Author: Peter Dimov

# Lambda2: A C++14 Lambda Library

Peter Dimov

## Overview

### Description

This is a simple, but functional, C++14 lambda library. It takes advantage of the fact that the standard `<functional>` header already provides `std::bind` customization points (`is_placeholder`, `is_bind_expression`), and function objects such as `std::plus`, `std::greater`, `std::logical_not`, and `std::bit_xor`, corresponding to arithmetic, relational, logical and bitwise operators.

This allows the library to provide a minimal implementation that still lets expressions such as `_1 + 5`, `_1 % 2 == 0`, `_1 > _2`, or `_1 == ' ' || _1 == '\t'` to be composed and used as function objects.

For example, `_1 + 5` is implemented as `std::bind(std::plus<>, _1, 5)`.

These "lambda" expressions can also be freely combined with `std::bind`. For example, `std::bind( f, _1 ) == std::bind( g, _1 )` and `std::bind( f, _1 + _2 )` both work and have the expected behavior.

### Usage Examples

#### Counting the Even Numbers

    #include <boost/lambda2.hpp>
    #include <algorithm>

    using namespace boost::lambda2;

    int count_even( int const * first, int const * last )
    {
        return std::count_if( first, last, _1 % 2 == 0 );
    }

#### Finding the First Whitespace Character

    #include <boost/lambda2.hpp>
    #include <algorithm>

    char const * find_whitespace( char const * first, char const * last )
    {
        using namespace boost::lambda2;

        return std::find_if( first, last,
            _1 == ' ' || _1 == '\t' || _1 == '\r' || _1 == '\n' );
    }

### Dependencies

None. A single, self-contained header.

### Supported Compilers

- GCC 5 or later with `-std=c++14` or above

- Clang 3.9 or later with `-std=c++14` or above

- Visual Studio 2015, 2017, 2019

Tested on Github Actions and Appveyor.

## Revision History

### Changes in 1.78.0

- Added special cases in `operator<<` and `operator>>` when the first argument is a stream, to allow `std::cout << _1`.

- Added `operator->*`.

- Added `first`, `second`.

## Reference

### <boost/lambda2/lambda2.hpp>

#### Synopsis

    namespace boost {
    namespace lambda2 {

    // placeholders

    template<int I> struct lambda2_arg;

    inline constexpr lambda2_arg<1> _1{};
    inline constexpr lambda2_arg<2> _2{};
    inline constexpr lambda2_arg<3> _3{};
    inline constexpr lambda2_arg<4> _4{};
    inline constexpr lambda2_arg<5> _5{};
    inline constexpr lambda2_arg<6> _6{};
    inline constexpr lambda2_arg<7> _7{};
    inline constexpr lambda2_arg<8> _8{};
    inline constexpr lambda2_arg<9> _9{};

    // arithmetic operators

    template<class A, class B> auto operator+( A && a, B && b );
    template<class A, class B> auto operator-( A && a, B && b );
    template<class A, class B> auto operator*( A && a, B && b );
    template<class A, class B> auto operator/( A && a, B && b );
    template<class A, class B> auto operator%( A && a, B && b );
    template<class A> auto operator-( A && a );

    // relational operators

    template<class A, class B> auto operator==( A && a, B && b );
    template<class A, class B> auto operator!=( A && a, B && b );
    template<class A, class B> auto operator>( A && a, B && b );
    template<class A, class B> auto operator<( A && a, B && b );
    template<class A, class B> auto operator>=( A && a, B && b );
    template<class A, class B> auto operator<=( A && a, B && b );

    // logical operators

    template<class A, class B> auto operator&&( A && a, B && b );
    template<class A, class B> auto operator||( A && a, B && b );
    template<class A> auto operator!( A && a );

    // bitwise operators

    template<class A, class B> auto operator&( A && a, B && b );
    template<class A, class B> auto operator|( A && a, B && b );
    template<class A, class B> auto operator^( A && a, B && b );
    template<class A> auto operator~( A && a );

    template<class A, class B> auto operator<<( A && a, B && b );
    template<class A, class B> auto operator>>( A && a, B && b );

    // additional unary operators

    template<class A> auto operator+( A && a );
    template<class A> auto operator*( A && a );
    template<class A> auto operator++( A && a );
    template<class A> auto operator--( A && a );
    template<class A> auto operator++( A && a, int );
    template<class A> auto operator--( A && a, int );

