definition error in visual studio C++

Posted on 2014-01-18
Last Modified: 2014-01-21

I seem to be getting the following error all of a sudden when trying to compile my code. I dont fully understand what it means. The part that is really confusing is that I have not made any changes to the A.h file or included it specifically anywhere between the last compiling stage and the current non-compilable stage. Any ideas how this might have come  up andhow i can  fix this?

c:\A.h(107) : error C2766: explicit specialization; '`anonymous-namespace'::ctti<int>' has already been defined
3>        c:\A.h(95) : see previous definition of 'ctti<int>'

Find Results 1, "Entire Solution"
  C:\A.h(95):template<> struct ctti<int> : ctti_base<int> {

line 95 is
template<> struct ctti<int> : ctti_base<int> {
  typedef unsigned int size_type;
  typedef int  signed_type;

line 107 is
template<> struct ctti<boost::int32_t> : ctti_base<boost::int32_t> {
  typedef boost::uint32_t size_type;
  typedef boost::int32_t signed_type;
Question by:LuckyLucks
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LVL 32

Expert Comment

ID: 39791769
I never heard of this error before so I looked it up.
// C2766.cpp
// compile with: /c
template<class T> 
struct A {};

struct A<int> {};                   // specialization #1a

struct A<int> {};   // C2766     // specialization #1b
// try the following line instead
// struct A<char> {};     // specialization #2

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Specializations 1a and 1b are obviously duplicates, so #2 could be used instead.

Now back to your case. My guess is that struct ctti<int>  and struct ctti<boost::int32_t> are seen by the compiler on your system as the same in that if your system has int as a 32 bit word, then that is the same as boost::int32_t.
Just a guess.

Other wild guess is to make sure that you have guards around all your header files.
LVL 33

Expert Comment

ID: 39793997
Other wild guess is to make sure that you have guards around all your header files.
the guess is good and by my experience the more reasonable issue.

if the a.h was not "protected" by a

#pragma once 

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statement or by macros like

// a.h
#ifndef A_H
#define A_H
// put here the current contents of the header 
#endif // A_H

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the template definition would be defined twice when the a.h was included in two source files.

// b.h
#include "a.h"

// b.cpp
#include "b.h"
#include "a.h"   

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when including a.h the second time the error could occur when a.h is not protected.


Author Comment

ID: 39795952
i put in #pragma once in A.h - did not change the error.

I notcied something interesting though.
I tried to reproduce the errror by slowly adding code that i thought might introduce the error in the first place.

Seems when I add the below line of code to one of my other header files C.h (not related or refering to A.h in any way) ,
std::tr1::unordered_map<size_t,std::vector<UINT64>> myhash;
I get this error.

What's strange is if i comment out the line, the error does not go away.

What causes the compiler to not let go of this error?
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LVL 32

Expert Comment

ID: 39796127
try changing line 107 to use <boost::uint32_t> instead of <boost::int32_t>.

Author Comment

ID: 39797686
is there an easier way to find out which header file it's definition is being picked up first? Going through all the headers in the headers  seem a really painful way to go in a big project.
LVL 32

Accepted Solution

phoffric earned 500 total points
ID: 39798654
Not sure if this will help you. In Visual Studio, there are two toggles with regards to headers. In project properties, under Configuration Properties -> C/C++:
Preprocessor -> Preprocess to a File -> Yes (/P)
The (/P) provides a text .i file which shows what the compiler processes.

Advanced -> Show Includes -> Yes (/showIncludes)
As you compile, the headers that are included are shown as they are included by the preprocessor.

I would work with them one at a time, and compare the results to make sure that you understand the outputs.

Author Comment

ID: 39799003
@phoffric : thanks! there is a certain third party header file that is being included at numerous places. I am surprised that the compiler has not complained at the 2nd instance it was included but instead much later.
LVL 32

Expert Comment

ID: 39799011
Glad you were able to figure it out. Lord knows, we tried, but without a little more source code, all we could do is WAG. You asked the right question to narrow down the problem. Nice going.


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