Say there is a single warning such as path_statements, unused_variables. Is there a way to ignore a single instant of this, without isolating them into a code block or function?
To be clear, when there is a single warning in the code. I would like the ability to quiet only that warning, without having to do special changes addressing the particular warning.
And without this quieting warnings anywhere else, even later on in the same function.
With GCC this can be done as follows:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat"
/* Isolated region which doesn't raise warnings! */
this_would_raise_Wformat(args);
#pragma GCC diagnostic pop
Does Rust have the equivalent capability?
Note, am asking about the general case of how to quiet warnings.
Am aware there are ways to resolve unused var warning for eg.
To silence warnings you have to add the allow(warning_type) attribute to the affected expression or any of its parents. If you only want to silence the warning on one specific expression, you can add the attribute to that expression/statement:
fn main() {
#[allow(unused_variables)]
let not_used = 27;
#[allow(path_statements)]
std::io::stdin;
println!("hi!");
}
However, the feature of adding attributes to statements/expressions (as opposed to items, like functions) is still a bit broken. In particular, in the above code, the std::io::stdin line still triggers a warning. You can read the ongoing discussion about this feature here.
Often it is not necessary to use an attribute though. Many warnings (like unused_variables and unused_must_use) can be silenced by using let _ = as the left side of your statement. In general, any variable that starts with an underscore won't trigger unused-warnings.
If you want to silence all warnings of a kind in a module, write e.g. #![allow(dead_code)] (note the exclamation mark) at the top of the module. This will disable all warnings of this kind in the whole module. You can also call rustc with e.g. -A dead_code.
You can disable all warnings by writing #![allow(warnings)] at the top of the module.
You can insert a module (as described in the Rust book) where the specific warnings are ignored.
As Lukas said, you can also write e.g. #[allow(dead_code)] on a statement or an expression.
I am getting this single error when I am linking my project,
COMMUNICATION.obj : fatal error LNK1179: invalid or corrupt file:
duplicate COMDAT '_IID_IXMLDOMImplementation'
What is the source of the problem?
This is a tricky one.
The issue is that the symbol(s)-generated is too-long, and an ambiguity exists:
//...
void MyVeryLongFunctionNameUnique_0(void);
void MyVeryLongFunctionNameUnique_1(void);
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// (example max-symbol-length-seen-by-linker)
In this case, the linker "sees" these two functions as the "same", because the part that makes them "unique" is longer-than-the-max-symbol length.
This can happen in at least three cases:
Your symbol names are "too-long" to be considered unique to the linker, but may have been fine for the compiler (such as when you expand-out from many nested templates)
You did some "trickery" that is invalid C++, and it passed the compiler, but you now have an invalid *.obj, and it chokes the linker.
You specified duplicate "unnamed" classes/structs, and the linker cannot resolve them.
===[UPDATE]===, It's not your fault, it's an internal problem with the compiler and/or linker (see below for possible work-arounds).
Depending on the issue (above), you can "increase" your symbol-length (by limiting-your-decrease-of-symbol-length), or fix your code to make it valid (unambiguous) C++.
This error is (minimally) described by Microsoft at:
http://msdn.microsoft.com/en-us/library/cddbs9aw(v=vs.90).aspx
NOTE: This max-symbol-length can be set with the /H option, see: http://msdn.microsoft.com/en-us/library/bc2y4ddf(v=vs.90).aspx
RECOMMEND: Check to see if /H is used on your command-line. If it is, delete it (do not specify max-symbol-length, it will default to 2,047, the /H can only DECREASE this length, not increase it).
