I have a function that performs a negative check on a value of a generic type (well I don't want to assume the type, it is not necessarily templated). However, with -Wtype-limits enabled, the compiler complains about the negative check if the type in question is an unsigned type. Removing the negative check is a bad idea if the type might change to be a signed type, and disabling the warnings is not ideal, as it might catch legitimate issues.
using example_t = uint8_t; // could be int8_t or int64_t or really whatever integer type that can store values 0 - 8.
void example(example_t value) {
if (value < 0 || value >= 9) { // <-- errors on the 'value < 0' check
throw std::invalid_argument("some error message");
}
// continue processing ...
}
Even if I use C++17 and use if constexpr(std::is_unsigned<example_t>::value) to guard the check that is warning, it still warns. I've noticed that GCC does not emit warning for if constexpr'd out code if it would trigger warnings because the value assigned into variables of those type would fall out of the range of the type. Also, GCC emits the warning even if I cast it to a signed int first. The warning behavior is not consistent with itself, so this seems like a bug. Clang issues no warning at all for these types of checks.
But I wondering if there was a way around this, my code base is strictly C++14 compliant, and we are stuck with old GCC compilers.
I am not sure it is worth the impact on readability, but you can move the comparison into a template context using a generic lambda. GCC doesn't warn about this in the templated code:
if ([](auto v){return v < 0;}(value) || value >= 9)
or maybe better write a function:
template<typename T>
constexpr bool is_negative(T t) { return t < 0; }
and use that:
if (is_negative(value) || value >= 9)
or just using std::less also wont warn:
if (std::less()(value, 0) || value >= 9)
GCC also has #pragmas to disable warnings temporarily, see this question:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wtype-limits"
if (value < 0 || value >= 9) {
throw std::invalid_argument("some error message");
}
#pragma GCC diagnostic pop
Sometimes a local variable is used for the sole purpose of checking it in an assert(), like so -
int Result = Func();
assert( Result == 1 );
When compiling code in a Release build, assert()s are usually disabled, so this code may produce a warning about Result being set but never read.
A possible workaround is -
int Result = Func();
if ( Result == 1 )
{
assert( 0 );
}
But it requires too much typing, isn't easy on the eyes and causes the condition to be always checked (yes, the compiler may optimize the check away, but still).
I'm looking for an alternative way to express this assert() in a way that wouldn't cause the warning, but still be simple to use and avoid changing the semantics of assert().
(disabling the warning using a #pragma in this region of code isn't an option, and lowering warning levels to make it go away isn't an option either...).
We use a macro to specifically indicate when something is unused:
#define _unused(x) ((void)(x))
Then in your example, you'd have:
int Result = Func();
assert( Result == 1 );
_unused( Result ); // make production build happy
That way (a) the production build succeeds, and (b) it is obvious in the code that the variable is unused by design, not that it's just been forgotten about. This is especially helpful when parameters to a function are not used.
I wouldn't be able to give a better answer than this, that addresses that problem, and many more:
Stupid C++ Tricks: Adventures in assert
#ifdef NDEBUG
#define ASSERT(x) do { (void)sizeof(x);} while (0)
#else
#include <assert.h>
#define ASSERT(x) assert(x)
#endif
As of C++17, the variable can be decorated with an attribute.
[[maybe_unused]] int Result = Func();
assert( Result == 1 );
See https://en.cppreference.com/w/cpp/language/attributes/maybe_unused for details.
This is better than the (void)Result trick because you directly decorate the variable declaration, rather than add something as an afterthought.
You could create another macro that allows you to avoid using a temporary variable:
#ifndef NDEBUG
#define Verify(x) assert(x)
#else
#define Verify(x) ((void)(x))
#endif
// asserts that Func()==1 in debug mode, or calls Func() and ignores return
// value in release mode (any braindead compiler can optimize away the comparison
// whose result isn't used, and the cast to void suppresses the warning)
Verify(Func() == 1);
int Result = Func();
assert( Result == 1 );
This situation means that in release mode, you really want:
Func();
But Func is non-void, i.e. it returns a result, i.e. it is a query.
Presumably, besides returning a result, Func modifies something (otherwise, why bother calling it and not using its result?), i.e. it is a command.
By the command-query separation principle (1), Func shouldn't be a command and a query at the same time. In other words, queries shouldn't have side effects, and the "result" of commands should be represented by the available queries on the object's state.
Cloth c;
c.Wash(); // Wash is void
assert(c.IsClean());
Is better than
Cloth c;
bool is_clean = c.Wash(); // Wash returns a bool
assert(is_clean);
The former doesn't give you any warning of your kind, the latter does.
So, in short, my answer is: don't write code like this :)
Update (1): You asked for references about the Command-Query Separation Principle. Wikipedia is rather informative. I read about this design technique in Object Oriented Software Construction, 2nd Editon by Bertrand Meyer.
