This C++11 library builds constant expression lookup tables given a generating function. The table class builds a std::array constant expression with element values initialized by mapping a generator function over the array index values. table<T,N>(f) is equivalent to array<T,N>{{f(0),f(1),...,f(N-1)}}. Here is an example:
#include <table.hpp>
struct times_two {
constexpr std::size_t operator()(std::size_t x) {return x*2;}
};
constexpr table<std::size_t, 2> test_table{times_two()};
static_assert( 0 == test_table[0], "test_table[0] should be equal 0 (2*0)" );
static_assert( 2 == test_table[1], "test_table[1] should be equal 2 (2*1)" );Note that the rules for constant expressions prohibit function pointers and lambda objects, so the generating function for a constexpr table must be given as a function object.
Lookup table constants were traditionally baked into C and C++ programs through code generation. They usually look something like this:
static uint32_t crc32_lut[256] = { 0x00000000, 0x77073096, 0xee0e612c, ... };The generated table adds some noise to the sources to the application. It can not be verified without its code generated. Furthermore, the code generation complicates the build. A constant expression lookup table generated at compile time solves these problems. The opaque values of the constant expression table do not need to appear in the source code. The table can be validated during the build using static assertions. The C++ compiler does all of the work, so there is no need to involve a custom code generation tool into the build process.
The implementation of this table library uses the "index trick" to build the initialized array values. A template argument pack is constructed with the index values <0, 1, 2, ..., N-1>. To avoid the compiler's maximum template instantiation depth, a divide-and-conquer approach is taken to cosntructing the index argument pack.
This library is available under the Boost Software License.
Copyright (C) 2013 Joshua Napoli Distributed under the Boost Software License, Version 1.0. (See Accompanying File LICENSE_1_0.Txt or copy at http://www.boost.org/LICENSE_1_0.txt)