constant<value> is a compile-time value like std::integral_constant<T,value> with additional arithmetic operators.
It is intended for use cases where it is important to treat values known at compile time uniformly with dynamic values.
For example, constant<1024> could encode the the size of a CUDA block:
constant<1024> block_size;
A user-defined literal operator is included to make it convenient to spell these constants.
// these two spellings are equivalent:
constant<1024> block_size_1;
auto block_size_2 = 1024_c;
We can use constant to index tuple-like types without needing to treat the index like a template parameter:
std::tuple<int,char,std::string> my_tuple = ...
char element_2 = element(my_tuple, 2_c);
This is sometimes convenient.