Tuesday, March 3, 2009

c++: lifetime of temporaries

In c++ you can encounter few types of temporary objects.

Implicitly temporary object is created by compiler when complex expression is being evaluated:

5 + a/4;
Here the result of expression of (a/4) has to be stored somewhere for further calculations.

An anonymous temporary object could be created by programmer when no name is given to an object. Such expression as
is an anonymous temporary object of type int.
When function is intend to return some value it would be also stored into the temporary object.

Of course smart compilers will make some optimizations here to avoid the creation of temporary objects.
The expression
int b = 5 + a/4
could be optimized as storing the result of a/4 to b then adding 5 to b.
The unused return value of the function won't be stored anywhere, moreover compiler may force function not to perform any 'return' actions.

The lifetime of temporary object is ended at the last step in evaluating the full-expression(or block of code) that contains the point where they were created. The temporary object of a/4 in expression 5 + a/4; is being destroyed just after the semicolon.

There is an exception in the standard when you can extend the lifetime of temporary object to the lifetime of const reference that is binded to it:
string get()
    return "string";
const string &obj = get();
The lifetime of string object created by function get on return is extended to the lifetime of const reference obj.

The temporaries are r-value objects, which means that you can't change their values. So the usage of const reference is mandatory. In pre-standard it's was allowed to use non-const references for temporaries but in ISO-C++ it's a compilation error. Changing value of const object could lead to unexpected results.

Using const references to catch the return value of the function could be used as an optimization of avoiding creating temporary object on function return. Consider this code:
string get()
    return "string";
string obj = get();
The temporary is created and its value is stored into the string object obj. Binding the return value to const reference could optimize memory usage and avoid extra calls to constructor and destructor in case if you are interested in the result of the function.

However modern compilers use RVO(Return Value Optimization) or NRVO(Named RVO) to avoid creation of temporary objects so in most cases you don't have to care about such optimizations.


Nick said...

Nice blog found some of the stuff really helpful/interesting.
Would you mind telling me how you were able to get your code to look nice in Blogger.

Ni@m said...

Hello, Nick!
Thank you.

To make the code look nicer I used tool from http://google-code-prettify.googlecode.com
I've linked its js directly from svn trunk, also I've linked jQuery from svn trunk and with it I've added to all tags 'pre' css class defined in code-prettify.