nuple: a Named Tuple

While browsing the Internets I came upon this blog post by Victor Laskin. In there he describes an implementation of a named tuple in C++. It just so happened that some time ago I was writing about useful properties of string interning in C++. I quickly realized that another good use of it is to implement a named tuple which I called nuple - a named tuple. By the way, you should check up Victor's blog if you're a fan of functional programming and C++.

So, the implementation turned out to be pretty simple. The usage is like this:

using nameid_t = nuple< $("name"), char const*, $("id"), int >;

The dollar sign is string interning which turns a string literal into a type. All we have to do is to sort this parameter type list into a names list and a type list. The type list is then passed to luple which is a lightweight (source code I mean) tuple I created for the C++ type loophole. The source code (GitHub) is short and easy to read (header is nuple.h).

So, here's an example of using it:

nameid_t n[] = { {"alex", 1}, {"ivan", 2} };

for( auto& v : n )      
  printf( "name: %s, id: %d\n", get<$("name")>(v), get<$("id")>(v) );

Because nuple inherits from luple (I'm not good at terminology) we can use all other tuple-like methods:

auto get_person(int i) { return nameid_t{"john", i}; }

auto p = get_person(3);

printf( "tuple size: %d\n", size(p) );

get<0>(p) = "irene";

get<int>(p) = 4;

printf( "name: %s, id: %d\n", get<$("name")>(p), get<$("id")>(p) );

See it in action online at tio.run (also Coliru or Wandbox).

Also a nuple-to-json example at tio.run (also Coliru or Wandbox).

There is no hacks. It's all valid C++ source code (C++14) and can be safely used in production code. There is a limit on string length of 10 characters (sort of arbitrary), you can increase it by editing the $(...) macro in the intern.h header. Or #define N3599 to use the N3599 proposal (adds string literal template to the language) which GCC and Clang implement as an extension. (Update: defined by default for GCC and Clang)

Update. Added syntax like this:

auto get_person( int id ) {

  return as_nuple( $name("name"), "Victoria", $name("id"), id );
}

auto p = get_person( 5 );

std::cout << get<$("name")>( p );

Here macro $name(...) returns a value of an interned string type ( $(...) gives a type ).

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