Last week, we discussed why we should sometimes use remove_cvref_t on our template parameters before applying concepts to them. We also saw that the solution is not super readable because we lose access to the terse, shorthand syntax.

C++26: Concept and variable-template template-parameters

by Sandor Dargo

From the article:

C++ already allows passing templates as template parameters, but only if they are class templates. A common reason for doing this is to allow higher-level abstractions. For instance, you may want to pass in a container template like std::vector, without specifying the type it contains.

Jason Turner explains this well in C++ Weekly - Ep 368 - The Power of template-template Parameters: A Basic Guide, but here’s his example for quick reference:

template<template <typename Contained, typename Alloc = std::allocator<Contained>>
		 typename ResultType>
auto get_data() {
	ResultType<double> result;
	// ...
	return result;
}

int main() {
	auto data = get_data<std::vector>();
}