In my code, I am using an array xyz of 10 objects. When I am trying to access an element of the array using an unsigned int index like this: xyz[level], I get 'Buffer overrun' warning. Logically, I am pretty sure that level won't exceed 10. How to avoid this warning?
I'm probably teaching my grandmother to suck eggs here, but do remember that "level won't exceed 10" is wrong for an array of size 10:
char a[10];
a[10] = '\0'; // Bug, and "Buffer Overrun" warning.
Are you really sure? I never got this warning until now. So, double check.
Anyway, you can use the
#pragma warning( disable: 6386 )
preprocessor directive. I usually push and pop this to the "pragma stack"
#pragma warning( push )
#pragma warning( disable : 6386 )
// Some code
#pragma warning( pop )
as advised here.
Related
I create a main.cpp in my vscode with clangd enabled, and put the following code in it.
clangd warns the first line with the warning message:
warning: unterminated ‘#pragma pack (push, …)’ at end of file
The whole content of main.cpp:
#pragma pack(push) // warning on this line
#pragma pack(1)
struct A
{
int a;
short b;
char c;
};
#pragma pack(pop)
See also: https://releases.llvm.org/13.0.0/tools/clang/docs/DiagnosticsReference.html#wpragma-pack
I think it's a very common usage of #pragma pack(push), I don't understand why the warning is generated.
More strange to me, if I add a semicolon before the first line, the warning disappears.
; // Add a semicolon
#pragma pack(push) // The warning disappears
#pragma pack(1)
struct A
{
int a;
short b;
char c;
};
#pragma pack(pop)
What's the reason behind?
This is a known bug in clangd, tracked in https://github.com/clangd/clangd/issues/1167.
Please see that issue for an explanation of why this currently happens and a potential workaround.
I'm having the same warning. The semi-colon trick works for me using Qt 6.4 as well. I have no idea why it should.
In Visual C++ you can temporarily disable a warning by using pragma:
#pragma warning(suppress: 4307)
How can I disable a warning within a macro, e.g., when I cause an "integral constant overflow" warning like this:
#define TIMES_A_MILLION(x) x * 1000000
int value = TIMES_A_MILLION(4711);
I don't want to repeat the warning at every place where the macro is used, but want the suppression to be part of the macro it self.
It is obviously not possible to do like this:
#define TIMES_A_MILLION(x) \
#pragma warning(suppress: 4307) \
x * 1000000
In your case you have to use the extension __pragma
__pragma
instead of
#pragma
I was trying to create a small define to work around this bug in QITABENT and I noticed peculiar behavior of the #pragma warning (disable: ...) statement.
In the following code the define QITABENT generates warning C4838
static const QITAB qit[] =
{
QITABENT(MediaPlayerCallback, IMFPMediaPlayerCallback)
};
I can easily suppress this warning, this works:
#pragma warning( push )
#pragma warning( disable: 4838 )
static const QITAB qit[] =
{
QITABENT(MediaPlayerCallback, IMFPMediaPlayerCallback)
//{ 0 },
};
return QISearch(this, qit, riid, ppv);
#pragma warning( pop )
Now I wanted to make a define QITABENTEX which automatically suppresses the warning generated by QITABENT. But it seems impossible because when I write the following code the warning C4838 is not suppressed.
static const QITAB qit[] =
{
#pragma warning( push )
#pragma warning( disable: 4838 )
QITABENT(MediaPlayerCallback, IMFPMediaPlayerCallback)
#pragma warning( pop )
//{ 0 },
};
How does the compiler interpret this code in such a way that the warning is not suppressed?
This probably has to with when the definition for QITABENT is fully resolved.
(Note that I'm not really interested in making the above code work, I'm just really curious how its interpreted by the compiler)
Addendum:
Regarding the close vote and clarification: I got into a discussion with someone giving a shotgun/link-only answer, presumably only reading the question halfway through (since the question explained how to use #pragma in a macro which is not what I'm asking) now that answer got (self) deleted and I got a close vote for being unclear. So let me reiterate my intentions with this question:
This question is not about finding a define to suppress this warning
This question is not about how to suppress a warning in VC++ in general
This question is about trying to understand what happens with the three lines of suppression code in the last code sample. Why do they not have effect in that exact position (in an array initialization) but do have effect outside the array initialization? This probably boils down to answering how and when the pragma statements and macross are resolved.
The initializer list terminates with the closing curly bracket }, and that is where warning is generated.
Try this:
static const QITAB qit[] =
{
QITABENT(Derived, Base)
#pragma warning( push )
#pragma warning( disable: 4365 )
}
#pragma warning( pop )
;
[edited]
Per Mr.C64's comment below, corrected the order of parameters in QITABENT(Derived, Base).
