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c++cvisual-studiopragma

error C2156: pragma must be outside function - why is it happening in this case?

Gives this set of 4 files:

file1.h:

#pragma once
#include <windows.h>

file1.cpp:

#include "file1.h"

file2.h:

#pragma once

#define N 666

file2.cpp:

#include "file2.h"
#include "file1.h"

When i try to compile file2.cpp with visual studio 2015 compiler cl -c file2.cpp i'll get a bunch of errors:

Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24215.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

file2.cpp
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(1179): error C2059: syntax error: 'constant'
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(2149): error C2598: linkage specification must be at global scope
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(4807): error C2598: linkage specification must be at global scope
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(4859): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(4991): error C2598: linkage specification must be at global scope
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5050): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5051): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5052): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5053): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5054): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5055): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5078): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5079): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5134): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5135): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5136): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(5137): error C2156: pragma must be outside function
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(15420): error C2059: syntax error: 'constant'
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(15420): error C2238: unexpected token(s) preceding ';'
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(15439): error C2059: syntax error: 'constant'
C:\Program Files (x86)\Windows Kits\8.1\include\\um\winnt.h(15439): error C2238: unexpected token(s) preceding ';'

But if i comment the line #define N 666 the file will be compiled ok. Could anyone explain me what's going on here? Cos I don't get it, docs aren't solving me anything.

Solution

  • The object-like macro N collides with windows.h, which includes winnt.h.

    For instance:

    extern "C++" // templates cannot be declared to have 'C' linkage
template <typename T, size_t N>
char (*RtlpNumberOf( UNALIGNED T (&)[N] ))[N];

    I would suggest to avoid using macros whenever you can. In both C and C++, you might consider to replace macro definition with const object:

    const int N = 666;

    (note that in C you may likely use static const, since the default linkage of const is external)