A local class cannot access local variables of the function in which it is defined.
Why is this? What is the reason for this rule in C++?
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First Learn What Is A Scope Of A Variable. – Shankar Shastri Oct 01 '16 at 13:45
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2I know scope of a variabe,this is a restriction that is put on local class by the author of C++,so get your facts straight first. – Harssh Chaudhry Oct 01 '16 at 13:52
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1In C++, classes don't inherit scope like Java classes do. – tkausl Oct 01 '16 at 14:03
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@tkausl Yet global variables are visible inside classes. So the question is why is there a restriction for local variables. – juanchopanza Oct 01 '16 at 14:07
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1May be it can be an exception made by the author of C++ but the question is why he made this exception ? – Harssh Chaudhry Oct 01 '16 at 14:11
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3I'm not sure why this is getting so many down-votes. Sure, it may be difficult to find an authoritative answer here, but it doesn't make this a bad question. And it doesn't mean an authoritative, non-speculative answer doesn't exist. – juanchopanza Oct 01 '16 at 14:14
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Why is this a problem? – Jesper Juhl Oct 01 '16 at 14:14
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@ShankarShastri: http://stackoverflow.com/q/3930204/560648 – Lightness Races in Orbit Oct 01 '16 at 14:15
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1@juanchopanza: There's not much meat to it, but I don't really know what more meat there could be. Anyway, I've edited out the equivocation and tidied it up a bit; perhaps that'll help. – Lightness Races in Orbit Oct 01 '16 at 14:17
1 Answers
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It's a reasonable question to ask, especially since some common languages, e.g. Python, allow this.
However, think about this situation if references to locals were allowed and hypothetically this code could compile.
struct MyClass {
virtual fetchValue() = 0;
};
MyClass* somefunction(){
int localVariable = 123;
struct HypotheticalClass : public MyClass {
virtual int fetchValue(){ return localVariable; }
};
return new HypotheticalClass()
}
Now, upon returning this MyClass derivative at the end of somefunction(), it maintains reference to a now-invalid stack location. Boom.
Of course there is a multitude of other ways to inappropriately reference invalid stack locations, for example, passing a reference to that local variable to your class to get the access anyway. And the same issue exists for lambdas which can have access to the local reference or do a copy.
As for an authoritative answer as to why, I skimmed through 4th edition Stroustrup and see nothing useful to report.
Thanks for the interesting question.
John Griffin
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