Re: include file question

On Tue, 24 Mar 2009 09:33:58 -0700 (PDT), Ajay <ajaykalra@yahoo.com> wrote:


I haven't made that mistake since VC4, and it did not detect it. Concerning
ODR violations, the main thing I remember from the standard is, "No
diagnostic required."

Here's a program which violates the ODR, compiles fine in VC 2008, but
either prints garbage (no optimizations) or crashes (/O2) when run:

// cl -EHsc a1.cpp a2.cpp or
// cl -EHsc -O2 a1.cpp a2.cpp

// a1.cpp
#include <string>

struct A
{
   std::string s;
};

static A a = { "s" };

void print();

int main()
{
   A a2 = a;
   print();
}

// a2.cpp
#include <string>
#include <stdio.h>

struct A
{
   std::string s1;
   int x;
   std::string s2;
};

static A a = { "s1", 2, "s2" };

void print()
{
   A a2 = a;
   puts(a2.s1.c_str());
   puts(a2.s2.c_str());
}

Note that both a2 initializations are necessary to elicit the problem, as
is the member s1 in a2.cpp's A. Putting the structs in anonymous namespaces
avoids the problem.

The linker may indeed detect the error for functions you write yourself,
but these compiler-generated functions (ctors) remain a problem after all
this time, and the solution is to use anonymous namespaces.

--
Doug Harrison
Visual C++ MVP