Is there a way to disable just a single warning line in a cpp file with visual studio?
For example, if I catch an exception and don't handle it, I get error 4101 (unreferenced local variable). Is there a way to ignore this just in that function, but otherwise report it in the compilation unit? At the moment, I put #pragma warning (disable : 4101) at the top of the file, but that obviously just turns it off for the whole unit.
#pragma warning( push )
#pragma warning( disable : 4101)
// Your function
#pragma warning( pop )
If you only want to suppress a warning in a single line of code (after preprocessing)[1], you can use the suppress warning specifier:
#pragma warning(suppress: 4101)
// here goes your single line of code where the warning occurs
For a single line of code, this works the same as writing the following:
#pragma warning(push)
#pragma warning(disable: 4101)
// here goes your code where the warning occurs
#pragma warning(pop)
[1] Others have noted in comments below that if the following statement is an #include statement that the #pragma warning(suppress: 4101) statement would not effectively suppress the warning for every line in the header file. If one were intending to do that, one would need to utilize the push/disable/pop method instead.
#pragma push/pop are often a solution for this kind of problems, but in this case why don't you just remove the unreferenced variable?
try
{
// ...
}
catch(const your_exception_type &) // type specified but no variable declared
{
// ...
}
Example:
#pragma warning(suppress:0000) // (suppress one error in the next line)
This pragma is valid for C++ starting with Visual Studio 2005.
https://msdn.microsoft.com/en-us/library/2c8f766e(v=vs.80).aspx
The pragma is NOT valid for C# through Visual Studio 2005 through Visual Studio 2015.
Error: "Expected disable or restore".
(I guess they never got around to implementing suppress ...)
https://msdn.microsoft.com/en-us/library/441722ys(v=vs.140).aspx
C# needs a different format. It would look like this (but not work):
#pragma warning suppress 0642 // (suppress one error in the next line)
Instead of suppress, you have to disable and enable:
if (condition)
#pragma warning disable 0642
; // Empty statement HERE provokes Warning: "Possible mistaken empty statement" (CS0642)
#pragma warning restore 0642
else
That is SO ugly, I think it is smarter to just re-style it:
if (condition)
{
// Do nothing (because blah blah blah).
}
else
Use #pragma warning ( push ), then #pragma warning ( disable ), then put your code, then use #pragma warning ( pop ) as described here:
#pragma warning( push )
#pragma warning( disable : WarningCode)
// code with warning
#pragma warning( pop )
as #rampion mentioned, if you are in clang gcc, the warnings are by name, not number, and you'll need to do:
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-variable"
// ..your code..
#pragma clang diagnostic pop
this info comes from here
One may also use UNREFERENCED_PARAMETER defined in WinNT.H. The definition is just:
#define UNREFERENCED_PARAMETER(P) (P)
And use it like:
void OnMessage(WPARAM wParam, LPARAM lParam)
{
UNREFERENCED_PARAMETER(wParam);
UNREFERENCED_PARAMETER(lParam);
}
Why would you use it, you might argue that you can just omit the variable name itself. Well, there are cases (different project configuration, Debug/Release builds) where the variable might actually be used. In another configuration that variable stands unused (and hence the warning).
Some static code analysis may still give warning for this non-nonsensical statement (wParam;). In that case, you mayuse DBG_UNREFERENCED_PARAMETER which is same as UNREFERENCED_PARAMETER in debug builds, and does P=P in release build.
#define DBG_UNREFERENCED_PARAMETER(P) (P) = (P)
Instead of putting it on top of the file (or even a header file), just wrap the code in question with #pragma warning (push), #pragma warning (disable) and a matching #pragma warning (pop), as shown here.
Although there are some other options, including #pramga warning (once).
This question comes up as one of the top 3 hits for the Google search for "how to suppress -Wunused-result in c++", so I'm adding this answer here since I figured it out and want to help the next person.
In case your warning/error is -Wunused (or one of its sub-errors) or -Wunused -Werror only, the solution is to cast to void:
For -Wunused or one of its sub-errors only1, you can just cast it to void to disable the warning. This should work for any compiler and any IDE for both C and C++.
