In short: it is a smart pointers in C question. Reason: embedded programming and need to ensure that if complex algorithm is used, then proper deallocation occurs with little effort on the developer side.
My favorite feature of C++ is ability to execute a proper deallocation of object allocated on stack and that goes out of scope. GO language defer provides same functionality and it is a bit closer in spirit to C.
GO defer would be the desired way of doing things in C. Is there a practical way to add such functionality?
The goal of doing so is simplification of tracking when and where object goes out of scope. Here is a quick example:
struct MyDataType *data = malloc(sizeof(struct MyDataType));
defer(data, deallocator);
if (condition) {
// dallocator(data) is called automatically
return;
}
// do something
if (irrelevant) {
struct DT *localScope = malloc(...);
defer(localScope, deallocator);
// deallocator(localScope) is called when we exit this scope
}
struct OtherType *data2 = malloc(...);
defer(data2, deallocator);
if (someOtherCondition) {
// dallocator(data) and deallocator(data2) are called in the order added
return;
}
In other languages I could create an anonymous function inside the code block, assign it to the variable and execute manually in front of every return. This would be at least a partial solution. In GO language defer functions can be chained. Manual chaining with anonymous functions in C is error prone and impractical.
Thank you
C does not have destructors (unless you think of the GCC specific variable attribute cleanup, which is weird and rarely used; notice also that the GCC function attribute destructor is not what other languages, C++ notably, call destructor). C++ have them. And C & C++ are very different languages.
In C++11, you might define your class, having a std::vector or std::function-s, initialized using a std::initialized_list of lambda expressions (and perhaps dynamically augmented by some push_back). Then its destructor could mimic Go's defer-ed statements. But this is not idiomatic.
Go have defer statements and they are idiomatic in Go.
I recommend sticking to the idioms of your programming languages.
(In other words: don't think in Go while coding in C++)
You could also embed some interpreter (e.g. Lua or Guile) in your application. You might also learn more about garbage collection techniques and concepts and use them in your software (in other words, design your application with its specific GC).
Reason: embedded programming and need to ensure that if complex algorithm is used, then proper deallocation occurs with little effort on the developer side.
You might use arena-based allocation techniques, and de-allocate the arena when suitable... When you think about that, it is similar to copying GC techniques.
Maybe you dream of some homoiconic language with a powerful macro system suitable for meta-programming. Then look into Common Lisp.
In C++, I've seen "stack based classes" that follow the RAII pattern. You could make a general purpose Defer class (or struct) that can take any arbitrary function or lambda.
For example:
#include <cstddef>
#include <functional>
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::function;
using std::string;
struct Defer {
function<void()> action;
Defer(function<void()> doLater) : action{doLater} {}
~Defer() {
action();
}
};
void Subroutine(int i) {
Defer defer1([]() { cout << "Phase 1 done." << endl; });
if (i == 1) return;
char const* p = new char[100];
Defer defer2([p]() { delete[] p; cout << "Phase 2 done, and p deallocated." << endl; });
if (i == 2) return;
string s = "something";
Defer defer3([&s]() { s = ""; cout << "Phase 3 done, and s set to empty string." << endl; });
}
int main() {
cout << "Call Subroutine(1)." << endl;
Subroutine(1);
cout << "Call Subroutine(2)." << endl;
Subroutine(2);
cout << "Call Subroutine(3)." << endl;
Subroutine(3);
return EXIT_SUCCESS;
}
I just implemented a very simple thing like defer in golang several days ago.
The only one behaviour different from golang is my defer will not be executed when you throw an exception but does not catch it at all. Another difference is this cannot accept a function with multiple arguments like in golang, but we can deal it with lambda capturing local variables.
The implementations are here.
class _Defer {
std::function<void()> __callback;
public:
_Defer(_Defer &&);
~_Defer();
template <typename T>
_Defer(T &&);
};
_Defer::_Defer(_Defer &&__that)
: __callback{std::forward<std::function<void()>>(__that.__callback)} {
}
template <typename T>
_Defer::_Defer(T &&__callback)
: __callback{
static_cast<std::function<void()>>(std::forward<T>(__callback))
} {
static_assert(std::is_convertible<T, std::function<void()>>::value,
"Cannot be convert to std::function<void()>.");
}
_Defer::~_Defer() {
this->__callback();
}
And then I defined some macros to make my defer like a keyword in C++ (just for fun)
#define __defer_concatenate(__lhs, __rhs) \
__lhs##__rhs
#define __defer_declarator(__id) \
if (0); /* You may forgot a `;' or deferred outside of a scope. */ \
_Defer __defer_concatenate(__defer, __id) =
#define defer \
__defer_declarator(__LINE__)
The if (0); is used to prevent defer a function out of a scope. And then we can use defer like in golang.
