variable<"name"> is a named placeholder for a value which is yet unknown.
For example, variable<"block_size"> could be a placeholder in an arithmetic expression which encodes a value dependent on the block size of a CUDA kernel launch before the shape of the launch has been decided:
std::size_t n = 12345;
variable<"block_size"> block_size;
auto num_blocks = ceil_div(block_size, n);
fmt::print("num_blocks: {}\n", num_blocks);
// "num_blocks: ((12345+block_size)-1)/block_size" is printed
std::tuple unevaluated_config(block_size, num_blocks);
// unevaluated_config is an unevaluated expression of a variable "block_size"
...
// later, evaluate the launch configuration after choosing a block size
int actual_block_size = choose_kernel_block_size();
std::tuple<int,int> config = evaluate(unevaluated_config, {"block_size", actual_block_size});
A user-defined literal operator is included to make it convenient to spell these variables.
// these two spellings are equivalent:
variable<"block_size"> block_size_1;
auto block_size_2 = "block_size"_v;