"memcopy" style construction of an object in C++

Question

In certain embedded situations, memory needs to be moved with memcopy style functions (such as from external memory, or using closed API calls).

When such a C++ object needs to be moved this way, however it doesn't have a default constructor, you can't do something like this, :

class Object { 
    //local data
public:
    Object(/* not a default constructor */){}
}

//elsewhere:
Object o; //compiler will complain here...
memcpy_like_function(src_address, &o, sizeof(o));

because Object doesn't have a default constructor, and thus the compiler will complain about creating Object o.

Some notes from things that have shown up in the comments:

• memcpy_like_function is like memcpy, it isn't actually memcpy. The src_address isn't a pointer to an address I can reach, or an int representing a pointer to an address I can reach. It is an int representing an address in a memory space I can't reach. The only way for me to access this memory space is with this function.
• Object doesn't have a default constructor, has no virtual functions, is neither inherited from, nor inherits anything. Object is trivially copyable.

What is the correct way to deal with creating such an object in this situation, without putting it on the heap? Preferably, I would like to get a stack allocated object that will behave correctly with scope and destructors. For the purposes of this question, Object is not inheriting from anything.

Answer 1

This seems like a horribly bad idea, but assuming that your memcpy_like_function actually works, then you can just add a constructor to Object

class Object { 
    //local data
public:
    Object(void* src_address)
    {
        memcpy_like_function(src_address, this, sizeof(*this));
    }
};

//elsewhere:
Object o(src_address);

Answer 2

You cannot safely copy an object using a memcpy-like function unless the object is a POD type. If the object is a POD type, you should be able to use:

char destination[sizeof(Object)];
memcpy_like_function(src_address, destination, sizeof(destination));
Object* ptr = reinterpret_cast<Object*>(destination);

My gut-feel says that that should work under all compilers for POD types. Whether it is cause for undefined behavior under some rules of the standard, I am not sure.

If that is cause for undefined behavior under some rules of the standard, you won't be able to save a POD-type to a binary file and read it from the binary file in a standards compliant manner. They rely on the bit-pattern written to a file and read from a file to represent the object.

The following program produces the expected result under g++.

#include <iostream>
#include <cstring>

struct Object
{
   int i;
   double d;
   Object(int ii, double dd) : i(ii), d(dd) {}
};

int main()
{
   Object o1(10, 20.34);
   char dest[sizeof(Object)];
   memcpy(dest, &o1, sizeof(dest));
   Object* ptr = reinterpret_cast<Object*>(dest);
   std::cout << o1.i << ", " << o1.d << std::endl;
   std::cout << ptr->i << ", " << ptr->d << std::endl;
}

Update, in response to OP's comments

The following program works as expected under g++.

#include <iostream>
#include <cstring>

struct Object
{
   int i;
   double d;
   Object(int ii, double dd) : i(ii), d(dd) {}
};

Object testFunction(Object o1)
{
   char dest[sizeof(Object)];
   memcpy(dest, &o1, sizeof(dest));
   Object* ptr = reinterpret_cast<Object*>(dest);
   return *ptr;
}

int main()
{
   Object o1(10, 20.34);
   Object o2 = testFunction(o1);
   std::cout << o1.i << ", " << o1.d << std::endl;
   std::cout << o2.i << ", " << o2.d << std::endl;
   o2.i = 25;
   o2.d = 39.65;
   std::cout << o2.i << ", " << o2.d << std::endl;
}

Answer 3

because Object doesn't have a default constructor

When Object doesn't have a default constructor,

//Object o;//NG

there is no way to construct Object unless call another ctor or factory function. Because of that, you cannot call memcpy-like function.

When you have way to construct Object, to use memcpy-like function, Object class must grantee that it is trivially copyable class and standard-layout class(not equal to POD class).

trivial class : trivially copyable class && has no default user-defined constructor
POD class : trivial class && standard-layout class

Answer 4

What you could do is simply use an array of bytes, then cast.

Note: what you are doing is not really good C++ practice, typically you should use assignment operators or copy constructors, and you should stick to the safer C++ casts rather than a brute-force C-style cast.

Anyway, that being said, this code will work:

class Object {
    int i;

public:
    Object(int i) : i(i) {}

    void foo() const {
        std::cout << i << std::endl;
    }
};

void bla() {
    Object one(1);
    char *bytes = new char[sizeof(Object)];
    memcpy(bytes, &one, sizeof(Object));
    Object &anotherOne = (Object &) *bytes;
    anotherOne.foo();
    const Object &oneMore = (Object) *bytes;
    oneMore.foo();
    Object *oneMoreTime = (Object *) bytes;
    oneMoreTime->foo();
    delete[] bytes;
}

The output is:

1
1
1

In summary, you need to allocate a region of memory on the stack or the heap that will become the Object instance.