C++ Virtual Functions Disassembled

Schmid — Tue, 31 Aug 2010 07:26:01 GMT

←Older revision		Revision as of 07:26, 31 August 2010
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	; (Derived::get_value) to eax		; (Derived::get_value) to eax
	call *%eax ; call Derived::get_value		call *%eax ; call Derived::get_value
-		+	== Performance ==
		+	Obviously, there is a small performance issue with virtual methods. If you do high-performance applications, note that this example performs perfectly in the time domain (although not in the space domain):
		+	<pre>
		+	struct Thing {
		+	virtual ~Thing() {}
		+	virtual int method() = 0;
		+	};
		+	struct Thing2 : public Thing {
		+	int method() { return 10; } // note: not virtual
		+	};
		+	int main() {
		+	Thing2 thing;
		+	return thing.method();
		+	}
		+	</pre>
		+	The 'main' code, when optimized, resolves to:
		+	<pre>
		+	main:
		+	pushl %ebp
		+	movl $10, %eax
		+	movl %esp, %ebp
		+	popl %ebp
		+	ret
		+	</pre>
		+	Which is equivalent to:
		+	<pre>
		+	int main() {
		+	return 10;
		+	}
		+	</pre>
		+	If we did:
		+	<pre>
		+	int main() {
		+	Thing2 thing2;
		+	Thing & thing = thing2;
		+	return thing.method();
		+	}
		+	</pre>
		+	We would have the usual virtual method overhead.
	[[Category:C++]]		[[Category:C++]]
	[[Category:Assembler]]		[[Category:Assembler]]
	[[Category:Gcc]]		[[Category:Gcc]]

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