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What are the differences between struct and class in C++

This question has been asked and answered a lot but every once in a while I come across something confusing.

To sum up, the difference between C++ structs and classes is the famous default public vs. private access. Apart from that, a C++ compiler treats a struct the same way it would treat a class. Structs could have constructors, copy constructors, virtual functions. etc. And the memory layout of a struct is same ad that of a class. And the reason C++ has structs is for backwward compatibility with C.

Now since people confuse as to which one to use, struct or class, the rule of thumb is if you have just plain old data, use a struct. Otherwise use a class. And I have read that structs are good in serialization but don't where this comes from.

Then the other day I came across this article: http://www.codeproject.com/Articles/468882/Introduction-to-a-Cplusplus-low-level-object-model

It says that if we have (directly quoting):

struct SomeStruct
{ 
    int    field1;
    char   field2;
    double field3;
    bool   field4; 
};

then this:

void SomeFunction()
{
    SomeStruct someStructVariable;
    // usage of someStructVariable
    ... 
}

and this:

void SomeFunction()
{ 
    int    field1;
    char   field2;
    double field3;
    bool   field4;      
    // usage of 4 variables
    ... 
}

are the same.

It says the machine code generated is the same if we have a struct or just write down the variables inside the function. Now of course this only applies if your struct if a POD.

This is where I get confused. In Effective C++ Scott Meyers says that there no such thing as an empty class.

If we have:

class EmptyClass { };

It is actually laid out by the compiler for example as:

class EmptyClass
{
    EmptyClass() {}
    ~EmptyClass() {}
    ...
};

So you would not have an empty class.

Now if we change the above struct to a class:

class SomeClass
{ 
    int  field1;
    char field2 
    double field3;
    bool  field4; 
};

does it mean that:

void SomeFunction()
{
    someClass someClassVariable;
    // usage of someClassVariable
    ... 
}

and this:

void SomeFunction()
{ 
    int  field1;
    char field2 
    double field3;
    bool  field4;      
    // usage of 4 variables
    ... 
}

are the same in terms of machine instructions? That there is no call to someClass constructor? Or that the memory allocated is the same as instantiating a class or defining the variables individually? And what about padding? structs and classes do padding. Would padding be the same in these cases?

I'd really appreciate if somebody can shed some light on to this.

Community
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madu
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  • Technically `struct` has a default inheritance access of `public` and `class` `private` as well. – chris Oct 08 '12 at 04:06
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    If, from reading that article, you get the impression that `struct` is in any way different from `class` except for the default member access protection level, then you got the wrong impression. Simple as that. – Seth Carnegie Oct 08 '12 at 04:14
  • -1: For lack of research. I typed just the *title* of your question into the "Ask Question" title box. And the very first question I got in the list of related questions (after this one of course) was a (closed) question that led the answer. That box with the list of related questions isn't just for show. – Nicol Bolas Oct 08 '12 at 07:58
  • @NicolBolas Thank you. I did my research. I have read that question number of times. However here my question is regarding the aforementioned article and the possible contradiction I felt. I asked about constructors and padding and assembly code. If you read the question you can see that I have already metioned public vs. private. My question was about code generated by compiler, which is not discussed in that question. I suggest you read the complete question before you type my title in to the "Ask Question". Please read some of the answers in this question and you will understand. Cheers. – madu Oct 08 '12 at 10:24

4 Answers4

2

I believe the author of that article is mistaken. Although there is probably no difference between the struct and the non-member variable layout version of the two functions, I don't think this is guaranteed. The only things I can think of that are guaranteed here is that since it's a POD, the address of the struct and the first member are the same...and each member follows in memory after that at some point.

In neither case, since it's a POD (and classes can be too, don't make THAT mistake) will the data be initialized.

I would recommend not making such an assumption anyway. If you wrote code that leveraged it, and I can't imagine why you'd want to, most other developers would find it baffling anyway. Only break out the legal books if you HAVE to. Otherwise prefer to code in manners that people are used to. The only important part of all this that you really should keep in mind that POD objects are not initialized unless you do so explicitly.

Edward Strange
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2

The only difference is that the members of structs are public by default, while the members of classes are private by default (when I say by default, I mean "unless specified otherwise"). Check out this code:

#include <iostream>
using namespace std;

struct A {
 int x;
 int y;
};

class A obj1;

int main() {
  obj1.x = 0;
  obj1.y = 1;
  cout << obj1.x << " " << obj1.y << endl;
  return 0;
}

The code compiles and runs just fine.

naxchange
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1

There is no difference between structs and classes besides the default for protection (note that default protection type for base classes is different also). Books and my own 20+ years experience tells this.

Regarding default empty ctor/dector. Standard is not asking for this. Nevertheless some compiler may generate this empty pair of ctor/dector. Every reasonable optimizer would immediately throw them away. If at some place a function that is doing nothing is called, how can you detect this? How this can affect anything besides consuming CPU cycles?

MSVC is not generating useless functions. It is reasonable to think that every good compiler will do the same.

Regarding the examples

struct SomeStruct
{ 
    int    field1;
    char   field2;
    double field3;
    bool   field4; 
}; 

void SomeFunction()
{ 
    int    field1;
    char   field2;
    double field3;
    bool   field4;      
   ... 
}

The padding rules, order in memory, etc may be and most likely will be completely different. Optimizer may easily throw away unused local variable. It is much less likely (if possible at all) that optimizer will remove a data field from the struct. For this to happen the struct should be in defined in cpp file, certain flags should be set, etc.

I am not sure you will find any docs about padding of local vars on the stack. AFAIK, this is 100% up to compiler for making this layout. On the contrary, layout of the structs/classes are described, there are #pargma and command line keys that control this, etc.

Kirill Kobelev
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are the same in terms of machine instructions?

There is no reason not to be. But there is no gurantee from the standard.

That there is no call to someClass constructor?

Yes there is a call to the constructor. But the constructor does no work (as all the members are POD and the way you declare someClass someClassVariable; causes value initialization which does nothing for POD members). So since there is no work to be done there is no need to plant any instructions.

Or that the memory allocated is the same as instantiating a class or defining the variables individually?

The class may contain padding that declaring the variables individually does not.
Also I am sure that the compiler will have an easier time optimizing away individual variables.

And what about padding?

Yes there is a possibility of padding in the structure (struct/class).

structs and classes do padding. Would padding be the same in these cases?

Yes. Just make sure you compare apples to apples (ie)

struct SomeStruct
{ 
    int    field1;
    char   field2;
    double field3;
    bool   field4; 
}; 
class SomeStruct
{ 
  public:           /// Make sure you add this line. Now they are identical.
    int    field1;
    char   field2;
    double field3;
    bool   field4; 
};
Martin York
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