and even worse, private inheritance, are the exceptions.
I have never had a case where private inheritance was the right thing to do. Yes I tried to invent problems to use private inheritance but it didn't work. And Java, the role model of Object Oriented programming defaults to public inheritance if you don't use the accessor keywords. And by the way, Java doesn't allow accessor keywords on inherited classes, they can only be publicly inherited. So you can see, the cpp team really fell down here.
Another frustrating thing about this, is that if you define as a class and declare as a struct you get compilation warning. As though this is something that impacted the performance or accuracy of your program. One answer also noted that MSVC may propogate a compiler error instead.
Those persons that use classes when it is raining and structs when it is shining are doing so based on what they have been taught. It's not something they discovered to be true. Java does not have a pair of names for classes, and only have the class keyword. If you want a data structure, simply make all your members public and don't add functions. This works in Java and I don't see any problem. What's the problem? You need 4 or 5 characters of BOM code to determine how to interpret the context of a class entity.
they're the same thing with different defaults (private by default for class, and public by default for struct), so in theory they're totally interchangeable.
so, if I just want to package some info to move around, I use a struct, even if i put a few methods there (but not many). If it's a mostly-opaque thing, where the main use would be via methods, and not directly to the data members, i use a full class.
Structs by default have public access and classes by default have private access.
Personally I use structs for Data Transfer Objects or as Value Objects. When used as such I declare all members as const to prevent modification by other code.
Just to address this from a C++20 Standardese perspective (working from N4860)...
A class is a type. The keywords "class" and "struct" (and "union") are - in the C++ grammar - class-keys, and the only functional significance of the choice of class or struct is:
The class-key determines whether ... access is public or private by default (11.9).
Data member default accessibility
That the class keyword results in private-by-default members, and `struct keyword results in public-by-default members, is documented by the examples in 11.9.1:
class X {
int a; // X::a is private by default: class used
...vs...
struct S {
int a; // S::a is public by default: struct used
Base class default accessibility
1.9 also says:
In the absence of an access-specifier for a base class, public is assumed when the derived class is defined with the class-key struct and private is assumed when the class is defined with the class-key class.
Circumstances where consistent use of struct or class is required...
There's a requirement:
In a redeclaration, partial specialization, explicit specialization or explicit instantiation of a class template, the class-key shall agree in kind with the original class template declaration (9.2.8.3).
...in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration (9.7.1), the union class-key shall be used to refer to a union (11.5), and either the class or struct class-key shall be
used to refer to a non-union class (11.1).
The following example (of when consistency is not required) is provided:
struct S { } s;
class S* p = &s; // OK
Still, some compilers may warn about this.
Interestingly, while the types you create with struct, class and union are all termed "classes", we have...
A standard-layout struct is a standard layout class defined with the class-key struct or the class-key class.
...so in Standardese, when there's talk of a standard-layout struct it's using "struct" to imply "not a union"s.
I'm curious if there are similar use of "struct" in other terminology, but it's too big a job to do an exhaustive search of the Standard. Comments about that welcome.
Technically both are the same in C++ - for instance it's possible for a struct to have overloaded operators etc.
However :
I use structs when I wish to pass information of multiple types simultaneously
I use classes when the I'm dealing with a "functional" object.
Hope it helps.
#include <string>
#include <map>
using namespace std;
struct student
{
int age;
string name;
map<string, int> grades
};
class ClassRoom
{
typedef map<string, student> student_map;
public :
student getStudentByName(string name) const
{ student_map::const_iterator m_it = students.find(name); return m_it->second; }
private :
student_map students;
};
For instance, I'm returning a struct student in the get...() methods over here - enjoy.
When would you choose to use struct
and when to use class in C++?
I use struct when I define functors and POD. Otherwise I use class.
// '()' is public by default!
struct mycompare : public std::binary_function<int, int, bool>
{
bool operator()(int first, int second)
{ return first < second; }
};
class mycompare : public std::binary_function<int, int, bool>
{
public:
bool operator()(int first, int second)
{ return first < second; }
};
I use structs when I need to create POD type or functor.
All class members are private by default and all struct members are public by default.
Class has default private bases and Struct has default public bases. Struct in case of C cannot have member functions where as in case of C++ we can have member functions being added to the struct. Other than these differences, I don't find anything surprising about them.
I use struct only when I need to hold some data without any member functions associated to it (to operate on the member data) and to access the data variables directly.
Eg: Reading/Writing data from files and socket streams etc. Passing function arguments in a structure where the function arguments are too many and function syntax looks too lengthy.
Technically there is no big difference between class and struture except default accessibility.
More over it depends on programming style how you use it.
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I thought that Structs was intended as a Data Structure (like a multi-data type array of information) and classes was inteded for Code Packaging (like collections of subroutines & functions)..
