I have been writing a program that has a rather large structure that is passed by reference to a few functions. However, there are a few other functions that need access to small pieces of information within the large structure. It's not being edited, just read.
I was thinking of creating a a second structure that just copies the specific pieces of information needed and passing that by reference, rather than passing the entire structure by reference.
What I am wondering is two things:
More specifically:
I have a configuration structure that sets up the programs configuration by calling a function and passing the structure by reference. There is some information (process name, command line arguments) that I want to use for informative purposes only. I'm asking if it's bad practice to pass around a structure that wasn't meant for the purpose of what I want to use it for.
1) Since I am passing the large structure by reference, there really is no performance impact. Correct?
Correct.
2) Even if 1) is correct, is it bad etiquette to be passing around a structure that shouldn't be edited (even though it wouldn't be edited, but still I'm talking about the principle here).
You could let your function accept a reference to const to make sure the function won't alter the state of the corresponding argument.
I'm asking if it's bad practice to pass around a structure that wasn't meant for the purpose of what I want to use it for.
I'm not sure what you mean by this. The way you write it, this definitely seems to be a bad practice: you shouldn't use something for doing what it wasn't meant for. That means distorting the semantics of an object. However, the rest of your question doesn't seem to imply this.
Rather, it seems like you are concerned with passing a reference to a function because that may allow the function to alter the argument's state; but provided the function takes a reference to const, it won't be able to alter the state of its argument. In that case, no it's not a bad practice.
If you are referring to the fact that the function only need to work with some of the data members or member functions of your structure, then again that is not necessarily a bad design. It would be silly to require that each function access every member of a data structure.
Of course, this is the best I can write without knowing anything concrete about the semantics of the function and the particular data structure.