Consider this template
// this is not a type
template<typename C> buffer {
// every instance of template shares this value
static inline mutable size_t count = 1024U ;
};
If we create few template definitions we get the same number of distinctive types, like so:
// 3 concrete types
using c_buffer = buffer<char>;
using w_buffer = buffer<wchar>;
using n_buffer = buffer<int> ;
So far so good. But. count is shared. It is the same for everyone. All concrete types defined from the same template and all the instances of those types.
// true
bool the_same_value = c_buffer::count == w_buffer::count == n_buffer::count ;
Since the count is shared on the type level (aka static user-defined type, member) one change applies to every other type.
w_buffer::count = 13;
//true
bool all_are_13 = 13 == c_buffer::count == w_buffer::count == n_buffer::count ;
This is not so good unless it is a design requirement.
The context
The other day I have developed a handy little template to hold and provide, a plain 2d array on the stack (aka the “matrix”). It is also fully “compile time capable” with the full static internal presentation. Very handy to enforce (gently) company-wide policies of static matrices in standard C++, and to give in the same time some useful functionality in return. I am calling this template “almost a pod”. Continue reading “C++ How to create unique types, from the same template”