1Note 1: see gcc documentation here, for example, for a list of these warnings: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html, then search for the phrase "All the above -Wunused options combined" and look there for the main -Wunused warning and above it for its sub-warnings. The sub-warnings that -Wunused contains include:
-Wunused-but-set-parameter
-Wunused-but-set-variable
-Wunused-function
-Wunused-label
-Wunused-local-typedefs
-Wunused-parameter
-Wno-unused-result
-Wunused-variable
-Wunused-const-variable
-Wunused-const-variable=n
-Wunused-value
-Wunused = contains all of the above -Wunused options combined
Example of casting to void to suppress this warning:
// some "unused" variable you want to keep around
int some_var = 7;
// turn off `-Wunused` compiler warning for this one variable
// by casting it to void
(void)some_var; // <===== SOLUTION! ======
For C++, this also works on functions which return a variable marked with [[nodiscard]]:
C++ attribute: nodiscard (since C++17)
If a function declared nodiscard or a function returning an enumeration or class declared nodiscard by value is called from a discarded-value expression other than a cast to void, the compiler is encouraged to issue a warning.
(Source: https://en.cppreference.com/w/cpp/language/attributes/nodiscard)
So, the solution is to cast the function call to void, as this is actually casting the value returned by the function (which is marked with the [[nodiscard]] attribute) to void.
Example:
// Some class or struct marked with the C++ `[[nodiscard]]` attribute
class [[nodiscard]] MyNodiscardClass
{
public:
// fill in class details here
private:
// fill in class details here
};
// Some function which returns a variable previously marked with
// with the C++ `[[nodiscard]]` attribute
MyNodiscardClass MyFunc()
{
MyNodiscardClass myNodiscardClass;
return myNodiscardClass;
}
int main(int argc, char *argv[])
{
// THE COMPILER WILL COMPLAIN ABOUT THIS FUNCTION CALL
// IF YOU HAVE `-Wunused` turned on, since you are
// discarding a "nodiscard" return type by calling this
// function and not using its returned value!
MyFunc();
// This is ok, however, as casing the returned value to
// `void` suppresses this `-Wunused` warning!
(void)MyFunc(); // <===== SOLUTION! ======
}
Lastly, you can also use the C++17 [[maybe_unused]] attribute: https://en.cppreference.com/w/cpp/language/attributes/maybe_unused.
If you want to disable unreferenced local variable write in some header
template<class T>
void ignore (const T & ) {}
and use
catch(const Except & excpt) {
ignore(excpt); // No warning
// ...
}
In certain situations you must have a named parameter but you don't use it directly.
For example, I ran into it on VS2010, when 'e' is used only inside a decltype statement, the compiler complains but you must have the named varible e.
All the above non-#pragma suggestions all boil down to just adding a single statement:
bool f(int e)
{
// code not using e
return true;
e; // use without doing anything
}
Is there a way to disable just a single warning line in a cpp file with visual studio?
For example, if I catch an exception and don't handle it, I get error 4101 (unreferenced local variable). Is there a way to ignore this just in that function, but otherwise report it in the compilation unit? At the moment, I put #pragma warning (disable : 4101) at the top of the file, but that obviously just turns it off for the whole unit.
#pragma warning( push )
#pragma warning( disable : 4101)
// Your function
#pragma warning( pop )
If you only want to suppress a warning in a single line of code (after preprocessing)[1], you can use the suppress warning specifier:
#pragma warning(suppress: 4101)
// here goes your single line of code where the warning occurs
For a single line of code, this works the same as writing the following:
#pragma warning(push)
#pragma warning(disable: 4101)
// here goes your code where the warning occurs
#pragma warning(pop)
[1] Others have noted in comments below that if the following statement is an #include statement that the #pragma warning(suppress: 4101) statement would not effectively suppress the warning for every line in the header file. If one were intending to do that, one would need to utilize the push/disable/pop method instead.
#pragma push/pop are often a solution for this kind of problems, but in this case why don't you just remove the unreferenced variable?
try
{
// ...
}
catch(const your_exception_type &) // type specified but no variable declared
{
// ...
}
Example:
#pragma warning(suppress:0000) // (suppress one error in the next line)
This pragma is valid for C++ starting with Visual Studio 2005.
https://msdn.microsoft.com/en-us/library/2c8f766e(v=vs.80).aspx
The pragma is NOT valid for C# through Visual Studio 2005 through Visual Studio 2015.
Error: "Expected disable or restore".
(I guess they never got around to implementing suppress ...)
https://msdn.microsoft.com/en-us/library/441722ys(v=vs.140).aspx
C# needs a different format. It would look like this (but not work):
#pragma warning suppress 0642 // (suppress one error in the next line)
Instead of suppress, you have to disable and enable:
if (condition)
#pragma warning disable 0642
; // Empty statement HERE provokes Warning: "Possible mistaken empty statement" (CS0642)
#pragma warning restore 0642
else
That is SO ugly, I think it is smarter to just re-style it:
if (condition)
{
// Do nothing (because blah blah blah).