However, you probably triggered it through the /Gy option (function-level-linking), which was probably implied through one of /Z7, /Zi, or /ZI: http://msdn.microsoft.com/en-us/library/958x11bc(v=vs.90).aspx
One MSDN thread that talks about this issue is:
http://social.msdn.microsoft.com/Forums/en-US/vcmfcatl/thread/57e3207e-9fab-4b83-b264-79a8a717a8a7
This thread suggests that it's possible to trigger this issue with "invalid-C++-code-that-compiles" (you get your *.obj), but that invalid-*.obj chokes the linker (this example attempts to use main as both a function and as a template):
http://www.cplusplus.com/forum/lounge/46361/
===[UPDATE]===
I should have said this before, because I suspected, but I now have more information: It might not be your fault, there seems to be an issue in the compiler and/or linker that triggers this error. This is despite the fact that the only common denominator in all your failed relationships is you.
Recall that the "above-list" applies (it MIGHT be your fault). However, in the case where, "it's not your fault", here's the current-running-list (I'm confident this list is NOT complete).
There is an internal error/corruption in your *.ilk file (intermediate-link-file). Delete it and rebuild.
You have /INCREMENTAL turned on for linking, but somehow that incremental-linking is not working for your project, so you should turn it off and rebuild (Project-Properties=>Configuration Properties=>Linker=>General=>Enable Incremental Linking [set to "No" (/INCREMENTAL:NO)]
There's a problem with "Optimization" for "COMDAT Folding" in your use. Your can "Remove Redundant COMDATs" by going to Project Proerties=>Configuration Properties=>Linker=>Optimization=>Enable COMDAT Folding, set to "Remove Redundant COMDATs (/OPT:ICF)
Here's an interesting thread from a guy who sometimes can link, and sometimes not, by commenting in/out a couple lines of code. It's not the code that is the problem -- he just cannot link consistently, and it looks like the compiler and/or linker has an internal problem under some obscure use case:
http://www.pcreview.co.uk/forums/fatal-error-lnk1179-t1430593.html
Other observations from a non-trivial web search:
this problem appears to be non-rare
it seems to be related to some form of template<> use
others seem to see this problem with "Release" build when it did not have this problem with "Debug" build (but it is also seen on the "Debug" build in many cases)
if the link "fails" on one machine, it may "succeed" on another build machine (not sure why, a "clean-build" appears to have no effect)
if you comment in/out a particularly significant couple-lines-of-code, and finish your build, and keep doing this until all the code is un-commented again, your link may succeed (this appears to be repeatable)
if you get this error with MSVC2008, and you port your code to MSVC2010, you will still get this error
===[PETITION TO THE GOOD PEOPLE OF THE WORLD]===
If you have other observations on this error, please list them below (as other answers, or as comments below this answer). I have a similar problem, and it's not my fault, and none of these work-arounds worked for me (although they did appear to work for others in their projects in some cases).
I'm adding a bounty because this is driving me nuts.
===[UPDATE+2]===
(sigh), Here's more things to try (which apparently work for others, but did not work for me):
this guy changed his compile settings, and it worked (from thread at http://forums.codeguru.com/showthread.php?249603.html):
Project->Settings->C++ tab, Debug cathegory: Inline function expansion: change from 'None' to 'Only _inline'.
the above thread references another thread where the had to re-install MSVC
it is possibly related to linking modules with "subtle-differences" in possibly-incompatible compiler and/or link switches. Check that all the "contributing libs" are built with the exact same switches
Here's some more symptoms/observations on this error/bug:
list(s) for above issues still apply
the issue seems to "start-showing-up" with MSVC2005, and continues with the same behavior for MSVC2008 and MSVC2010 (error still occurs after porting code to newer compilers)
restarting IDE, rebooting machine doesn't seem to work for anybody
one guy said an explicit "clean" followed by a recompile worked for him, but many others say it did not work for them
is often related to "incremental linking" (e.g., turn it off)
Status: No joy.
===[UPDATE+3 : LINK SUCCESS]===
Super-wacky-makes-no-sense fix to successfully link discovered!
This is a variation on (above), where you "fiddle-with-the-code-until-the-compiler-and/or-linker-behaves". NOT GOOD that one might need to do this.
