Question

I’m trying to understand exact C++ (pre C++0x) behavior with regards to references and rvalues. Is the following valid?

void someFunc ( const MyType & val ) { 
    //do a lot of stuff
    doSthWithValue ( val);
} 

MyType creatorFunc ( ) { 
    return MyType ( "some initialization value" ); 
} 

int main () { 
    someFunc ( creatorFunc() ); 
    return 0; 
}

I found similar code in a library I’m trying to modify. I found out the code crashes under Visual Studio 2005.
The way I understand the above, what is happening is:
creatorFunc is returning by value, so a temporary MyType object obj1 is created. (and kept… on the stack?)
someFunc is taking a reference to that temporary object, and as computation progresses the temporary object is overwritten/freed.
Now, what is mind-boggling is that the code most often works fine. What is more, with a simple piece of code I cannot reproduce the crash.
What is happening/supposed to happen here? Does is matter if the reference (val) is const or not? What is the lifespan of the object returned from creatorFunc?

Answer 1

The return value has the lifetime of a temporary. In C++ that means the complete expression that created the type, so the destructor of MyType shouldn’t be called until after the call to someFunc returns.

I’m curious at your ‘is overwritten/freed’. Certainly calling delete on this object is not OK; it lives on the stack and deleting it would probably cause heap corruption. Also, overwriting/modifying it could also be bad. Your example uses a constant “C string”; on many compilers values like this are stored in read-only memory, so attempting to modify it later could cause a crash/access violation. (I’m not sure if Visual C++ does this optimization, however).

There is a big difference between passing a temporary by const versus mutable references. Creating a mutable reference to a temporary is not allowed by standard C++, and most compilers (including GCC) will reject it, though at least some versions of Visual C++ do allow it.

If you are passing it with a mutable reference, what you want to write is:

   MyType t = creatorFunc();
   someFunc(t);