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From: "terminator(jam)" <farid.mehrabi@gmail.com>
Newsgroups: comp.std.c++,comp.lang.c++
Subject: pre return optimization
Date: Tue,  9 Oct 2007 09:56:13 CST
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consider:

struct memory_pig{//a really large type:

	memory_pig(){
		std::cout<<"mem pig default\n";
		//etc...
	};

	memory_pig(memory_pig const&){
		std::cout<<"mem pig copy\n";
		//etc...
	};

	~memory_pig(){
		std::cout<<"mem pig finish\n";
		//etc...
	};

	//etc...

};///struct memory_pig

memory_pig foo(){
	memory_pig result;
	result=something;
	//etc...
        result=something_else;
	return result;
};

any time 'foo' is called the output will contain the following
sequence:


mem pig default
mem pig copy
mem pig finish

the last line of output may repeat based on how the result is
stored(rvo) or not.
So,two objects of a large type will be constructed and at least one is
destructed on every call to 'foo' in PASCAL you can write:

function foo:memory_pig
begin
	foo:=something;
	{etc...}
	foo:=somthing_else;
end

that is you can refrence the returned object inside the function and
decrease the overhead for copying large objects.C++ lacks such syntax
and IMHO we should be able to mark the result object as referencing
the actual return so that there is no need for the extra copy
construction;this is espesifically beneficall when dealing with
operator definitions .We can declare the return itself as an object.I
suggest the following syntax:

class ret_type funxn (paramlist){
	ret_type return /*optional:*/(initiallizer params);

        //etc...

        if(false)return;//return like void functions

        //return something;//error:named return accepts no param.

        return=something;//ok;
        return=something_else;
        return.member_funxn();
	another_funcxn(return);

};

Provided that 'return' is declared as an object { whenever you use the
'return' keyword inside pharantesis or accompanied via an operator ?
the return object is referenced :otherwise a parameterless 'return'
returns to the caller.
The return object is an lvalue inside the function and can be aliased
for readability:

memory_pig foo(){
	memory_pig return , & result=return;
	result=something;
	//etc...
        result=something_else;
};

and whenever 'foo' is called the output looks like this:

mem pig default

the unnessesary copy/move has vanished and the destruction of
temporary depends on what you do with it.

regards,
FM.

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