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lambda-tuple

An implementation of std::tuple based on variadic lambda capture (P0780R2) added in C++20. The idea is to store all elements of the tuple in such a lambda:

[... v = std::move(v)](auto f) mutable -> decltype(auto)
{
    return f(v...);
}

I would consider it more of a toy implementation. Nonetheless, there is an extensive test suite including several tests from the Microsoft STL amounting to 75+ tests in total.

Play with it on godbolt.

Advantages

  • Triviallity of copy/move construction of types is preserved.
  • No template or function recursion used in the implementation.
  • Automatic pretty-printing in the debugger. E.g. for tuple<int, double>
    Lambda-tuple pretty-print
  • An empty tuple is std::is_trivial_v.
  • Roughly 2.6 times faster to compile with GCC and 2 times faster with Clang. (build-bench)

Disadvantages

  • Cannot be passed across DLL boundaries.
  • No in-place construction, just like std::tuple.
  • Triviallity of copy/move assignment of types is not preserved, just like std::tuple.
  • The compiler is allowed to re-order elements captured in the lambda to minimize its size (by ordering elements by their sizeof or performing empty-class optimization). In practice, however, none of the major compilers seem to make use of it. (godbolt)

Installation

Copy the single header from src/ltpl/tuple.hpp into your project.

Alternatively, use CMake to install the project. From the root of the repository:

cmake -B build -S .
cmake --install build --prefix build/out

And in your CMake project's CMakeLists.txt:

# Add build/out to the CMAKE_PREFIX_PATH
find_package(lambda-tuple)
target_link_libraries(my_app PRIVATE lambda-tuple::lambda-tuple)

Usage

Include the single header:

#include <ltpl/tuple.hpp>

And use ltpl::Tuple just like std::tuple.

Requirements

The only requirement is a small subset of C++20.

The following compilers are continuously tested by Github Actions:

  • GCC 10,
  • Clang 10
  • MSVc 19.32
  • AppleClang 13