Frequently asked questions: the pitfalls and responses of sizeof in C + +

In C + +, sizeof is a frequently asked concept, such as the following several aspects of sizeof's face test repeatedly appeared in the major IT companies in the technical interview, we need to fully understand and grasp.
Note: I was asked this question when I was interviewing a big company.

Interview Question: define an empty data type, there are no member variables and member functions, to the type of sizeof, the result is how much? Answer: is 1, why not 0? An instance of an empty type does not contain any information, and it would have been expected that sizeof should be 0, but when we declare an instance of that type, it must occupy a certain amount of space in memory, otherwise it will not be able to be used (and cannot be asked for sizeof), and as to how much memory is consumed, the compiler decides , an instance of each empty type in Visual Studio occupies 1 bytes of space.   Extension 1: If you add a constructor and destructor to the type and then ask sizeof, what is the result? Answer: It's still 1. Calling constructors and destructors requires only knowing the address of the function, which is related only to the type, and not to the instance of the type , and the compiler does not add any additional information to the instance because of the two functions. Note: This is the result regardless of whether you are adding a constructor or destructor or any other type of function.   Extension 2: What if the destructor is marked as a virtual function ? Answer:the compiler for C + + once found a virtual function in a type, generates a virtual function table for that type, and adds a pointer to the virtual function table in each instance of the type, on a 32-bit machine, a pointer occupies 4 bytes of space, Therefore, sizeof gets 4, or 8if it is 64 bits. test Case:

1#include <stdio.h>2 3 structNulltype {};4 5 structtype16 {7 type1 () {}8~type1 () {}9     intPrint () {printf ("Alexia");return 0; }Ten }; One  A structtype2 - { - type2 () {} the     Virtual~type2 () {} - }; -  - intMain () + { -printf"sizeof (Nulltype) =%d\n",sizeof(Nulltype)); +printf"sizeof (type1) =%d\n",sizeof(type1)); Aprintf"sizeof (type2) =%d\n",sizeof(type2)); at  -     return 0; -}

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In C , sizeof (' a ') =4 instead of 1 because the character constants in C are of type int , so sizeof (' a ') is sizeof (int), which is different from C + +.

Frequently asked questions: the pitfalls and responses of sizeof in C + +