Can I use memcpy in C++ to copy classes that have no pointers or virtual functions

Question

Say I have a class, something like the following;

class MyClass
{
public:
  MyClass();
  int a,b,c;
  double x,y,z;
};

#define  PageSize 1000000

MyClass Ar         


        

Answer 1

According to the Standard, if no copy constructor is provided by the programmer for a class, the compiler will synthesize a constructor which exhibits default memberwise initialization. (12.8.8) However, in 12.8.1, the Standard also says,

A class object can be copied in two ways, by initialization (12.1, 8.5), including for function argument passing (5.2.2) and for function value return (6.6.3), and by assignment (5.17). Conceptually, these two operations are implemented by a copy constructor (12.1) and copy assignment operator (13.5.3).

The operative word here is "conceptually," which, according to Lippman gives compiler designers an 'out' to actually doing memberwise initialization in "trivial" (12.8.6) implicitly defined copy constructors.

In practice, then, compilers have to synthesize copy constructors for these classes that exhibit behavior as if they were doing memberwise initialization. But if the class exhibits "Bitwise Copy Semantics" (Lippman, p. 43) then the compiler does not have to synthesize a copy constructor (which would result in a function call, possibly inlined) and do bitwise copy instead. This claim is apparently backed up in the ARM, but I haven't looked this up yet.

Using a compiler to validate that something is Standard-compliant is always a bad idea, but compiling your code and viewing the resulting assembly seems to verify that the compiler is not doing memberwise initialization in a synthesized copy constructor, but doing a memcpy instead:

#include 

class MyClass
{
public:
    MyClass(){};
  int a,b,c;
  double x,y,z;
};

int main()
{
    MyClass c;
    MyClass d = c;

    return 0;
}

The assembly generated for MyClass d = c; is:

000000013F441048  lea         rdi,[d] 
000000013F44104D  lea         rsi,[c] 
000000013F441052  mov         ecx,28h 
000000013F441057  rep movs    byte ptr [rdi],byte ptr [rsi] 

...where 28h is the sizeof(MyClass).

This was compiled under MSVC9 in Debug mode.

EDIT:

The long and the short of this post is that:

1) So long as doing a bitwise copy will exhibit the same side effects as memberwise copy would, the Standard allows trivial implicit copy constructors to do a memcpy instead of memberwise copies.

2) Some compilers actually do memcpys instead of synthesizing a trivial copy constructor which does memberwise copies.