Question in basic C++

LordsAngel

Assume i have a base class A (which contains a virtual function "test") and its derived class B (implement the virtual function). Now i do :- A* a = new A; memset( a, 0, sizeof( A ) ); a->test(); // here it crashes?? Why? Thanks... Heaven is a girl I know so well She makes me feel good when I feel like hell Heaven is a girl that I've got to have And she makes me feel better when I'm feelin' bad Heaven is a girl that makes dreams come true Oh, no one does it good like heaven do, yeah

Cedric Moonen

I'm not totally sure but I think the reason is because once you have a virtual function in your class, you will have a virtual function table added in the class. That is, a table that will redirect the call to a function to the specific function (base or inherit) in function of the type of the object. So, if you 'erase' this table with 0, if you try to call a function within this table, you will try to call a function at memory location 0, which causes a crash.

Cédric Moonen Software developer
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Javagal Srinath

I think main idea behind virtual functions is they r there only so that derived classes could make use of it. If u had written A* a = new B ; ur code would have worked fine. I have not refreshed c++ from last 8 months if I am wrong anybody cancorrect me bye.

toxcct

all appologies...

Javagal Srinath

I posted before his reply. anyway i will make use of information

toxcct

[Message Deleted]

Stephen Hewitt

You are correct. This is a classic example of what goes wrong when you mix low level constructs with high level ones without a well developed understanding of how the high-level constructs are implemented. Every class that has virtual functions has a virtual function table; one table is shared between all instances of that class. It contains pointers to virtual functions (or sometimes adjustor thunks to the virtual functions if multiple inheritance is used). When the constructor of a class with virtual functions is called it initializes a pointer in the instance data to point to the class’s vtable: this is the pointer that's being overwritten. In short, if you use simple structs and classes (no virtual functions or multiple inheritance) the layout of the class is what you would naively expect; if you have a struct or class with high-level constructs such as virtual functions or multiple inheritance the layout of the class as a whole will not be so predictable and will contain the book keeping info needed to implement them. Steve

Javagal Srinath

u r almost right, I was in my own world to be honest I had written my suggestion when nobody had presented reply to the original question, but I failed to press submit. If I was totally wrong I would not have replied. why waste our time unnecessarily on useless point. bye(this time really)

Emilio Garavaglia

Just to be more precise: the v-table is not in the object itself, but somewhere in the system. Its leading address is in the object, and part of the object size, as the virtual base table address (if any). What is erased is not that table itself, but the table address, hence, by invoking a virtual function, you'll try to find the address by seeking the corresponding entry considering the memory "0" to be the table. The crash is not the function invoke, but the attempt to read an invalid memory page to detect wich function to invoke. 2 bugs found. > recompile ... 65534 bugs found. :doh:

Stephen Hewitt

Sometimes when you start a reply you're the first but you take some time to write it and by the time you send it it's out of date; it has happened to me before today. Steve

Maximilien

LordsAngel wrote:

memset( a, 0, sizeof( A ) );

Why the heck are you doing this ?

Maximilien Lincourt Your Head A Splode - Strong Bad

Cedric Moonen

A plain C habbit maybe ? In C, for initalizing all data members of a struct, it is quite practical to use such mechanism.

Cédric Moonen Software developer
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Nish Nishant

toxcct wrote:

man, either you have a problem in your time management, or you didn't refresh your browser :

It's happened to me a few times that I've replied and found someone's replied before me. Regards, Nish

Nish’s thoughts on MFC, C++/CLI and .NET (my blog)
Currently working on C++/CLI in Action for Manning Publications. Also visit the Ultimate Toolbox blog (New)

Gary R Wheeler

As a rule in C++, never initialize a struct/class using memset. That's what constructors are for.

Software Zen: delete this;

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Stephen Hewitt

There is nothing wrong with initializing a class/struct with memset if you know what you're doing. The basic rule is only use memset on classs/structs which: - Don't have any virtual functions. - Don't have any virtual base classes. The above two restrictions are also recursive into the classes members. For example a class itself may not have any virtual functions but some its members may and so it is NOT safe to use memset on it. While you are correct that "that's what constructors are for" there is little doubt that when it is safe to do so it will be more efficient. Most times the efficiency gain will not be worth it but, as always, there will be exceptions. If you really want to use memset but still want to use have virtual functions you can do so like this: struct CMyClass_Data { CMyClass_Data() { memset(this, 0, sizeof(*this)); // Safe as no virtuals. } // Put members here... }; class CMyClass : private CMyClass_Data { public: // Stuff goes here... virtual VFun1() { /* ... */ } virtual VFun2() { /* ... */ } // etc... // Stuff goes here... } Steve

Gary R Wheeler

While what you say is true, it still skates closer to the edge than I prefer. I use memset to initialize simple buffers and other types of POD (plain old data), and that's about it. I do use this construct, however:

struct MyData {
...
};
MyData data = { 0 };

in preference to memset most of the time. In this case, the compiler will complain if anything in type MyData has a non-trivial constructor, which guarantees at compile time that the operation is safe.

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Stephen Hewitt

I've got nothing against this technique and use it myself; but it can't achieve exactly the same result as the example I gave. If you want the members that you're ={0}ing to be members of the class with virtual functions this technique can't be used. I reiterate - in general I wouldn't bother with such confusing constructs - but there are exceptions to every rule. Steve

Gary R Wheeler

Stephen Hewitt wrote:

exceptions to every rule

Well, as long as you catch them properly...

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