golang-style “defer” in C++ [duplicate]

Answer 1

This already exists, and it's called scope guard. See this fantastic talk: https://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Andrei-Alexandrescu-Systematic-Error-Handling-in-C. This lets you easily create an arbitrary callable to be called at exit. This is the newer version; it was developed originally long before go existed.

It works perfectly in general, but I'm not sure what you mean by it handling exceptions. Throwing exceptions from a function that has to be called at scope exit is a mess. The reason: when an exception is thrown (and not immediately caught), current scope exits. All destructors get run, and the exception will continue propagating. If one of the destructors throws, what do you do? You now have two live exceptions.

I suppose there are ways a language could try to deal with this, but it's very complex. In C++, it's very rare that a throwing destructor would be considered a good idea.

Answer 2

This already exists in C++, and it's a tremendously bad idea and the example you gave exemplifies why it's a pointless thing to do and I hope that the Committee never introduces it.

For example, if you have a file handle, then write a class to do it for you and then you won't have to write a defer statement for every single use case, which you could easily forget to do. Or just plain get it wrong. You write one destructor, once. That's it. Then you're guaranteed for all uses of the class that it's safe. It's much safer and much easier.

Answer 3

Boost discuss this in Smart Pointer Programming Techniques:

• http://www.boost.org/doc/libs/1_59_0/libs/smart_ptr/sp_techniques.html#handle

You can do, for example:

#include <memory>
#include <iostream>
#include <functional>

using namespace std;
using defer = shared_ptr<void>;    

int main() {
    defer _(nullptr, bind([]{ cout << ", World!"; }));
    cout << "Hello";
}

Or, without bind:

#include <memory>
#include <iostream>

using namespace std;
using defer = shared_ptr<void>;    

int main() {
    defer _(nullptr, [](...){ cout << ", World!"; });
    cout << "Hello";
}

You may also as well rollout your own small class for such, or make use of the reference implementation for N3830/P0052:

• N3830: https://github.com/alsliahona/N3830
• P0052: https://github.com/PeterSommerlad/scope17

The C++ Core Guidelines also have a guideline which employs the gsl::finally function, for which there's an implementation here.

There are many codebases that employ similar solutions for this, hence, there's a demand for this tool.

Related SO discussion:

• Is there a proper 'ownership-in-a-package' for 'handles' available?
• Where's the proper (resource handling) Rule of Zero?