A function that accepts only compile time known expressions?

问题

Compile time expressions are good because you can use them to specialize templates. So for example, tuples can be accessed by using a compile time expression with the std::get method.

std::cout << std::get<0>(my_tuple) << std::endl;

Now, the above expression is pretty ugly. I am trying to develop some sort of tuples myself (hoping to make it to turn them into compile time dictionaries), so that, say, they expose a method in the form:

my_dict.get<0>();

Now, what I would like to do is to substitute that with a [] operator. I was wondering if that was even possible at all. First of all, I wouldn't know how to select only constant, compile time known expressions as parameters for my operator. Moreover, the return type would depend on the value of the constant expression.

With a define, however, I can get closer to what I want with something like

#define item(x) get<x>()

so that I can then use

my_dict.item(0)

Is there a way to get something better than this?

回答1:

This approach uses types to convey the index, which are then passed to operator[], which extracts the index.

template<std::size_t n>
using idx_t=std::integral_constant<std::size_t, n>;
template<std::size_t n>
idx_t<n> idx;

constexpr int square( int x ) { return x*x; }
constexpr int power( int base, size_t pow ) {
  return (pow==0)?1:
    (pow==1)?base:
      (
        square(power(base, pow/2))
          *( (pow%2)?base:1 )
      );
}

template<char... cs>
struct helper:idx_t<0>{};
template<char c0, char...cs>
struct helper<c0, cs...>:idx_t< (c0-'0') * power(10, sizeof...(cs)) + helper<cs...>{} > {};

template<char...cs>
constexpr idx_t< helper<cs...>{} > operator""_idx() { return {}; }

struct foo {
  template<std::size_t n>
  void operator[](idx_t<n>) const {
    char arr[n];
    std::cout << sizeof(arr) << '\n';
  }
};

There are 3 (equivalent, up to syntactic sugar) ways to use this:

foo f;
f[idx_t<1>{}];
f[idx<2>];
f[3_idx];
f[1337_idx];

live example. Support for 0xff_idx etc is left as an exercise.

来源：https://stackoverflow.com/questions/30549431/a-function-that-accepts-only-compile-time-known-expressions

标签

c++

templates

operators

compile-time-constant