Specific single linker-error (LNK1179) was for MyMainBody<>():
#include "MyClassA.hpp"
#include "MyClassB.hpp"
#include "MyClassC.hpp"
#include "MyClassD.hpp"
#include "MyMainBody.hpp"
int main(int argc, char* argv[])
{
// Use a function template for the "main-body",
// implementation is "mostly-simple", instantiates
// some local "MyClass" instances, they reference
// each other, and do some initialization,
// (~50 lines of code)
//
// !!! LNK1179 for `MyMainBody<>()`, mangled name is ~236 chars
//
return MyMainBody<MyClassA,MyClassB,MyClassC,MyClassD>(argc,argv);
}
THE FIX:
Convert MyMainBody<>() from a "template<>" to an explicit function, LINK SUCCESS.
THIS FIX SUX, as I need the EXACT-SAME-CODE for other types in other utilities, and the MyMainBody<>() implementation is non-trivial (but mostly simple) instantiations-and-setups that must be done in a specific way, in a specific order.
But hey, it's a temporary work-around for now: Confirmed on MSVC2008 and MSVC2010 compilers (same LNK1179 error for each, successful link on each after applying the work-around).
THIS IS A COMPILER AND/OR LINKER ERROR, as the code is "simple/proper-C++" (not even C++11).
So, I'm happy (that I got a link after suffering full-time for 2+weeks). But, disappointed (that the compiler and/or linker has a STUPID GLARING PROBLEM with linking a SIMPLE TEMPLATE<> in this use-case that I couldn't figure out how to address).
FURTHER, the "Bounty Ended", but nobody else wanted to take this on (no other answers?), so looks like "+100" goes to nobody. (heavy-sigh)
This question has a lot of answers but none of them quite capture what was happening in my codebase, and what I suspect the OP was seeing back in 2012 when this question was asked.
The Problem
The COMDAT error on an IID_* type is easy to accidentally reproduce by using the #import directive with both the rename_namespace and named_guids attributes.
If two #imported type libraries contain the same interface, as is likely the case for OP's IXMLDOMImplementation, then the generated .tlh files will declare IID_IXMLDOMImplementation in both namespaces, leading to the duplicate.
For example, the code generated for:
#import <foo.tlb> rename_namespace("FOO") named_guids;
#import <bar.tlb> rename_namespace("BAR") named_guids;
...could be simplified into something like this:
namespace FOO {
extern "C" __declspec(selectany) const GUID IID_IFOOBAR = {0};
}
namespace BAR {
extern "C" __declspec(selectany) const GUID IID_IFOOBAR = {0};
}
Here's a simple RexTester reproduction of the problem: https://rextester.com/OLAC10112
The named_guids attribute causes the IID_* to be generated and the rename_namespace attribute wraps it in the namespace.
Unfortunately, in this case, extern "C" does not seem to work as expected when it appears inside a C++ namespace. This causes the compiler to generate multiple definitions for IID_FOOBAR in the same .obj file.
DUMPBIN /SYMBOLS or a hex editor confirms the duplicate symbols.
The linker sees these multiple definitions and issues a duplicate COMDAT diagnostic.
A Solution
Knowing that rename_namespace doesn't play well with named_guids, the obvious solution is to simply not use them together. It's probably easiest to remove the named_guids attribute and instead use the _uuidof() operator.
After removing named_guids from #import directives and touching up the code, replacing all uses of FOO::IID_IFooBar with _uuidof(FOO::IFooBar), my COM-heavy codebase is back to building again.
This issue is reported as a bug in some specific versions of Visual Studio 2017. Try patching 15.9.1 or later to fix this issue
Reported Issue in VS 15.8 Preview 4
Resolved patchs in VS 15.9 Preview 2
I encountered this problem whilst porting some code (1) from MSVC to GCC. To get the build to link on GCC, I had to provide empty implementations for some specialised templated functions (2), and this resulted in LNK1179 on MSVC. I was able to resolve by inlining the functions (3), i.e.
template<> template<> void LongName1<LongName2>::FunctionName(boost::library::type1 & a, const unsigned int b);
template<> template<> void LongName1<LongName2>::FunctionName(boost::library::type1 & a, const unsigned int b) {};
template<> template<> inline void LongName1<LongName2>::FunctionName(boost::library::type1 & a, const unsigned int b) {};
I had to do
c++ -> code generation -> enable function - level linking -> no
Hopefully my lame workaround will help someone: I make sure to manually delete ALL .obj AND intermediate build files (including at least .pch, .pdb, .tlog, .lastbuildstate and anything else just hanging out looking suspicious) and rebuild from scratch.
