CODEWITHSUNDEEP
c++classsizeof
pushE
pushE

Reputation: 414

What is the size of this class and Why?

I have two classes as follows

class A
{

};

class B
{
    int a[];
};


int main()
{
    cout << sizeof(A) <<endl;      //outputs 1
    cout << sizeof(B) <<endl;      //outputs 0
    return 0;
}

I am familiar that size of empty class is 1,but why is the size of class B coming to be ZERO??

Upvotes: 9

Views: 229

Answers (3)

6502
6502

Reputation: 114461

The size of an empty class is not 1. It's AT LEAST 1 in a C++ system. The reason is that you need to be able for example to allocate an instance with new and having a non-null pointer directed at it.

The second case instead is simply invalid C++.

Often compiler makers take some freedom by allowing non-standard "extensions" by default and try to make you use them unconsciously (a paranoid would say to lock you in by making your code unportable to other compilers).

Upvotes: 2

Michael Burr
Michael Burr

Reputation: 340178

GCC permits zero length arrays as an extension: http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

And:

As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero.

Upvotes: 6

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361302

Your code is ill-formed as far as C++ language is concerned. In particular, the class B shouldn't compile in C++ Standard Conformant compiler. Your compiler has either bug, or it provides this feature as extension.

GCC with -pedantic-errors -std=c++11 gives this error:

cpp.cpp:18:11: error: ISO C++ forbids zero-size array 'a' [-Wpedantic]
     int a[];
           ^

Upvotes: 4

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