Update (2): j_random_hacker comments "OTOH, every "command" function f() that previously returned a value must now set some variable last_call_to_f_succeeded or similar". This is only true for functions that don't promise anything in their contract, i.e. functions that might "succeed" or not, or a similar concept. With Design by Contract, a relevant number of functions will have postconditions, so after "Empty()" the object will be "IsEmpty()", and after "Encode()" the message string will be "IsEncoded()", with no need to check. In the same way, and somewhat symetrically, you don't call a special function "IsXFeasible()" before each and every call to a procedure "X()"; because you usually know by design that you're fulfilling X's preconditions at the point of your call.
You could use:
Check( Func() == 1 );
And implement your Check( bool ) function as you want. It may either use assert, or throw a particular exception, write in a log file or to the console, have different implementations in debug and release, or a combination of all.
With C++17 we can do:
[[maybe_unused]] int Result = Func();
though it involves a bit of extra typing compared to a assert substitution. See this answer.
Note: Added this because is the first google hit for "c++ assert unused variable".
You should move the assert inside the function before the return value(s). You know that the return value is not an unreferenced local variable.
Plus it makes more sense to be inside the function anyway, because it creates a self contained unit that has its OWN pre- and post-conditions.
Chances are that if the function is returning a value, you should be doing some kind of error checking in release mode on this return value anyway. So it shouldn't be an unreferenced variable to begin with.
Edit, But in this case the post condition should be X (see comments):
I strongly disagree with this point, one should be able to determine the post condition from the input parameters and if it's a member function, any object state. If a global variable modifies the output of the function, then the function should be restructured.
Most answers suggest using static_cast<void>(expression) trick in Release builds to suppress the warning, but this is actually suboptimal if your intention is to make checks truly Debug-only. The goals of an assertion macro in question are:
Perform checks in Debug mode
Do nothing in Release mode
Emit no warnings in all cases
The problem is that void-cast approach fails to reach the second goal. While there is no warning, the expression that you've passed to your assertion macro will still be evaluated. If you, for example, just do a variable check, that is probably not a big deal. But what if you call some function in your assertion check like ASSERT(fetchSomeData() == data); (which is very common in my experience)? The fetchSomeData() function will still be called. It may be fast and simple or it may be not.
What you really need is not only warning suppression but perhaps more importantly - non-evaluation of the debug-only check expression. This can be achieved with a simple trick that I took from a specialized Assert library:
void myAssertion(bool checkSuccessful)
{
if (!checkSuccessful)
{
// debug break, log or what not
}
}
#define DONT_EVALUATE(expression) \
{ \
true ? static_cast<void>(0) : static_cast<void>((expression)); \
}
#ifdef DEBUG
# define ASSERT(expression) myAssertion((expression))
#else
# define ASSERT(expression) DONT_EVALUATE((expression))
#endif // DEBUG
int main()
{
int a = 0;
ASSERT(a == 1);
ASSERT(performAHeavyVerification());
return 0;
}
All the magic is in the DONT_EVALUATE macro. It is obvious that at least logically the evaluation of your expression is never needed inside of it. To strengthen that, the C++ standard guarantees that only one of the branches of conditional operator will be evaluated. Here is the quote:
5.16 Conditional operator [expr.cond]
logical-or-expression ? expression : assignment-expression
Conditional expressions group right-to-left. The first expression is
contextually converted to bool. It is evaluated and if it is true, the
result of the conditional expression is the value of the second
expression, otherwise that of the third expression. Only one of these
expressions is evaluated.
I have tested this approach in GCC 4.9.0, clang 3.8.0, VS2013 Update 4, VS2015 Update 4 with the most harsh warning levels. In all cases there are no warnings and the checking expression is never evaluated in Release build (in fact the whole thing is completely optimized away). Bare in mind though that with this approach you will get in trouble really fast if you put expressions that have side effects inside the assertion macro, though this is a very bad practice in the first place.
Also, I would expect that static analyzers may warn about "result of an expression is always constant" (or something like that) with this approach. I've tested for this with clang, VS2013, VS2015 static analysis tools and got no warnings of that kind.
The simplest thing is to only declare/assign those variables if the asserts will exist. The NDEBUG macro is specifically defined if asserts won't be effected (done that way round just because -DNDEBUG is a convenient way to disable debugging, I think), so this tweaked copy of #Jardel's answer should work (cf. comment by #AdamPeterson on that answer):
#ifndef NDEBUG
int Result =
#endif
Func();
assert(Result == 1);
or, if that doesn't suit your tastes, all sorts of variants are possible, e.g. this:
#ifndef NDEBUG
int Result = Func();
assert(Result == 1);
#else
Func();
#endif
In general with this stuff, be careful that there's never a possibility for different translation units to be build with different NDEBUG macro states -- especially re. asserts or other conditional content in public header files. The danger is that you, or users of your library might accidentally instantiate a different definition of an inline function from the one used inside the compiled part of the library, quietly violating the one definition rule and making the runtime behaviour undefined.
