Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>This may be a bit off topic, but there are differences between G++ (MinGW 4.4.1) and MSCV++ 10 in the scope of returned temporary variables created on the stack.</p> <p>I just created an object, called say myObj and class members are coordinates, vectors, whatever ...</p> <p>The class has a default constructor, another constructor that initializes class members, a copy constructor, and overloaded +, += and = operators.</p> <pre><code>myObj &amp; myObj::operator+(myObj &amp; mO) { myObj retO = myObj(mO); // using a non-default constructor defined retO += *this; // Uses the other overloaded operator defined. return retO; } </code></pre> <p>The + operator allocates a myObj object on the stack (NOT using new) and adds the objects *this and mO (via retO) and assigns them to the retO object created on the stack.</p> <p>G++ allows you to call:</p> <pre><code>myObj2 = myObj1 + myObj2; </code></pre> <p>However, Visual Studio 10.0 destroys retO that was created on the stack immediately after the overloaded operator member function exits (verified using static member increment and decrement with output inside all constructors and the destructor).</p> <p>I do not know how the standard defines this behavior. Oddly enough, MinGW gives a compiler warning whereas MSVC++ 10 does not.</p> <p>Of course the problem with creating a new myObj object with the new keyword is memory leakage as the above construct does not keep a reference to the new myObj instantiation, which is allocated to myObj2.</p> <p>Regards,</p> <p>Navid</p>
    singulars
    1. This table or related slice is empty.
    1. POLifetime of temporary within operator+ sequence
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USuser998198
    1. USuser998198
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. COg++ is right to issue a compiler warning. The `myObj::operator+(myObj&)` member function, which really should be declared as `myObj::operator+(const myObj&) const`, is returning a reference to a temporary object. Per the standard, the code you posted would obtain a reference to the local `myObj` object `ret0`, destroy `ret0` when exiting the block (returning), and return the reference. But notice that the reference is to a destructed object *and* it is a reference to an object allocated on the stack. Very bad.
      singulars
      1. PO
      1. USDaniel Trebbien
    2. COI totally agree - as I said a bit off topic as not a simple type but an object reference is returned. But the temp references' persistence differ to address the questions about compiler differences. The solution is of course to return a new object and use the overloaded += operator for assignments to the same object and to use the overloaded + operator only for assignments to a new object in order to avoid memory leaks. I still can't figure out why I get no warning from MSCV++. Already looked through all options in the IDE ...
      singulars
      1. PO
      1. USuser998198
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload