Initializing const members of structs in C/C++... compiler dependent?

Question

Recently I ran into a compiler error in a legacy environment using Borland C++ 5.2. I had a .cpp file which included a header from some C source which I don't control. The header contained a struct definition which included const members, and the compiler complained about a "constant member in class without constructors". On investigation, this error seems to be compiler-dependent. Here's some sample code w/ results from various compilers:

#include <stdio.h>

typedef struct {
   const float a;
} _floater;

int main()
{
   _floater f = {5.1F};

   printf("%f\r\n",f.a);

   return 0;
}

Borland 5.2

E:\Projects\Scratchpad>bcc32 -P const_float.c
Borland C++ 5.2 for Win32 Copyright (c) 1993, 1997 Borland International
const_float.c:
Error const_float.c 13: Constant member ' ::a' in class without constructors
*** 1 errors in Compile ***

Microsoft VS 2003 .NET:

E:\Projects\Scratchpad>cl /TP const_float.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86
Copyright (C) Microsoft Corp 1984-1998. All rights reserved.

const_float.c
const_float.c(19) : error C2552: 'f' : non-aggregates cannot be initialized with
initializer list

Microsoft VS 2008:

C:\Projects\Scratchpad>cl /TP const_float.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.21022.08 for 80x86
Copyright (C) Microsoft Corporation.  All rights reserved.

const_float.c
Microsoft (R) Incremental Linker Version 9.00.21022.08
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:const_float.exe
const_float.obj

C:\Projects\Scratchpad>const_float.exe
5.100000

G++ 3.3.3

$ g++ const_float.c -o const_float.exe
const_float.c:25:2: warning: no newline at end of file

$ ./const_float.exe
5.100000

Note that Borland fails at the declaration of the struct, because it has a const member but no constructors, while VS 2003 is ok w/ the declaration, but complains when you try to instantiate it with an initializer list – considering the struct a non-aggregate type. VS2008 and g++ are perfectly happy. [Apologies.. I just realized that the line #s in the errors are wrong because I stripped some commented-out lines before posting.]

Microsoft’s definition of aggregates is here: http://msdn.microsoft.com/en-us/library/0s6730bb.aspx. It’s not apparent to me that const members would make a struct non-aggregate, but maybe they did back in 2003.

It also appears that the latest Borland (Embarcadero) compiler treats this as a warning rather than an error: http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/devwin32/wrnmembnocons_xml.html .

So, 2 questions I guess:

1. Why the variance in compilers? Is the standard ambiguous on this point?
2. Any workarounds? Given that I'm stuck w/ the compiler version and the header file, I don't see any.

Thanks!

Answer 1

The standard is pretty clear. Having a const member doesn't bar a class from being an aggregate.

8.5.1 [dcl.init.aggr]

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

It is legal to copy-intialize a const object and this is the initialization that aggregate initialization performs on the members of the aggregate. The restrictions on not naming a const object with no user-declared constructor in the mem-initializer-list in 12.6.2 apply only to initialization by a constructor which doesn't apply because aggregate initialization happens instead.

As to why the older compilers fail, I don't know. I can only say that they don't conform to the standard in this respect.