there is something that's bugging me.
In a non-threaded program, is it better to have local static variables(inside methods) or static class members?
In this example:
class C{ public: C(){}; void foo(); }; void C::foo(){ static int bar = 0; bar++; printf("%d\n",bar); } Is it considered a bad practice if bar will solely be used in C::foo()?
Neither is better. They serve very different use cases
foo would be a copy constructor and bar would be called numberOfCopies, it would be a good candidate for a static class member. So it depends on how it's used.I usually try to limit the scope of variables as much as possible, as long as it doesn't become weird or tedious.
If you have 1000 lines of code in class C, of them 100 lines of code in the function foo, any change you do to bar (for example, changing the name or type) requires going over 100 lines of code to make sure the change is OK. If you had bar a static class member, you might have to go over 1000 lines of code, just to make sure bar is not used there. This would be a waste of time.
If you think you might need bar in another function foo2 (for example, when counting call count for foo and foo2 together), you might want to make bar a static class member.
If it's a public class, a static class member requires editing the header file. This is not always desirable.
Another option is a file-scoped variable, in an anonymous namespace. This is sufficient if you need access only in one method, and if you need it in multiple.
Both local and non-local global variables are "bad" by the virtue of being global. But initialization and access for those two cases are different, so the answer which one to use depends on your needs regarding those requirements.
As a side note, dynamic initialization of local variables with static storage duration may not be thread-safe, depending on your compiler. The good news, in C++0x it's guaranteed to be thread-safe.
Both local and non-local global variables are "bad". On its own this statement is wrong. The issue (as with the question) is global mutable state.Object-oriented-speaking, bar is part of the state of class C. This is the reason why I usually prefer using fields rather than static local variables.
bar is part of the class' state. It may be part of the member function's state. In which case, a local static is appropriate.I would consider the use of static members for anything but const values (that aren't eligible for constexpr) to be a bad practice. The scope of a static member or method local is global, it just happens to be accessible only by (all instances of) a class.
Non-const class static is at the top of my list for bad practices. It's worse than simply declaring a global variable - because it's hidden. A singleton class (or even just a class that you construct only once and pass to dependent classes) eliminates this practice.
But otherwise I would recommend exposing the static via a global accessor (with big ugly comments around it). If at any point the code becomes concurrent, you can drop in a critical section.
// Danger, global state. int inc_bar() { static int bar = 0; return bar++; } class my_class { public: my_class() { } void inc() { printf("%d\n", inc_bar()); } }; 