pointers issue

hello!

I'm writing a C++ program and I have to use memcpy at some point.
I will explain my problem:
I have a simple struct named A and a class named B and I do the following:

Code:

class B{
//private variables
public:
A* a1;
void* pVoid;
}


A a2;
a2=//assign a2 with a value

Now,at some point of my program I have an object of type B
and memcpy works fine.

Code:

B b1;
memcpy((void*)((char*)b1.pVoid,(void*)&a2,sizeof(A));

In another part of my program I have a pointer to an object of type B
as I call a function in which I send an object of type B like this:

function(&b1);

and when I get the results of memcpy it returns garbage or memory addresses,but not the value of a2 as it does above.So I think it must be pointer's fault..

Code:

function(B *b2){
memcpy((void*)((char*)b2->pVoid,(void*)&a2,sizeof(A));}

Does anybody have any idea how to fix it?

Originally Posted by quo

I'm writing a C++ program and I have to use memcpy at some point.

Fail (especially in this case !)
1.Define a Copy constructor. (and also assignment operator and destructor)
2.Use std::copy.

Originally Posted by manasij7479

Fail (especially in this case !)
1.Define a Copy constructor. (and also assignment operator and destructor)
2.Use std::copy.

No I have to use memcpy,that's not the issue.
I have to write something in memory,I don't want a copy constructor.
What I don't get is why it's not working with the pointer and works with the object.
Thank you for your time,hope you can help me

Your code looks very suspicious, but unless you post ALL of your code, it's hard to say whether it's just bad style or actually an error.

Originally Posted by quo

No I have to use memcpy,that's not the issue.
I have to write something in memory,I don't want a copy constructor.\

Sorry, I misunderstood your question.

In both the cases, B's pointers are not initialized... and the first one only works by luck.
(Well, it could be initialized, for the 2nd one but you likely didn't)

Now, would you tell me that you do not need to allocate memory ?

Originally Posted by antred

Your code looks very suspicious, but unless you post ALL of your code, it's hard to say whether it's just bad style or actually an error.

Unfortunately I can't do that as it is very very big

Then at least show us where you actually point that pVoid member variable at something.

std::memcpy - cppreference.com

Note that it's undefined behavior to memcpy classes.

It's probably a problem with parentheses. This works:

Code:

#include <iostream>
#include <cstring>

struct A {
    int n;
};

class B {
public:
    A* a1;
    void* pVoid;
};

void func(B *b) {
    A a;
    a.n = 2;
    std::memcpy(b->pVoid, &a, sizeof(A));
    std::cout << ((A*)(b->pVoid))->n << '\n';
}

int main() {
    A a;
    a.n = 1;
    B b;
    std::memcpy(b.pVoid, &a, sizeof(A));
    std::cout << ((A*)(b.pVoid))->n << '\n';
    func(&b);
}

Originally Posted by msh

std::memcpy - cppreference.com

Note that it's undefined behavior to memcpy classes.

It's perfectly defined for "C-compatible structs" (of course!).

Originally Posted by oogabooga

It's probably a problem with parentheses. This works:

Works?
Do you realize that the pVoid s can be anything when you copy stuff into locations pointed by it?

Aha. I missed that A is a struct. My mistake.

Originally Posted by oogabooga

It's probably a problem with parentheses. This works:

Code:

#include <iostream>
#include <cstring>

struct A {
    int n;
};

class B {
public:
    A* a1;
    void* pVoid;
};

void func(B *b) {
    A a;
    a.n = 2;
    std::memcpy(b->pVoid, &a, sizeof(A));
    std::cout << ((A*)(b->pVoid))->n << '\n';
}

int main() {
    A a;
    a.n = 1;
    B b;
    std::memcpy(b.pVoid, &a, sizeof(A));
    std::cout << ((A*)(b.pVoid))->n << '\n';
    func(&b);
}

thank you for your answer,it doesn't work though
&a has to be casted to void*

Originally Posted by msh

Aha. I missed that A is a struct. My mistake.

That means nothing. The only difference between a struct and a class is the default access level (public vs private). It's impossible to tell whether memcpy on a class / struct is safe without knowning the definition of said struct or class.

Originally Posted by antred

Then at least show us where you actually point that pVoid member variable at something.

points to data of a block file where objects of type A are stored