Skip to main content

Modifying temporaries

Temporary objects are created and destroyed all the time in a C++ program. A simple example would be a function that returns by value. A temporary is as good as a const object because it makes little sense (usually) to change a temporary object, which is unnamed and has a very short time span. (Note: A temporary can be bound to a const reference in which case the scope of the temporary is the same as that of the reference.) However, as it turns out, in C++ you can change temporaries ... if they are of class type! You can call non-const member functions on a temporary. This is quite similar to binding a temporary to a non-const reference and changing it. Section 3.10.10 in C++ ISO/IEC 14882:1998 standard clearly mentions this exception. There are at least two practical use of such an exception. One is the "Named Parameter" idiom and the other one is the Move Constructor idiom. In case of the named parameter idiom, the member functions might prefer to return the object by non-const reference instead of a by value. Here is an example:

class X
int a;
char b;
X() : a(0), b(0) {}
X setA(int i) { a = i; return *this; } // non-const function
X setB(char c) { b = c; return *this; } // non-const function

std::ostream & operator << (std::ostream & o, X const & x)
o << x.a << " " << x.b;
return o;

X createX() // returns X by value.
return X();

int main (void)
// The following code uses the named parameter idiom.
std::cout << createX().setA(10).setB('Z') << std::endl;


Keith said…
You'd be surprised how often you take advantage of this without thinking about it. We have some old Sun Solaris compile that insists temporaries are const, and we often find ourselves unable to compile our latest changes on that box for that very reason.
Keith said…
By the way, returning an auto_ptr from a function requires modifying the temporary return value, or else you wouldn't be able to use it on the right side of an equals.
Sumant said…
@Wriiight: In case of auto_ptr, move constructor/assignment idiom is used. This idiom partially depends on the fact that non-const member functions can be called on temporary objects. A non-const conversion function is called in the case of this idiom. Colvin-Gibbons were the first to come up with this subtle technique.
Cool stuff, can you check out my C++ Code Samples too?
>> Will VC10 support variable length array (VLA)?

> No; that is a C99 language feature.

Where do I put in a feature request for C99 support in VC10?

I really don't want to have to move to mingw (from VC9).
Chris said…
I think more useful would be using the command pattern:
class A
SetX(int x);
SetY(int y);
B b = CreateBFromA().SetX(1).SetY(2);

ie. Using class A to make a B object.
Bose said…
I can't thank you enough for all you've done to help
Resume template
Phone sex uk said…
I have Interest in C++, You have helped me to providing me.
Anonymous said…
This comment has been removed by the author.
Anonymous said…
Thank you for such a fantastic blog. Where else could anyone get that kind of info written in such a perfect way? I have a presentation that I am presently working on, and I have been on the lookout for such information.
sell online

Popular Content

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example. struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for