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Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?
Why is the do while(false) necessary in the macros below?
#define LOG(message, ...) \
do { \
Lock<MutualExclusion> lock (logMutex); \
.... a lot of code ...
} while (false)
I dont think it serves any functional purpose. Am I overlooking something?
It turns a block into a single statement. If you just use a block (i.e. code enclosed in {}) strange things can happen, for example
#define STUFF() \
{ do_something(); do_something_else(); }
if (cond)
STUFF();
else
//...
the extra semi-colon breaks the syntax. The do {} while(false) instead is a single statement.
You can find more about this and other macro tricks here.
So you are forced to add semicolon at the end of the macro, when you use it. This is a common idiom and only way to enforce it.
If somebody has code that does this:
if (something)
LOG("My log message");
That would expand to:
if (something)
Lock<MutualExclusion> lock (logMutex);
// A bunch of other code
Which is incorrect (only the first line would be under the if statement).
The macro makes sure that the macro call is inside of a block of code.
People use it because otherwise, you can screw up your ifs with compound statements. Imagine
#define hai int x; \
x = 0;
if (condition)
hai;
else
func();
Imagine what the preprocessed source looks like.
if (condition)
int x;
x = 0;
else
func();
Oh wait- now our else doesn't work.
Macros like that however are typically unnecessary in C++.
The reason for this weird practice in #define's is to encapsulate the different assignments within a loop that is executed exactly once, so one may use the macro like a function. For example, with the code you posted, one can write:
if(...)
LOG(x, y);
else
// Something else
and it is expanded as
if(...)
do {...} while(false);
else
// Something else
This would not work without the do...while(false) surrounding the different assignments, because that would be expanded as
if(...)
Lock<MutualExclusion> lock (logMutex);
// Other code... Outside the if statement!
Also forcing a semicolon after the macro makes it look like a function and you wont get errors because you added an semicolon like after a normal function.
It provides local scope to that which is inside the macro.
It looks to me like it is only used for scoping rules, so that Lock<MutualExclusion> falls out of scope at the end of the block.
If that's the reason for it, then it's completely unnecesarry:
// some other code...
string s = "oh hai";
{
Lock<MutualExclusion> lock(logMutex);
// MAGIC HAPPENS
}
s = "oh bai";
Related
im a beginner starting to learn c++ i have a question.. Can i write scripts in do while loop i mean like this...
//you type do then like
do{
// your code here
}while(condition{ // <-- the question is here
then the code of the script
} ) closing Parenthesis and curly braces
yeah if you didn't understand that my question was that can i expand my condition in the while Parenthesis?? please answer because I'm learning C++ and I wanna improve.
The while condition takes an expression. That includes things like variables (foo), operators (1 + 2), and function calls. But it excludes things like if statements and additional loops. If you need to do something complicated inside of a while block, you should put it in a function.
do {
// ...
} while (should_continue(foo, bar));
bool should_continue(int foo, int bar) {
// ... complicated code goes here ...
}
Technically speaking, in C++11 and onward, you can create and evaluate a lambda in the same line, allowing arbitrary statements in expression context, but this is not very readable and should generally be a sign that your code needs to be split up into more functions.
// Don't do this; your coworkers will despise you.
do {
// ...
} while (([&]() {
// ... complicated code goes here ...
})());
Some non-portable compiler extensions will also allow the syntax you suggested in the question, where you can just throw braces { ... } with arbitrary statements in expression context. But, again, this is non-portable and not very readable code to begin with. So just write a function.
For example, I'd like to have something like this:
If(condition) object.start();
// Do some unrelated processing
If(condition) object.stop();
As a simple example of what I would like to happen. Are there any language agnostic ways to organize this, especially as you have more conditional lines of code?
I suppose you're asking this because the condition is expensive or non-repeatable.
One typical solution would be to set a boolean variable in the first IF, and let the second IF check that boolean variable instead of re-executing the condition.
boolVar=false
if (condition) {
boolVar=true
object.start()
}
// do unrelated stuff
if (boolVar) {
object.stop()
}
Another typical solution would be something like this :
if (condition) {
object.start()
doSomething()
object.end()
} else {
doSomething()
}
Don't be afraid of simple solutions, if they're good enough. :)
When refactoring some code, I often encounter this :
bool highLevelFunc foo()
{
// ...
bool result = LesserLevelFunc();
if (!result) return false;
// ... Keep having fun if we didn't return
}
Is there any way to make this a little more sexy and less verbose ? Without any overhead or pitfall of course.
I can think of a macro
#define FORWARD_IF_FALSE(r) if (!r) return r;
bool highLevelFunc foo()
{
// ...
FORWARD_IF_FALSE(LesserLevelFunc());
// ...
}
Anything better, i.e without preprocessor macro?
To me, "readable" code is sexy. I find the original code more readable than your proposal, since the original uses standard C++ syntax and the latter uses a macro which I'd have to go and look up.
If you want to be more explicit, you could say if (result == false) (or better yet, if (false == result) to prevent a possible assignment-as-comparison bug) but understanding the ! operator is a fairly reasonable expectation in my opinion.
That said, there is no reason to assign the return value to a temporary variable; you could just as easily say:
if (!LesserLevelFunc()) return false;
This is quite readable to me.
EDIT: You could also consider using exceptions instead of return values to communicate failure. If LesserLevelFunc() threw an exception, you would not need to write any special code in highLevelFunc() to check for success. The exception would propagate up through the caller to the nearest matching catch block.
Because you might be continuing if LesserLevelFunc returns true, I suggest keeping it pretty close to how it is now:
if (!LesserLevelFunc())
return false;
First of all introducing the macro you are making the code unsafe. Moreover your macro is invalid.
