How to override “no non-const reference to temporary object” correctly

问题

I have a class Foo. Foo has a few non-const methods. I am okay with calling non-const methods on a temporary Foo object; I am only interested in what the methods actually return, than what they do to the Foo object itself.

First question: Does this, by itself, necessary indicate that the class Foo is not well-designed?

Second question: If I want to continue with Foo as-is, but I still want to be able to pass Foo objects by reference to functions that will call non-const methods on it, what would be the best way to do it?

This is what I arrived at:

// The function that performs work on a Foo object.
int TakeFoo (Foo& FooBar) { ... }

// An overload just to accept temporary Foo objects.
int TakeFoo (Foo&& FooBar)
{
    // Delegate
    return TakeFoo(FooBar);
}

The alternative approach is just doing this:

int TakeFoo (const Foo& FooBar)
{
    Foo& MyFooBar = const_cast<Foo&>(FooBar);
    // Do work on MyFooBar
}

But this approach has the problem that you may be const-cast'ing away the const on an object that was actually declared const, which would put me in undefined-behavior-land.

Edit:

Code examples that use TakeFoo:

Foo GimmeFoo() { ... }

cout << TakeFoo(GimmeFoo()) << endl;

Foo ConstructedFoo(...);

cout << TakeFoo(ConstructedFoo) << endl;

// Continue to use ConstructedFoo

回答1:

Answer to your second question:

If your function TakeFoo is meant to call non const members of Foo then use

int TakeFoo (Foo& FooBar);

If you are sure that TakeFoo only takes an rvalue as argument then use

int TakeFoo (Foo&& FooBar);

If you want to do some changes to Foo in order to compute the int return value then use

int TakeFoo (const Foo& FooBar)
{
    Foo FooBar MyFooBar = FooBar;
    // do something with MyFooBar and return int
}

int TakeFoo (Foo FooBar);

Answer to your first question:

int TakeFoo (FooBar) should not change FooBar in order to compute the int result. A better design would be

Foo transform_foo(Foo const& foo);
int compute_result(Foo const& foo);
int TakeFoo(Foo const& FooBar)
{
    return compute_result( transform_foo(FooBar) );
}

来源：https://stackoverflow.com/questions/17199695/how-to-override-no-non-const-reference-to-temporary-object-correctly

标签

c++11

pass-by-rvalue-reference