Assign return value from function to a reference c++?

Question

This is a two part question. Is it ok to assign the return value of a function to a reference? Such as

Foo FuncBar()
{
    return Foo();
}

// some where          


        

Answer 1

You're not "assigning" to a reference, you're binding to a reference.

That's only proper when the type is const and the context is one where there is automatic lifetime extension of the temporary.

In general, when Foo isn't a const type your examples should fail to compile. Unfortunately, they may compile with one common compiler, due to language extensions implemented by that compiler. It is a good idea to try out examples (and also ordinary code!) with at least two compilers.

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EDIT: as an example of investigative work before posting a question, you should have compiled the following (or very similar) with highest warning level with at least two compilers, with and without CONST defined.

struct Bar {};

#ifdef CONST
    typedef Bar const Foo;
#else
    typedef Bar Foo;
#endif

Foo FuncBar()
{
    return Foo();
}

int main()
{
    // som where else
    Foo &myFoo = FuncBar();
}

If you haven't already done so, it can be a good idea to do that now.

Answer 2

Foo &myFoo = FuncBar();

Will not compile. it should be:

const Foo &myFoo = FuncBar();

because FuncBar() returns a temporary object (i.e., rvalue) and only lvalues can be bound to references to non-const.

Is it safe?

Yes it is safe.

C++ standard specifies that binding a temporary object to a reference to const lengthens the lifetime of the temporary to the lifetime of the reference itself, and thus avoids what would otherwise be a common dangling-reference error.

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Foo myFoo = FuncBar();      

Is Copy Initialization.
It creates a copy of the object returned by FuncBar() and then uses that copy to initalize myFoo. myFoo is an separate object after the statement is executed.

const Foo &myFoo = FuncBar();

Binds the temporary returned by FuncBar() to the reference myFoo, note that myFoo is just an alias to the returned temporary and not a separate object.