C++ streamsize prec = cout.precision(3) - How does it work?

Question

I am kind of newbie in using c++. I have a quick question, probably a dumb question.

streamsize prec = cout.precision(3);

As I understand correctly this declaration works like that: set the cout precision to 3, but assign the previous precision value to prec.

Also, simply, we can assign a function result (say a math addition function) to a variable:

int z = addition(3,4);

In the second one, it does the calculation and assigns the results to the variable z, not the previous value or a default value. Is my understanding correct? What is the difference between them?

Answer 1

What value a function returns depends entirely on that particular function. Most functions simply return a result of their operation.

The state-setting functions in standard library streams (such as precision) are a bit unusual in their interface of "I set a new value and return the old one," but it's still perfectly valid, as long as the function's behaviour is documented (which it is in their case).