Immediately calling destructor after setting in the fluent interface in C++ [closed]

Question

I want to create a class only for setting params for function. I'm using a fluent interface for that. Some function returns the object to setting params and real code will execute in the destructor of this object.

First of all, is it a good idea? Is this a common way to improve code readability when I have a lot of parameters? Or maybe I should use another way to do that? I would be glad to advise on this.

If it's okay, how can I prevent accidentally saving an object to a local variable? In this case, the destructor will be called at end of the variable scope.

Example:

class MyClass { public: ~MyClass() noexcept { // some code to execute immediately after setting params A and B } // abstract setting methods MyClass &SetA() { return *this; } MyClass &SetB() { return *this; } }; MyClass SomeFunction() { return MyClass(); } int main() { // Correct usage: SomeFunction() .SetA() .SetB(); // destructor here // I want to prevent it: auto my_class = SomeFunction(); my_class .SetA() .SetB(); { auto my_class2 = SomeFunction(); my_class2 .SetA() .SetB(); // destructor of `my_class2` here } // destructor of `my_class` here. return 0; }

To be honest, this looks like an XY Problem

PaulMcKenzie
– PaulMcKenzie

2020-11-21 20:43:36 +00:00
Commented Nov 21, 2020 at 20:43 — PaulMcKenzie
– PaulMcKenzie, Commented Nov 21, 2020 at 20:43

Bernd · Accepted Answer · 2020-11-21 21:10:53Z

In general the destructor is a good place if you want force that something happens even if an exception occured (but be aware that destructros are declared noexcept(true) by default).

I personally use it for "finally" blocks (with noexcept(false)). But usually in C++ you don't need finally because of RAII.

I think you can't prevent the life-time extension / local variables. And we have no attribute [[discard]], only [[nodiscard]].

But why you don't do the work in SetA()? For me it would be more clear?

A good reason would be if you need all the param data to do the work...

Update

There is one option. You could make your object not copyable / moveable:

 MyClass() = default; MyClass(const MyClass&) = delete; MyClass(MyClass&&) = delete;

But with such approach you can't have something like 'SomeFunction'. And of course such a factory function might causes a copy which call your destructor, too.

As a suggestion I would simply do it explicit:

SomeFunction() .SetA() .SetB() .Exec() ;

I believe there is no guarantee that Exec() will be called last.
No that is guaranteed, what is not guaranteed is the evaluation order of parameters...

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