Single line class declaration and instantiation

Question

I've stumbled upon code using the following syntax.

int main(){    
  class foo{
    public:
      int x;
      foo(int y){x=y;}
  }
  * bar = new foo(1);    
}

Is there any purpose/consequence of using it compared to the more common

int main(){    
  class foo{
    public:
      int x;
      foo(int y){x=y;}
  };

  foo * bar = new foo(1);    
}

Personally I prefer the latter but it would have to be `auto bar = std::make_unique(1);` — NathanOliver, Aug 07 '18 at 18:19
This is why people think C++ is complicated, don't use the first one — JVApen, Aug 07 '18 at 18:21

score 3 · Accepted Answer · answered Aug 07 '18 at 18:29

3

This is probably a matter of opinion but I think the first method is poor coding practice.

answered Aug 07 '18 at 18:29

R Sahu

Do you believe that putting class definitions inside functions is ever good coding practice? _(honest question)_ – Aug 07 '18 at 18:32
2

@gooroo7, I don't recall using it at my work over the last 20+ years. I can't tell whether others have a legitimate use for it. My assessement is that with the advent of `lambda` expressions, local classes/structs are probably not needed. You can get by with `std::pair` and `std::tuple` for most `struct`-like needs in a function. – R Sahu Aug 07 '18 at 18:36
@gooroo7 For context, the original code had multiple "maker" functions, each returning a pointer to a different implementation of a virtual class. – xvan Aug 07 '18 at 18:43
https://stackoverflow.com/questions/10072208/when-do-i-need-anonymous-class-in-c As @RSahu described, there is really no reason for anonymous classes using modern C++, but you may need a construct like that when also using existing C code. – Ben Jones Aug 07 '18 at 19:31
2

In the past, I used local classes to profit from RAII, e. g. as cleanup guards with tasks specific to the function. Today, though, I'd have a generic template guard accepting a lambda in constructor and executing it in destructor... – Aconcagua Aug 07 '18 at 20:01

1 Answers1