:(
I never use "struct" in C++.
I can't ever imagine a scenario where you would use a struct when you want private members, unless you're willfully trying to be confusing.
It seems that using structs is more of a syntactic indication of how the data will be used, but I'd rather just make a class and try to make that explicit in the name of the class, or through comments.
E.g.
class PublicInputData {
//data members
};
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What are the differences between struct and class in C++
class and struct in c++
It looks like that struct can has constructor and destructor and members, and looks very simple, so can we use struct instead class, if not, when shall we use struct with functions when shall we use class ?
https://github.com/developmentseed/node-sqlite3/blob/master/src/database.h#L32
struct Baton {
uv_work_t request;
Database* db;
Persistent<Function> callback;
int status;
std::string message;
Baton(Database* db_, Handle<Function> cb_) :
db(db_), status(SQLITE_OK) {
db->Ref();
uv_ref(uv_default_loop());
request.data = this;
callback = Persistent<Function>::New(cb_);
}
virtual ~Baton() {
db->Unref();
uv_unref(uv_default_loop());
callback.Dispose();
}
};
struct OpenBaton : Baton {
std::string filename;
int mode;
OpenBaton(Database* db_, Handle<Function> cb_, const char* filename_, int mode_) :
Baton(db_, cb_), filename(filename_), mode(mode_) {}
};
There's absolutely no technical reason to prefer one over the other, but I've noticed a certain convention regarding the use of class or struct.
If your datatype is something that is meant to be used by other parts of your program (ie. it's part of the 'interface'), then usually people make it a class to indicate its importance. If the datatype is used only in the implementation of a function or a class and it is not visible outside of a certain scope, then make it a struct.
These are some very rought guidelines, but no one will complain if you don't follow them.
Edit: In C++ there's no real difference between the two, but other newer languages that are inspired by C++ have actually made struct and class different. In C# and in D, for example, class and struct are both used to define datatypes, but they are not the same: struct is implemented such that it should be used for 'small' types.
The only difference is the default access-level (private for a class, public for a struct). Other than that, they are completely interchangeable. You should decide which one you like better, and use that all the time (consistency makes your code more readable).
when shall we use struct with functions when shall we use class ?
It is completely your choice.
There is nothing that one can do with classes and not with structures in C++.
Only difference between structure and class are:
access specifier defaults to private for class and public for struct
inheritance defaults to private for class and public for struct
So just use the one of your choice and stick to using it consistently, do not mix classes and structures.
While as stated by other struct & class does not have any difference besides default access level. However, it's common practice to use structs mostly for data aggregation, as that is what structs are reduced to in C. For example user defined PODs are almost always created as structs in my experience.
The only difference between class and struct is the default accessibility to its members and base classes. For struct, it is public and for class, it is private.
As others have said, the main difference is the default access level of member data and functions, namely private for class and public for structs. The same goes for default inheritance access levels: private for classes and public for structs.
As for when to use which, that is a matter of what is normal for the company to do. In my experience, most companies, and indeed individuals, use structs to hold packets of pure data and classes for storing a collection of functions that operate on its own data and/or structs.
This method is a throwback to C programming where structs can only store data and not functions and so most people like to stick to this definition in C++ too.
Note that it is common to use structs for functors, which would seem to break consistency through the code of structs not containing functions, but since functors usually only overload the () operator we retain some form of consistency anyway. Plus, it saves us having to type public for one function and/or inherited structures... Oh the typing we allow ourselves to avoid ;)
A class is a reference type. When an object of the class is created, the variable to which the object is assigned holds only a reference to that memory. When the object reference is assigned to a new variable, the new variable refers to the original object. Changes made through one variable are reflected in the other variable because they both refer to the same data.
A struct is a value type. When a struct is created, the variable to which the struct is assigned holds the struct's actual data. When the struct is assigned to a new variable, it is copied. The new variable and the original variable therefore contain two separate copies of the same data. Changes made to one copy do not affect the other copy.
In general, classes are used to model more complex behavior, or data that is intended to be modified after a class object is created. Structs are best suited for small data structures that contain primarily data that is not intended to be modified after the struct is created.
I am running a simulation in which I have objects of a class which use different models. These models are randomly selected for some objects of the class and specifically decided for some objects too. These objects communicate with each other for which I am using structures (aka struct) in C++ which has some
standard variables and
some additional variables which depends on models which the objects communicating with each other have.
So, how can I do this?
Thanks in advance.
You can hack around with:
the preprocessor;
template meta-programming;
inheritance/polymorphism.
Each gives a different way of producing a different user-defined type, based on different kinds of conditions.
Without knowing what you're trying to accomplish, this is the best I can do.
All instances of a structure or class have the same structure. Luckily, there are some tricks that can be used to 'simulate' what you try to do.
