When to use x3::lit in Boost spirit

Question

I am beginning with Boost Spirit x3 parsing library - and I'm very excited about it.

One thing unclear to me is when and why one should use x3::lit.

From what I understood, it's because we can not expect an expression like ']' >> double_ to be interpreted as intended within C++ syntactic rules.

But my interpretation seems incorrect, since the official documentation displays numerous examples of simple strings followed by the >> operator, and others instances where parenthesis and/or brackets have to be specified as lit(']')

What am I missing?

Answer 1

You can not generally expect '[' >> x to see the X3 expression overload.

Overload resolution depends on the types of both operands. Since '[' is char, there cannot be user-defined overloads. So in this case, only if x is already an x3 parser expression the x3 overload of operator>> will be found.

In generic code, if both operands are unknown you should use x3::as_parser instead of x3::lit because it will leave other types of expressions intact, but promotes string literals like you would expect:

 auto combine(auto a, auto b) {
      return x3::as_parser(a) >> x3::as_parser(b);
 }

The mechanism for overload resolution at play in e.g. 'x' >> x3::double_ is ADL: https://en.cppreference.com/w/cpp/language/adl (Incidentally it's the same mechanism that makes std::cout << "Hello world\n" find the std::operator>> overload)