Barrett Adair Barrett Adair - 5 months ago 27
C++ Question

What is the name of this unusual C++ template feature used by Boost.Spirit?

The code below is from the Boost.Spirit x3 documentation. It uses an interesting C++ syntax that I've never seen before, which is nearly impossible to describe in a search query without knowing the proper terminology. Is this shorthand for the forward declaration of a class? Where is this feature mentioned in the C++ standard?

namespace parser
using x3::eps;
using x3::lit;
using x3::_val;
using x3::_attr;
using ascii::char_;

auto set_zero = [&](auto& ctx){ _val(ctx) = 0; };
auto add1000 = [&](auto& ctx){ _val(ctx) += 1000; };
auto add = [&](auto& ctx){ _val(ctx) += _attr(ctx); };

// What is this? This is the very first use of the identifier `roman`.
x3::rule<class roman, unsigned> const roman = "roman";
// ^^^^^^^^^^^

auto const roman_def =
eps [set_zero]
-(+lit('M') [add1000])
>> -hundreds [add]
>> -tens [add]
>> -ones [add]



The arguments to a template do not necessarily have to be defined to be used. The use of "class roman" actually declares the class roman.

Here is some example code:

#include <iostream>
template <class T> void foo();
template<> void foo<class roman>()
    // allowed because roman is declared
    roman* pointer1;
    // not allowed because romania is not declared
    // romania* pointer2;
    std::cout << "Hello world!" << std::endl;
int main(int argc, char** argv) {
    return 0;

As people have pointed out in the comments above, this distinguishes this instantiation of the template.