}
else
Use #pragma warning ( push ), then #pragma warning ( disable ), then put your code, then use #pragma warning ( pop ) as described here:
#pragma warning( push )
#pragma warning( disable : WarningCode)
// code with warning
#pragma warning( pop )
as #rampion mentioned, if you are in clang gcc, the warnings are by name, not number, and you'll need to do:
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-variable"
// ..your code..
#pragma clang diagnostic pop
this info comes from here
One may also use UNREFERENCED_PARAMETER defined in WinNT.H. The definition is just:
#define UNREFERENCED_PARAMETER(P) (P)
And use it like:
void OnMessage(WPARAM wParam, LPARAM lParam)
{
UNREFERENCED_PARAMETER(wParam);
UNREFERENCED_PARAMETER(lParam);
}
Why would you use it, you might argue that you can just omit the variable name itself. Well, there are cases (different project configuration, Debug/Release builds) where the variable might actually be used. In another configuration that variable stands unused (and hence the warning).
Some static code analysis may still give warning for this non-nonsensical statement (wParam;). In that case, you mayuse DBG_UNREFERENCED_PARAMETER which is same as UNREFERENCED_PARAMETER in debug builds, and does P=P in release build.
#define DBG_UNREFERENCED_PARAMETER(P) (P) = (P)
Instead of putting it on top of the file (or even a header file), just wrap the code in question with #pragma warning (push), #pragma warning (disable) and a matching #pragma warning (pop), as shown here.
Although there are some other options, including #pramga warning (once).
This question comes up as one of the top 3 hits for the Google search for "how to suppress -Wunused-result in c++", so I'm adding this answer here since I figured it out and want to help the next person.
In case your warning/error is -Wunused (or one of its sub-errors) or -Wunused -Werror only, the solution is to cast to void:
For -Wunused or one of its sub-errors only1, you can just cast it to void to disable the warning. This should work for any compiler and any IDE for both C and C++.
1Note 1: see gcc documentation here, for example, for a list of these warnings: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html, then search for the phrase "All the above -Wunused options combined" and look there for the main -Wunused warning and above it for its sub-warnings. The sub-warnings that -Wunused contains include:
-Wunused-but-set-parameter
-Wunused-but-set-variable
-Wunused-function
-Wunused-label
-Wunused-local-typedefs
-Wunused-parameter
-Wno-unused-result
-Wunused-variable
-Wunused-const-variable
-Wunused-const-variable=n
-Wunused-value
-Wunused = contains all of the above -Wunused options combined
Example of casting to void to suppress this warning:
// some "unused" variable you want to keep around
int some_var = 7;
// turn off `-Wunused` compiler warning for this one variable
// by casting it to void
(void)some_var; // <===== SOLUTION! ======
For C++, this also works on functions which return a variable marked with [[nodiscard]]:
C++ attribute: nodiscard (since C++17)
If a function declared nodiscard or a function returning an enumeration or class declared nodiscard by value is called from a discarded-value expression other than a cast to void, the compiler is encouraged to issue a warning.
(Source: https://en.cppreference.com/w/cpp/language/attributes/nodiscard)
So, the solution is to cast the function call to void, as this is actually casting the value returned by the function (which is marked with the [[nodiscard]] attribute) to void.
Example:
// Some class or struct marked with the C++ `[[nodiscard]]` attribute
class [[nodiscard]] MyNodiscardClass
{
public:
// fill in class details here
private:
// fill in class details here
};
// Some function which returns a variable previously marked with
// with the C++ `[[nodiscard]]` attribute
MyNodiscardClass MyFunc()
{
MyNodiscardClass myNodiscardClass;
return myNodiscardClass;
}
int main(int argc, char *argv[])
{
// THE COMPILER WILL COMPLAIN ABOUT THIS FUNCTION CALL
// IF YOU HAVE `-Wunused` turned on, since you are
// discarding a "nodiscard" return type by calling this
// function and not using its returned value!
MyFunc();
// This is ok, however, as casing the returned value to
// `void` suppresses this `-Wunused` warning!
(void)MyFunc(); // <===== SOLUTION! ======
}
Lastly, you can also use the C++17 [[maybe_unused]] attribute: https://en.cppreference.com/w/cpp/language/attributes/maybe_unused.
If you want to disable unreferenced local variable write in some header
template<class T>
void ignore (const T & ) {}
and use
catch(const Except & excpt) {
ignore(excpt); // No warning
// ...
}
In certain situations you must have a named parameter but you don't use it directly.
For example, I ran into it on VS2010, when 'e' is used only inside a decltype statement, the compiler complains but you must have the named varible e.
All the above non-#pragma suggestions all boil down to just adding a single statement:
bool f(int e)
{
// code not using e
return true;
e; // use without doing anything
}