I suggest without evidence that having some files left over from a previous build tends to cause the problem to happen more frequently. In my specific build system, I delete and recreate the .vcxproj and .sln files from scratch as well.
My own personal suspicion is that some kind of race condition exists in the build/link process between the time that intermediate files are read and the time they are written in a large project. Again, I have no evidence this is true, but this is my only guess that seems to fit all the known facts of the bug.
I wrote Outlook addins years ago and I was asked to write another. Right off the bat, I ran into this problem and through a little process of elimination, I fixed mine.
It turns out that when you choose an extensibity project(I hand coded mine back in the day), it creates and save 2 objects that I was unaware of: DTE and DTE80. To create the interfaces that manipulate these objects, they import directly from the DLLs in stdafx.h. Being that I'm working on Outlook, I also needed to import a couple of interfaces: Office and Outlook.
So, seeing as this error popped up almost immediately after writing my first tidbits of code, I started over, and added one thing at a time. The project blew chunks in the described way right after I added:
//Added mvc
//The following #import imports MSO based on it's LIBID
#import "libid:2DF8D04C-5BFA-101B-BDE5-00AA0044DE52" version("2.2") lcid("0") rename_namespace("Office") raw_interfaces_only named_guids
using namespace Office;
//The following #import imports Outoloks Object lib based on it's LIBID
#import "libid:00062FFF-0000-0000-C000-000000000046" rename_namespace("Outlook") raw_interfaces_only named_guids
using namespace Outlook;
So, seeing as I had no intention of figuring out the DTE stuff, I just commented out them and anything having to do with them:
//The following #import imports VS Command Bars based on it's LIBID
// #import "libid:1CBA492E-7263-47BB-87FE-639000619B15" version("8.0") lcid("0") raw_interfaces_only named_guids
//The following #import imports DTE based on it's LIBID
// #import "libid:80cc9f66-e7d8-4ddd-85b6-d9e6cd0e93e2" version("8.0") lcid("0") raw_interfaces_only named_guids
//The following #import imports DTE80 based on it's LIBID
// #import "libid:1A31287A-4D7D-413e-8E32-3B374931BD89" version("8.0") lcid("0") raw_interfaces_only named_guids
After wandering around fixing the compile errors, it compiled and linked just fine. I'm not suggesting this will work for everybody, but it worked for me. Good luck to any who pass by here....
I got this error and was really confused about it. Ended commenting out everything in the referenced cpp and reintroduce things in small batches until the file was back in the same state as when I started. And I don't get the error anymore. To me this points to this in my case being a bug in the compiler but since I can't reproduce it anymore I can't get help further than that.
I'm on:
Microsoft Visual Studio Professional 2019
Version 16.11.3
I'm starting a new BREW project, and I'd like to compile with Warning Level 4 (/W4) to keep the application code nice and clean. The problem is that the BREW headers themselves don't compile cleanly with /W4.
In gcc you can differentiate between application and system headers by using -I and -isystem, and then by default gcc doesn't report any compilation warnings in system headers. Is there an equivalent mechanism in Visual C++?
Use this method around (a) header(s) that you cannot or don't want to change, but which you need to include.
You can selectively, and temporarily disable all warnings like this:
#pragma warning(push, 0)
// Some include(s) with unfixable warnings
#pragma warning(pop)
Instead of 0 you can optionally pass in the warning number to disable, so something like:
#pragma warning(push)
#pragma warning(disable : 4081)
#pragma warning(disable : 4706)
// Some code
#pragma warning(pop)
Visual C++ team has just added support for warning levels in external headers. You can find the details in their blog post: Broken Warnings Theory.