#include <iostream>
void foo() {
std::cout << "foo" << std::endl;
}
int main() {
defer []() {
std::cout << "bar" << std::endl;
};
defer foo;
}
This will print
foo
bar
to screen.
This implementation avoids dynamic allocation and most limitations of other implementations shown here
#include<type_traits>
#include<utility>
template<typename F>
struct deferred
{
std::decay_t<F> f;
template<typename G>
deferred(G&& g) : f{std::forward<G>(g)} {}
~deferred() { f(); }
};
template<typename G>
deferred(G&&) -> deferred<G>;
#define CAT_(x, y) x##y
#define CAT(x, y) CAT_(x, y)
#define ANONYMOUS_VAR(x) CAT(x, __LINE__)
#define DEFER deferred ANONYMOUS_VAR(defer_variable) = [&]
And use it like
#include<iostream>
int main()
{
DEFER {
std::cout << "world!
";
};
std::cout << "Hello ";
}
Now, whether to allow exceptions in DEFER is a design choice bordering on philosophy, and I'll leave it to Andrei to fill in the details.
Note all such deferring functionalities in C++ necessarily has to be bound to the scope at which it is declared, as opposed to Go's which binds to the function at which it is declared.
GO defer would be the desired way of doing things in C. Is there a practical way to add such functionality?
The goal of doing so is simplification of tracking when and where object goes out of scope.
C does not have any built-in mechanism for automatically invoking any kind of behavior at the end of an object's lifetime. The object itself ceases to exist, and any memory it occupied is available for re-use, but there is no associated hook for executing code.
For some kinds of objects, that is entirely satisfactory by itself -- those whose values do not refer to other objects with allocated storage duration that need to be cleaned up as well. In particular, if struct MyDataType in your example is such a type, then you get automatic cleanup for free by declaring instances as automatic variables instead of allocating them dynamically:
void foo(void) {
// not a pointer:
struct MyDataType data /* = initializer */;
// ...
/* The memory (directly) reserved for 'data' is released */
}
For objects that require attention at the end of their lifetime, it is generally a matter of code style and convention to ensure that you know when to clean up. It helps, for example, to declare all of your variables at the top of the innermost block containing them, though C itself does not require this. It can also help to structure your code so that for each object that requires custom cleanup, all code paths that may execute during its lifetime converge at the end of that lifetime.
Myself, as a matter of personal best practices, I always try to write any cleanup code needed for a given object as soon as I write its declaration.
In other languages I could create an anonymous function inside the code block, assign it to the variable and execute manually in front of every return. This would be at least a partial solution. In GO language defer functions can be chained. Manual chaining with anonymous functions in C is error prone and impractical
C has neither anonymous functions nor nested ones. It often does make sense, however, to write (named) cleanup functions for data types that require cleanup. These are analogous to C++ destructors, but you must call them manually.
The bottom line is that many C++ paradigms such as smart pointers, and coding practices that depend on them, simply do not work in C. You need different approaches, and they exist, but converting a large body of existing C++ code to idiomatic C is a distinctly non-trivial undertaking.
Many different answers, but a few interesting details was not said.
One big difference between C++ and go is when deffered is called. In C++ when your program leaving scope, where is was created. But in go when your program leaving function. That means, this code won't work at all:
for i:=0; i < 10; i++ {
mutex.Lock()
defer mutex.Unlock()
/* do something under the mutex */
}
Of course C does not pretends that is object oriented and therefore there are no destructors at all. It help a lot of readability of code, because you know that your program at line X do only what is written in that line. In contrast of C++ where each closing curly bracket could cause calling of dozens destructors.
In C you can use hated statement goto. Don't use it for anything else, but it is practical to have cleanup label at the end of function call goto cleanup from many places. Bit more complicated is when more than one resource you want do release, than you need more that one cleanup. Than your function finish with
cleanup_file:
fclose(f);
cleanup_mutex:
pthread_mutex_unlock(mutex);
return ret;
}