This is a bad use of assert, IMHO. Assert is not meant as an error reporting tool, it's meant to assert preconditions. If Result is not used elsewhere, it's not a precondition.
Certainly you use a macro to control your assert definition, such as "_ASSERT". So, you can do this:
#ifdef _ASSERT
int Result =
#endif /*_ASSERT */
Func();
assert(Result == 1);
int Result = Func();
assert( Result == 1 );
Result;
This will make the compiler stop complaining about Result not being used.
But you should think about using a version of assert that does something useful at run-time, like log descriptive errors to a file that can be retrieved from the production environment.
I'd use the following:
#ifdef _DEBUG
#define ASSERT(FUNC, CHECK) assert(FUNC == CHECK)
#else
#define ASSERT(FUNC, CHECK)
#endif
...
ASSERT(Func(), 1);
This way, for release build, the compiler don't even need to produce any code for assert.
If this code is inside a function, then act on and return the result:
bool bigPicture() {
//Check the results
bool success = 1 != Func();
assert(success == NO, "Bad times");
//Success is given, so...
actOnIt();
//and
return success;
}
// Value is always computed. We also call assert(value) if assertions are
// enabled. Value is discarded either way. You do not get a warning either
// way. This is useful when (a) a function has a side effect (b) the function
// returns true on success, and (c) failure seems unlikely, but we still want
// to check sometimes.
template < class T >
void assertTrue(T const &value)
{
assert(value);
}
template < class T >
void assertFalse(T const &value)
{
assert(!value);
}
I haven't succeeded in using [[maybe_unused]] but
you can use the unused attribute
int Result __attribute__((__unused__)) = Func();
gcc Variable-Attributes
I am looking at the code below which comes from JavascriptCore and I don't know what the meaning of the && is in the context below. An address of an address does not really make sense.
So can someone explain what the && means in the context below.
(the bitwise_cast uses a union to avoid strict aliasing problems that come with a reinterpret_cast)
The code below compiles on clang (and presumably gcc) but does not compile on our own proprietary C++ compiler.
The full source can be found here.
#if ENABLE(COMPUTED_GOTO_OPCODES)
Opcode* opcodeMap = LLInt::opcodeMap();
#define OPCODE_ENTRY(__opcode, length) \
opcodeMap[__opcode] = bitwise_cast<void*>(&&__opcode); //<---- The double &&
FOR_EACH_OPCODE_ID(OPCODE_ENTRY)
#undef OPCODE_ENTRY
#define LLINT_OPCODE_ENTRY(__opcode, length) \
opcodeMap[__opcode] = bitwise_cast<void*>(&&__opcode);
FOR_EACH_LLINT_NATIVE_HELPER(LLINT_OPCODE_ENTRY)
#undef LLINT_OPCODE_ENTRY
#endif
That's a GCC extension: computed goto.
Given a goto label
label:
in standard C++, you can only jump to it directly:
goto label;
but GCC allows you to store its address with a non-standard use of && as a unary operator (analogous to & for taking the address of an object, function, or member):
void * ptr = &&label;
and use that pointer later:
goto *ptr;
It looks like you can disable this through the preprocessor, for your compiler which doesn't have this extension. It will use some scheme based on a switch statement instead of computed jump label.
I am trying to understand the use of static_assert and assert and what the difference is between them, however there are very little sources/explantions about this
here is some code
// ConsoleApplication3.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "conio.h"
#include "cassert"
#include "iostream"
int main()
{
assert(2+2==4);
std::cout << "Execution continues past the first assert\n";
assert(2+2==5);
std::cout << "Execution continues past the second assert\n";
_getch();
}
comments on redundency will be appreciated(since I am learning "how to c++")
output in cmd
Execution continues past the first assert
Assertion failed: 2+2==5, file c:\users\charles\documents\visual studio 2012\pro
jects\consoleapplication3\consoleapplication3\consoleapplication3.cpp, line 14
I have been trying to find out the different mehtods and uses of it, but as far as I understand it is a runtime check and another "type" of if statement
could someone just clarify the use and explain what each one does and they difference?
You can think of assertions as sanity checks. You know that some condition should be true unless you've screwed something up, so the assertion should pass. If you have indeed screwed something up, the assertion will fail and you'll be told that something is wrong. It's just there to ensure the validity of your code.
