固定容量的自定义STL容器的max_size（）的正确值是什么？

Question

I'm writing a custom circular buffer implementation with fixed capacity (fixed at runtime by call to constructor, no resize methods) and want it to be STL-compatible. My goal is to make it a Random Access Container . Additionally I want it to have the specific interface of an Back Insertion Sequence , but without being a sequence (no resize and inserting/erasing at arbitrary positions, ...). So it would only be an extended Random Access Container. After reading some questions about max_size() and size() , I'm still a little confused.

My current idea: size() : number of elements contained in buffer
max_size() : capacity of buffer (maximum number of elements it can hold)

Is this correct (conforming to standard / STL)? Or do I have to handle it like (std::)array with size() == max_size()?

Answer 1

The standard is quite clear on the meaning of max_size() in C++11 Table 96:

distance(begin(), end()) for the largest possible container

If, like std::array , the size is a property of the container type (eg specified by a template parameter), then that should be the same as size() . If you can instantiate the same type with various sizes, then it should be the largest allowed size.

I would follow the example of the standard containers, and have a capacity() function to tell you the capacity.

Answer 2

max_size doesn't relate to a specific instance of the container type. It is the maximum size that the type can handle, according to whatever implementation details limit it (the max value of size_type if nothing else).

vector can reserve space that isn't currently used, and for that reason it has a capacity() function. This isn't part of any of the container concepts though, because other containers ( deque , list ) don't reserve space.