The expression after the negation operator shall be enclosed in parentheses.
#define FORWARD_IF_FALSE(r) if (!( r ) ) return r;
Secondly the macro calls r twice. Sometimes two calls of a function is not equivalent to one call of the same function. For example the function can have some side effects or internal flags that are switched on/off in each call of the function.
So I would keep the code as is without introducing the macro because the macro does not equivalent to the symantic of the original code.
Sun Studio 12.1 prints the warning
Warning: The last statement should return a value.
frequently for functions like that:
int f()
{
/* some code that may return */
// if we end up here, something is broken
throw std::runtime_error("Error ...");
}
It is perfectly clear that we do not need a return value at the end of the function. I hesitate to insert something like
// Silence a compiler warning
return 42;
at the end of such a function, since it is dead code anyway. For more complicated return types, it might actually be difficult to construct a 'sensible' bogus value.
What is the recommended way to silence such a warning?
Can you reorganize the code in the function in such a way (hopefully more logical as well) that the normal path happens at the end of the function so that a return can be used, and the exceptional path happens earlier, NOT as the last statement?
EDIT: If reorganizing the function really doesn't make sense, you can always just put a dummy return 0; with a comment. It's better to squelch the warning that way than more globally.
If you really want to quiet the warning permanently, you can use #pragma error_messages (off, wnoretvalue) but note that the warning really is useful most of the time so I absolutely don't suggest turning it off. You can use the on version of the pragma to re-enable the warning after the function, but the compiler will still emit the warning if your function is ever inlined. If you put the function in its own source file and use the pragma that should shush the warning relatively safely though, since it can't affect other translation units.
Another really wacky possibility is to switch to g++. Unless you're compiling for SPARC g++ may actually generate better code than Sun studio.
I find it a perfect spot for abort(). You should never end there, according to you, so something like:
UNREACHABLE("message")
which expands into:
#ifdef NDEBUG
#define UNREACHABLE(Message_) abort();
#else
#define UNREACHABLE(Message_) assert(0 && Message_);
#endif
Looks appropriate
Since you know the exception will be systematically called, why don't you simply return a 0?
Perhaps encapsulate the contents in a do { } while (false); construct:
int my_function()
{
int result = DEFAULT_VALUE;
do
{
result = /*...*/
// Whatever
if (error)
{
throw std::runtime_error("Error ...");
}
} while (false);
return result;
}
The idea is for normal operation to set the result value then let the execution flow to the end or use a break to jump to the return statement.
I don't know of a "recommended" way to deal with it, but to answer your question about coping with more complex types, what about:
ComplexType foo()
{
...
throw std::runtime( "Error..." );
return *(ComplexType*)(0);
}
This would then work with any return type. I realise it looks evil, but its there just to silence the warning. As you say, this code will never be executed, and it may even be optimised out.
I am trying to create a macro that takes a scope as a parameter.
I know, it is probably not a good thing etc etc.
I was trying this and got the problem that preprocessor looks for commas and parentheses... the problem is with enum.
How would I declare a enum inside a scope that is a parameter of a macro?
when the compiler see the comma between enum itens, it takes it as a separator.
If you are curious to know why I entered into this, is because I need to register my namespaces and classes, for namespaces I need to know when they are closed, so I was thinking to create a macro that initially calls a static function that register the namespace, encapsulate its contents and finally call a static function that removes the namespace from the registry.
With a macro it would be easier for the coder to do this and make sure he doesn't forget to remove the namespace in the end of the bracket.
Thanks,
Joe
EDIT:
I want a macro that accepts a scope as parameters:
#define MYMACRO(unkownscope) unknownscope
class MYMACRO({
// please, don't take this code seriously, it is just an example so you can understand my question
});
now, if I try:
#define MYMACRO(unkownscope) unknownscope
class MYMACRO({
enum {
anything = 1,
everything = 2
};
});
it won't compile because of the comma inside the enum, because the compiler thinks it is a separator of the macro. It doesn't happen with commas inside parentheses, example:
int a(){
int x = anyfunction(1, 2);
}
would compile normally because the comma is inside a double parentheses.
Sorry for not being able to explain earlier... my english is not that good and the words just keep skipping me =[
Ty for the answers!
Joe
It sounds like you are pushing the preprocessor beyond where it's willing to go. While it's not as elegant, how about breaking your macro in two (one pre- and one post-) and rather then passing a "scope" as parameter, you surround your scope with you pre- and post- macros.
So, if your macro looks something like:
SOMACRO({ ... });
You would instead do something like:
PRESOMACRO();
{ ... };
POSTSOMACRO();
#define SCOPED_STUFF(pre,post) pre; STUFF; post;
#define STUFF enum {a,b,c}
SCOPED_STUFF(a,b)
#undef STUFF
#define STUFF enum {q,r}
SCOPED_STUFF(c,d)
#undef STUFF
You are attempting to replicate RAII with a macro.
#define SCOPE(ns) NamespaceRegistrar _ns_rar(ns);
struct NamespaceRegistrar {
std::string _ns;
NamespaceRegistrar(const std::string& ns) : _ns(ns) { AcquireResource(_ns); }
~NamespaceRegistrar() { ReleaseResource(_ns); }
};
{
SCOPE("Foo")
// stuff
}
I have no idea what you are talking about with regard to enums.
You already noticed what the problem is, an article on boostpro.com sums the problem up.
There are work-arounds, but i'd go for utilizing Boost.Preprocessor.
Without knowing exactly what you're trying to achieve syntactically, something like this might be what you are looking for (edited to PP_SEQ):
#define MAKE_ENUM(Name, Seq) enum Name { BOOST_PP_SEQ_ENUM(Seq) }
MAKE_ENUM(foo, (a)(b)(c));