The first trick (which can also be used in C), is to use a union, e.g.:
struct MyStruct
{
int field1;
char field2;
int type;
union
{
int field3a;
char field3b;
double field3c;
} field3;
};
In a union, all members take up the same space in memory. As a programmer you have to be careful. You can only get out of the union what you put in. If you initialize one member of a union, but you read another member, you will probable get garbage (unless you want to do some low-level hacks, but don't do this unless you are very experienced).
Unions often come together with another field (outside the union) that indicates which member is actually used in the union. You could consider this your 'condition'.
A second trick is use the 'state' pattern (see http://en.wikipedia.org/wiki/State_pattern). From the outside world, the context class looks always the same, but internally, the different states can contain different kinds of information.
A somewhat simplified approach for state is to use simple inheritance, and to use dynamic casts. Depending on your 'condition', use a different subclass, and perform a dynamic cast to get the specific information.
E.g., suppose that we have a Country class. Some countries have a president, others have a king, others have an emperor. You could something like this:
class Country
{
...
};
class Republic : public Country
{
public:
const string &getPresident() const;
const string &getVicePresident() const;
};
class Monarchy : public Country
{
public:
const string &getKing() const;
const string &getQueen() const;
};
In your application you could work with pointers to Country, and do a dynamic cast to Republic or Monarchy where the president or king is needed.
This example can be easily transformed into one using the 'state' pattern, but I leave this as an exercise for you.
Personally, I would go for the state pattern. I'm not a big fan of dynamic casts and they always seem to be kind-of-hack for me.
If it's at compile-time, a simple #ifdef or template specialization will serve this purpose just fine. If it's at run-time and you need value semantics, you can use a boost::optional<my_struct_of_optional_members>, and if you're fine with reference semantics, inheritance will solve the problem at hand.
A union and that kind of dirty trick is not necessary.
There are several common approaches for "dynamic" attributes/properties in languages, and a few that tend to work well in C++.
For example, you can make a C++ class called "MyProperties" that has a sparse set of values, and your MyStructureClass would have its well-known members, plus a single MyProperties instance which may have zero-or-more values.
Similarly, languages like Python and Perl make extensive use of Associative Arrays/Dictionaries/Hashes to achieve this: The (string) key uniquely identifies the value. In C++, you can index your MyProperties class with a string or any type you want (after overloading the operator[]()), and the value can be a string, a MyVariant, or any other pointer-or-type that you want to inspect. The values are dynamically added to the parent container as they are assigned (e.g., the class "remembers" the last value it is given, uniquely identified by key).
Finally, in the "olden days", what you describe was commonly done for distributed application processing: You defined a C-struct with "well-known" (typed) fields/members, and the last field was a char* member. Then, that char* member would identify the start of a serialized stream of bytes that were also part of that struct (you merely serialized that array of chars when you marshalled the struct across systems). In the context of C++, you could similarly extract your values dynamically from that char* stream buffer on-access-demand (which logically should be "owned" by the class). This worked for marshalling across systems because the size of the struct was the size of everything (including the last char* member), but the "allocation" for that struct was much larger (e.g., the size of the struct itself, which was logically a "header", plus a certain number of bytes after that header, which represented the "payload" and which was indexed by the last member, the char* member.) Thus, it was a contiguous-block-of-memory struct, with dynamic size. (This would also work in C++ as long as you passed-by-reference, and never by value.)
embed an union into your structure, and use a flag to tell which part of the union is valid.
enum struct_type
{
cool,
fine,
bad
};
struct demo
{
struct_type type;
union
{
struct
{
double cool_factor;
} cool_part;
struct
{
int fineness;
} fine_part;
struct
{
char *bad_stuff;
} bad_part;
};
struct
{
int life_is_cool;
} common_part;
};
The pure and simple C++ answer is: use classes.
I can't determine from your question what you are trying to achieve: runtime variation or compile time variation, but either way, I doubt you'll get a workable implementation any other way. (Template metaprogramming aside... which isn't for the faint of heart.)
The following function writes a struct to a file.
#define PAGESIZE sizeof(BTPAGE)
#define HEADERSIZE 2L
int btwrite(short rrn, BTPAGE *page_ptr)
{
long addr;
addr = (long) rrn * (long) PAGESIZE + HEADERSIZE;
lseek(btfd, addr, 0);
return (write(btfd, page_ptr, PAGESIZE));
}
The following is the struct.
typedef struct {
short keycount; /* number of keys in page */
int key[MAXKEYS]; /* the actual keys */
int value[MAXKEYS]; /* the actual values */
short child[MAXKEYS+1]; /* ptrs to rrns of descendants */
} BTPAGE;
What would happen if I changed the struct to a class, would it still work the same?
If I added class functions, would the size it takes up on disk increase?
There's a lot you need to learn here.
