What is the use of temporaries in c++

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Hi Friends,

I have some doubt in temporaries in C++.
I did not exactly understand the concept of temporaries. I already searched in google, but I did not get any satisfactory explanation of this and most importantly why we use this? So guys please let me give some good explanation with example for that i can understand easily.

...Waiting for your replies...

Regards.
Johny...

Posted 2-Sep-12 8:11am

Johny_sa

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A simple way to "tune" on them is thinking on how compute an expression like

z = (a+b)*c;

What does multiply c?
"The result of a+b", but it has to be stored somehow as result of + just for the time neede to * to compute.

Now what about

z = operator*(operator+(a,b),c);

This is the same as above: the function named "operator+" return a result,
that is then passed to another function - operator*, in this case - in the form of a temporary value.

Much like
z = f(g(a,b),c);

expressed as

tmp = g(a,b);
z = f(tmp,c);

Of course, this is not completely exhaustive about temporaries, but is just a way to imagine them in a case they have to come to existence.

Posted 2-Sep-12 9:47am

Emilio Garavaglia

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pasztorpisti 2-Sep-12 16:21pm

5ed, this explanation is better for begginner C++ coders. Maybe the other beast - the function return value copying would worth mentioning, thats easy to understand and might happen often in regular c++ code.

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pasztorpisti · Accepted Answer · 2012-09-02T08:24:00

Temporaries are called rvalues and non-temporaries are called lvalues. Named objects in your code are lvalues, and temporary object that are the results of your (sub)expressions are temporary objects - rvalues. For example (x+y) is an rvalue and (x) is an lvalue. Every expression is either an lvalue or rvalue. You can easily distinguis between the two: you can not get a pointer to an rvalue so you can not say &(x+y) while you can do this with &x. The C++11 introduces a very cool feature to handle temporaries more efficiently, its the rvalue reference. To read about temporaries and this new C++11 feature read this lengthy but superb explaining article: http://blogs.msdn.com/b/vcblog/archive/2009/02/03/rvalue-references-c-0x-features-in-vc10-part-2.aspx[^]