A static_assert can be used when the condition is a constant expression. This basically means that the compiler is able to evaluate the assertion before the program ever actually runs. You will be alerted that a static_assert has failed at compile-time, whereas a normal assert will only fail at run time. In your example, you could have used a static_assert, because the expressions 2+2==4 and 2+2==5 are both constant expressions.
static_asserts are useful for checking compile-time constructs such as template parameters. For example, you could assert that a given template argument T must be a POD type with something like:
static_assert(std::is_pod<T>::value, "T must be a POD type");
Note that you generally only want run-time assertions to be checked during debugging, so you can disable assert by #defineing NDEBUG.
assert() is a macro whose expansion depends on whether macro NDEBUG is defined. If so, assert() doesn't expand to anything - it's a no-op. When NDEBUG is not defined, assert(x) expands into a runtime check, something like this:
if (!x) cause_runtime_to_abort()
It is common to define NDEBUG in "release" builds and leave it undefined in "debug" builds. That way, assert()s are only executed in debug code and do not make it into release code at all. You normally use assert() to check things which should always be true - a function's preconditions and postconditions or a class's invariants, for example. A failed assert(x) should mean "the programmer thought that x holds, but a bug somewhere in the code (or in their reasoning) made that untrue."
static_assert() (which was introduced in C++11) is a keyword - similar to e.g. typedef. It can only be used on compile-time expressions, and if it fails, it results in a compilation error. It does not result in any object code and is not executed at all.
static_assert() is primarily useful in templates, if you want to prevent instantiating a template incorrectly. For example:
template <class IntType>
IntType foo(IntType x)
{
static_assert(std::is_integral<IntType>::value, "foo() may be used with integral types only.");
// rest of the code
}
That way, trying to call foo() with e.g. a float will result in a compile-time error with a sensible message.
Occasionally, static_assert() can also be useful outside of templates, e.g. like this:
static_assert(sizeof(void*) > 4, "This code does not work in 32 bits");
a static_assert is evaluated at compile time, an assert is evaluated an runtime.
I doubt you cannot find any sources for this, but I’ll nevertheless give some explanation.
assert is for runtime checks which should never ever fail. If they fail, the program will terminate ungracefully. You can disable assertions for a release build by specifying a compile time flag. As assertions should never fail (they are assertions after all), in a working program, that should not make any difference. However, the expressions inside an assert must not have side effects, because there is no guarantee that they are executed. Example:
unsigned int add_non_negative_numbers(int a, int b)
{
// good
assert(a > 0);
assert(b > 0);
return (unsigned int)a + (unsigned int)b;
}
void checked_read(int fd, char *buffer, int count)
{
// BAD: if assertions are disabled, read() will _not_ be called
assert(read(fd, buffer, count) == count);
/* better: not perfect though, because read can always fail
int read_bytes = read(fd, buffer, count);
assert(read_bytes == count);
*/
}
static_assert is new and can be used to perform compile time checks. They will produce a compiler error when they fail and block the program from compiling:
static_assert(sizeof(char) == 1, "Compiler violates the standard");
In a coding style question about infinite loops, some people mentioned they prefer the for(;;) style because the while(true) style gives warning messages on MSVC about a conditional expression being constant.
This surprised me greatly, since the use of constant values in conditional expressions is a useful way of avoiding #ifdef hell. For instance, you can have in your header:
#ifdef CONFIG_FOO
extern int foo_enabled;
#else
#define foo_enabled 0
#endif
And the code can simply use a conditional and trust the compiler to elide the dead code when CONFIG_FOO isn't defined:
if (foo_enabled) {
...
}
Instead of having to test for CONFIG_FOO every time foo_enabled is used:
#ifdef CONFIG_FOO
if (foo_enabled) {
...
}
#endif
This design pattern is used all the time in the Linux kernel (for instance, include/linux/cpumask.h defines several macros to 1 or 0 when SMP is disabled and to a function call when SMP is enabled).
What is the reason for that MSVC warning? Additionally, is there a better way to avoid #ifdef hell without having to disable that warning? Or is it an overly broad warning which should not be enabled in general?
A warning doesn't automatically mean that code is bad, just suspicious-looking.
Personally I start from a position of enabling all the warnings I can, then turn off any that prove more annoying than useful. That one that fires anytime you cast anything to a bool is usually the first to go.
I think the reason for the warning is that you might inadvertently have a more complex expression that evaluates to a constant without realizing it. Suppose you have a declaration like this in a header:
const int x = 0;
then later on, far from the declaration of x, you have a condition like:
if (x != 0) ...
You might not notice that it's a constant expression.
I believe it's to catch things like
if( x=0 )
when you meant
if( x==0 )
A simple way to avoid the warning would be:
#ifdef CONFIG_FOO
extern int foo_enabled;
#else
extern int foo_enabled = 0;
#endif