First of all, you're treating a structure as an array of bytes. This is strictly undefined behavior due to the strict aliasing rule. Anything can happen. So don't do it. Use proper serialization (for example via boost) instead. Yes, it's tedious. Yes, it's necessary.
Even if you ignore the undefinedness, and choose to become dependant on some particular compiler implementation (which may change even in the next compiler version), there's still reasons not to do it.
If you save a file on one machine, then load it on another, you may get garbage, because the second machine uses a different float representation, or a different endianness, or has different alignment rules, etc.
If your struct contains any pointers, it's very likely that saving them verbatim then loading them back will result in an address that doesn't not point to any meaningful place.
Typically when you add a member function, this happens:
the function's machine code is stored in a place shared by all the class instances (it wouldn't make sense to duplicate it, since it's logically immutable)
a hidden "this" pointer is passed to the function when it's called, so it knows which object it's been called on.
none of this requires any storage space in the instances.
However, when you add at least one virtual function, the compiler typically needs to also add a data chunk called a vtable (read up on it). This makes it possible to call different code depending on the current runtime type of the object (aka polymorphism). So the first virtual function you add to the class likely does increase the object size.
In C++, the difference between a struct and a class is simply that the members and base classes of a struct are public by default, whereas for a class they are private by default.
The technique of simply writing the bytes of the struct to a file and then reading them back in again only works if the struct is a plain old data, or POD, type. If you modify your struct such that it is no longer POD, this technique is not guaranteed to work (the rules describing what makes a POD struct are listed in answers to thet linked question).
If the class has any virtual function, then you're in trouble; if no virtual functions, you should still be OK (the same applies to a struct, of course, since it, too, could have virtual functions: the difference between struct and class is just that the default visibility in struct is public, in class it's private).
If you are doing more serialisation of classes consider using google protocol buffers, or something similar see this question
Structure's member are public by default ans class's members are private by default. We can access private data members through a proper channel (using member function). If we have access to member functions we can read/write data in private data member, so how it is secure...we are accessing it and we are changing data too.....
Access specifiers, such as private and public have nothing to do with security. In C++, for example, these specifications get compiled away and don't even exist in the compiled binary.
Rather, access specifiers are designed to make types easy to use correctly, and difficult to use incorrectly.
There are only two syntactic differences between class and struct:
In a class, members and base classes are by default private, whereas in a struct, they are public by default.
For historical reasons, class can be used instead of typename to declare a template type parameter.
In fact, there's no difference between
struct X {
void f() {}
private:
int i;
};
and
class Y {
int i;
public:
void f() {}
};
The only way that a class is more "secure" than a struct is that it gently "presses" you towards better encapsulation by defaulting to private members.
There really was no need to introduce a new keyword class, Stroustrup himself said that on a few occasions. (Read his book The Design and Evolution of C++ if you're interested in such things.) Basically, class was introduced to emphasize the fact that a struct with member functions isn't really a "structure" anymore in the way the term was used in C: a collection of loosely coupled objects of built-in types. It's a "class" in the sense the term is used in statically typed object-oriented languages, even though syntactically, it's no real difference from a struct.
primarily because the member functions can validate the values before storing them. eg say you have a field called 'age', which must be between 0 and 150. in a structure (or a class with a public field), you could just do obj.age = 200. whereas if you only had a setAge(int) method, that method could sanity check the value is between 0 and 150 before storing, possibly throwing an exception if necessary or just clamping the value if not.
the public/private/protected keywords are not meant for security but rather for encapsulation. you design your class by separating the implementation (where the private/protected members are used - those actually used to implement the functionality) from the interface (where you expose the public members - which users of the class -other objects- will access/call) and thus you are later able to change the private parts without changing the interface. the outside objects only need to know about the public members and use these safely regardless of the implementation.
if you are speaking about security, think that anyone could change the private members of your class if they really wanted to and knew the internal structure of the class , just by overwriting the appropriate memory locations with new values - but the problem here is not about security, it's at a lower level
Taking that classes and structs are exactly the same besides the default access, the question is how encapsulating the internal representation from the interface helps build more robust code.
The main difference is that you control when and how your class data is modified, and as such you can control your class invariants. Consider a simple vector class that has a pointer and a size. By having them private you can control how they change: if the resize method is called, both the pointer and the internal size variable will be coherently updated, keeping the invariant that accessing any position in the range [0..size) is well defined. If the member attributes were public, user code would update the storage or size without actually updating the other field, and that invariant could be broken.
Many classes have internal invariants for the correct usage. If you write a class that contains a string with user email addresses, by providing an accessor method you can control that the value passed in is a valid email address, while if the email field was public user code could reset it to anything...
The whole thing is that you control how your members are accessed and modified and that reduces the number of places where mistakes can be made and/or your chances of detecting it.