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Asked: 2026-05-25T17:48:30+00:00

I’ve read that the C++ standard allows optimization to a point where it can

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I’ve read that the C++ standard allows optimization to a point where it can actually hinder with expected functionality. When I say this, I’m talking about return value optimization, where you might actually have some logic in the copy constructor, yet the compiler optimizes the call out.

I find this to be somewhat bad, as in someone who doesn’t know this might spend quite some time fixing a bug resulting from this.

What I want to know is whether there are any other situations where over-optimization from the compiler can change functionality.

For example, something like:

int x = 1;
x = 1;
x = 1;
x = 1;

might be optimized to a single x=1;

Suppose I have:

class A;

A a = b;
a = b;
a = b;

Could this possibly also be optimized? Probably not the best example, but I hope you know what I mean…

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1 Answer

  1. Editorial Team

    Eliding copy operations is the only case where a compiler is allowed to optimize to the point where side effects visibly change. Do not rely on copy constructors being called, the compiler might optimize away those calls.

    For everything else, the “as-if” rule applies: The compiler might optimize as it pleases, as long as the visible side effects are the same as if the compiler had not optimized at all.

    (“Visible side effects” include, for example, stuff written to the console or the file system, but not runtime and CPU fan speed.)

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