I was reading some articles and in many of them mentioned about expression templates can avoid using temporary objects. But none of them mentioned how this is done. As far as I know, due to the design architecture operations are done using temporary object. For example, if a, b and c are two matrix and if we do a = b+c then the result of b+c is kept in a temporary object like temp = b+c and then result is copied back to a like a = temp.
But if we use expression templates then this addition(+) operation returns reference of b and c and then the main calculation happen when assignment operator(=) is evaluated. This is the simple general concept of template expression. But I don't understand how it get rid of temporary objects. It would be nice if someone could give just the general idea how this temporary is avoided.
Take the example of
Vector a,b,c,d;
a = b + c + d;
Usually this would translate into something like
a = b.operator+( c.operator+(d) );
where each call to operator+ would have to loop through the entries. However, the more natural way would be to loop through all elements once and do some addition like
a_i = b_i + c_i + d_i
and thats what expression templates effectively do by evaluating an epxression only when the result is really needed.
Note that the temporary (ie intermediate result of c+d) is only required because each operator+ seperately loops through all elements. Once the seperate loops are combined, there is no need for a temporary anymore.
For a less amateurish explanation and more details I can suggest you this talk from the CppConf 2015.