In essence it does automatically what the suggestions here were recommending to do manually: pushes new warning level right before #include directive and pops it up right after. There are additional flags to specify locations of external headers, flag to treat all <> includes as external, #pragma system_header and a feature not available in Clang or GCC (as of this writing) to see warnings in external headers across template instantiation stack when the template was instantiated in the user code.
Besides the comments under that post, you can also find some useful discussion in a reddit announcement for that post.
I don't believe Visual C++ lets you differentiate. You can fake it by using #pragma warning around the include:
#pragma warning(push, 0)
#include "mywarningheader.h"
#pragma warning(pop)
It seems like there is an answer to this.
this post talks about /external:I that can be used to include headers with a special set of warnings.
I have not tested it myself, but the blog post is from 2017.
The /external:anglebrackets /external:W0 compiler flags disable warnings on the headers imported with #include <...>.
You can change the W0 to W1, W2, W3, or W4, to set a different warning level for those.
Does anybody know how to switch off exception handling option in MSVC? I tried to set the option 'Enable C++ exceptions' to 'NO' and I got warning:
warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc.
I would like to switch off the exception handler, too, but I don't know how.
In my application I basically need more speed than stability, therefore I chose switching off the exception handling. I do not have any try/catch blocks, but I do use STL. When I switch the option 'Enable C++ exceptions' to 'NO' is there any way how to get rid of those warnings?
Most likely you're including one or more standard C++ headers that contain try/catch. The most typical case would be <iostream> - you will get this error on a file which consists of a single line that just includes that. Any other stream header will also do, as will locales.
If you look closely at the error message, it should reference two filenames, not one - your file, and the included file with the error. E.g. in my sample case of #include <iostream>, I get this:
except.cpp
C:\Program Files\Microsoft Visual Studio 9.0\VC\INCLUDE\xlocale(342) : warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
If you really don't want exceptions in the STL that will be linked to your program, define _HAS_EXCEPTIONS=0 in your whole project. Better than compiling your code with /EHsc if you don't plan on adding exception handling.
That warning means that you told the compiler you're not going to use exceptions yet you have a try {} catch() {} block in the code. It informs you that although you have that block, if an exception is thrown no desctructors are going to be executed. Turning exceptions off means exactly that - the compiler doesn't produce code for automatic destruction when the stack is unwound in the event of an exceptions.
In Visual Studio, /EH is found at Configuration Properties ➔ C/C++ ➔ Code Generation ➔ Enable C++ Exceptions.
Switching off exceptions is quite hard, as you're dealing with C++ here. It's really in the same category as switching off NULL pointers - how are you going to handle memory allocation failure, for instance?
That said, /EH specifies which exception handling model you want, and "none" is not an option. You can pick /EHa, /EHs, /EHac and /EHsc - [a]ynchronous with or without support for throwing extern "C" functions.
Do you still have try/catch block(s) in your code?
The first thing to do when you get stuck is look up the error on MSDN and/or Google for it. That usually helps. This is what MSDN says:
When the /EHsc option has not been enabled, an object with automatic storage in the frame, between the function doing the throw and the function catching the throw, will not be destroyed. However, an object with automatic storage created in a try or catch block will be destroyed. [...]
Using MSVC aka CL one can simply use the /kernel switch or not have any /EH switch. Windows C/C++ MSVC builds always have SEH. SEH is inbuilt in Windows.
If #if _HAS_EXCEPTIONS == 0 , MS STL will simply switch to SEH.
Is that standard C++? No it is not. Three keywords will be missing: try, throw and catch. And you need to learn about SEH.
So is that a "good thing"? Well for Windows low level C++, SEH is a good thing.
ps: It is not advisable to use /kernel switch, outside of kernel developments.