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Dice Template Library

This template library is a collection of template-oriented code that we, the Data Science Group at UPB, found pretty handy. It contains:

  • switch_cases: Use runtime values in compile-time context.
  • integral_template_tuple: Create a tuple-like structure that instantiates a template for a range of values.
  • integral_template_variant: A wrapper type for std::variant guarantees to only contain variants of the form T<ix> where $\texttt{ix}\in [\texttt{first},\texttt{last}]$ (inclusive).
  • for_{types,values,range}: Compile time for loops for types, values or ranges
  • polymorphic_allocator: Like std::pmr::polymorphic_allocator but with static dispatch
  • DICE_DEFER/DICE_DEFER_TO_SUCCES/DICE_DEFER_TO_FAIL: On-the-fly RAII for types that do not support it natively (similar to go's defer keyword)
  • overloaded: Composition for std::variant visitor lambdas
  • flex_array: A combination of std::array, std::span and a vector with small buffer optimization
  • tuple_algorithms: Some algorithms for iterating tuples
  • generator: The reference implementation of std::generator from P2502R2
  • channel: A single producer, single consumer queue
  • variant2: Like std::variant but optimized for exactly two types

Usage

switch_cases

Use runtime values in compile-time context. This is realised by instantiating ranges of values at compile-time and dispatching to the correct version at runtime. You can add fallbacks for when the runtime value lies outside the range defined. By using switch_cases inside of switch_cases multidimensional ranges can be handled as well. Examples can be found here.

integral_template_tuple

Create a tuple-like structure that instantiates a template for a range of values. Let's say you have a type like

template <std::size_t N> struct my_type{...};

Then you can create a tuple consisting of my_type<i>, my_type<i+1>, ... up to my_type<j> for i<=j with this code. Negative indices, recasting to fewer values and non-default construction is also possible. Examples can be found here.

integral_template_variant

Creates a variant-like structure that instantiates a template for a range of values. Let's say you have a type like

template <std::size_t N> struct my_type{...};

Then you can create a variant consisting of my_type<i>, my_type<i+1>, ..., my_type<j> with the help of integral_template_variant<my_type, i, j>. Negative indices, and j <= i are also possible. Examples can be found here.

for_{types,values,range}

Different flavors of compile time loops that allow to iterate types, values or ranges at compile time. Types and values are provided as template arguments and a lambda to be called for each of them is passed as function argument, e.g. for_types<uint8_t, uint64_t>([]<typename T>() {}) and for_values<1, 1.1, 'c'>([](auto x) {}). Ranges are defined by template parameters for start and exclusive end and receive a function to be applied to each range element as function argument, e.g. for_range<3, 5>([](auto x) {}), including support for decreasing ranges and negative indices, e.g. for_range<2, -4>([](auto x) {}). Examples can be found here.

polymorphic_allocator

A std::pmr::polymorphic_allocator-like type that uses static dispatch instead of dynamic dispatch to choose the allocator. This allocator is primarily useful for situations where you have inhomogeneous memory and one of the memory types does not allow dynamic dispatch using vtables; but you still want to mix and match values from both memory types.

For example, you might have some allocations in persistent or shared memory (or generally: memory-mapped allocations) and others on the heap. The problem with mmap allocations is that they will be placed at an arbitrary position in virtual memory each time they are loaded, therefore absolute pointers will cause segfaults if the segment is reloaded. Which means: vtables will not work (because they use absolute pointers) and therefore you cannot use std::pmr::polymorphic_allocator.

DICE_DEFER/DICE_DEFER_TO_SUCCES/DICE_DEFER_TO_FAIL

A mechanism similar to go's defer keyword, which can be used to defer some action to scope exit. The primary use-case for this is on-the-fly RAII-like resource management for types that do not support RAII (for example C types). Usage examples can be found here.

tuple algorthims

Some algorithms for iterating tuples, for example tuple_fold a fold/reduce implementation for tuples.

flex_array

A combination of std::array, std::span and a vector with small buffer optimization where the size is either statically known or a runtime variable depending on the extent/max_extent template parameters

generator

The reference implementation of std::generator from P2502R2. By default, the generator and corresponding utilities are exported under the dice::template_library:: namespace. If you want this generator to serve as a drop in replacement for std::generator until it arrives use #define DICE_TEMPLATELIBRARY_GENERATOR_STD_COMPAT 1 before including the generator header. That will export all generator related things under namespace std::.

channel

A single producer, single consume queue. This can be used to communicate between threads in a more high level fashion than a mutex+container would allow.

variant2

Like std::variant but specifically optimized for usage with two types/variants. The internal representation is a union of the two types plus a 1 byte (3 state) discriminant. Additionally, visit does not involve any virtual function calls.

Further Examples

Compilable code examples can be found in examples. The example build requires the cmake option -DBUILD_EXAMPLES=ON to be added.

Requirements

A C++20 compatible compiler. Code was only tested on x86_64.

Include it into your projects

CMake

add

FetchContent_Declare(
        dice-template-library
        GIT_REPOSITORY "https://github.com/dice-group/dice-template-library.git"
        GIT_TAG v1.9.1
        GIT_SHALLOW TRUE)

FetchContent_MakeAvailable(dice-template-library)

to your CMakeLists.txt

You can now add it to your target with:

target_link_libraries(your_target
        dice-template-library::dice-template-library
        )

conan

You can use it with conan. To do so, you need to add dice-template-library/1.9.1 to the [requires] section of your conan file.

Build and Run Tests and Examples

# get it 
git clone https://github.com/dice-group/dice-template-library.git
cd dice-template-library
# build it
mkdir build
cd build
cmake -DBUILD_TESTING=ON -DBUILD_EXAMPLES=ON ..
make -j$(nproc)
# run tests
make run_tests
# run examples
./examples/examples_integral_template_tuple
./examples/examples_switch_cases