Constness of STL containers and their elements - when to use const?

Question

I have been overthinking (some may say underthinking, let's see what happens) the const-ness of STL containers and their elements.

I have been looking for a discussion of this, but the results have been surprisingly sparse. So I'm not necessarily looking for a definite answer here, I'd be just as happy with a discussion that gets the gears in my head moving again.

Let's say I have a class that keeps std::strings in a std::vector. My class is a dictionary that reads words from a dictionary file. They will never be changed. So it seems prudent to declare it as

std::vector<const std::string> m_myStrings;

However, I've read scattered comments that you shouldn't use const elements in a std::vector, since the elements need to be assignable.

Question:

• Are there cases when const elements are used in std::vector (excluding hacks etc)?

• Are const elements used in other containers? If so, which ones, and when?

I'm primarily talking about value types as elements here, not pointers.

Answer 1

My class is a dictionary that reads words from a dictionary file. They will never be changed.

Encapsulation can help here.

Have your class keep a vector<string>, but make it private. Then add an accessor to your class that returns a const vector<string> &, and make the callers go through that.

The callers cannot change the vector, and operator [] on the vector will hand them const string &, which is exactly what you want.

Answer 2

No, for the reason you state.