    // compound assignment operators

    template<class A, class B> auto operator+=( A && a, B && b );
    template<class A, class B> auto operator-=( A && a, B && b );
    template<class A, class B> auto operator*=( A && a, B && b );
    template<class A, class B> auto operator/=( A && a, B && b );
    template<class A, class B> auto operator%=( A && a, B && b );
    template<class A, class B> auto operator&=( A && a, B && b );
    template<class A, class B> auto operator|=( A && a, B && b );
    template<class A, class B> auto operator^=( A && a, B && b );
    template<class A, class B> auto operator<<=( A && a, B && b );
    template<class A, class B> auto operator>>=( A && a, B && b );

    // additional binary operators

    template<class A, class B> auto operator->*( A && a, B && b );

    // projections

    inline constexpr /unspecified/ first{};
    inline constexpr /unspecified/ second{};

    } // namespace lambda2
    } // namespace boost

#### Placeholders

    template<int I> struct lambda2_arg
    {
        template<class... A> decltype(auto) operator()( A&&... a ) const noexcept;
        template<class T> auto operator[]( T&& t ) const;
    };

`lambda2_arg<I>` is the type of the library-provided placeholders `_I`. The standard customization point `std::is_placeholder` is specialized for it, enabling the use of Lambda2’s placeholders with `std::bind`.

The placeholders define `operator()`, which permits their direct use as function objects. E.g. `_1(x, y)` returns `x`.

`operator[]` is also defined to allow expressions like `_1[x]` or `_1[_2]`.

    template<class... A> decltype(auto) operator()( A&&... a ) const noexcept;

- Returns:

    `std::get<std::size_t{I-1}>( std::tuple<A&&…​>( std::forward<A>(a)…​ ) );`

    template<class T> auto operator[]( T&& t ) const;

- Returns:

    `std::bind( fn, *this, std::forward<T>(t) );`, where `fn` is a function object such that `fn(x, y)` returns `x[y]`.

#### Common Requirements

All operators defined in the subsequent sections only participate in overload resolution if at least one of their operands is such that for its unqualified type `T`, the expression `std::is_placeholder<T>::value || std::is_bind_expression<T>::value` is `true`.

#### Arithmetic Operators

    template<class A, class B> auto operator+( A && a, B && b );

- Returns:

    `std::bind( std::plus<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator-( A && a, B && b );

- Returns:

    `std::bind( std::minus<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator*( A && a, B && b );

- Returns:

    `std::bind( std::multiplies<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator/( A && a, B && b );

- Returns:

    `std::bind( std::divides<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator%( A && a, B && b );

- Returns:

    `std::bind( std::modulus<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A> auto operator-( A && a );

- Returns:

    `std::bind( std::negate<>(), std::forward<A>(a) );`

#### Relational Operators

    template<class A, class B> auto operator==( A && a, B && b );

- Returns:

    `std::bind( std::equal_to<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator!=( A && a, B && b );

- Returns:

    `std::bind( std::not_equal_to<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator>( A && a, B && b );

- Returns:

    `std::bind( std::greater<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator<( A && a, B && b );

- Returns:

    `std::bind( std::less<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator>=( A && a, B && b );

- Returns:

    `std::bind( std::greater_equal<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator<=( A && a, B && b );

- Returns:

    `std::bind( std::less_equal<>(), std::forward<A>(a), std::forward<B>(b) );`

#### Logical Operators

    template<class A, class B> auto operator&&( A && a, B && b );

- Returns:

    `std::bind( std::logical_and<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator||( A && a, B && b );

- Returns:

    `std::bind( std::logical_or<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A> auto operator!( A && a );

- Returns:

    `std::bind( std::logical_not<>(), std::forward<A>(a) );`

#### Bitwise Operators

    template<class A, class B> auto operator&( A && a, B && b );

- Returns:

    `std::bind( std::bit_and<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator|( A && a, B && b );

- Returns:

    `std::bind( std::bit_or<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A, class B> auto operator^( A && a, B && b );

- Returns:

    `std::bind( std::bit_xor<>(), std::forward<A>(a), std::forward<B>(b) );`

    template<class A> auto operator~( A && a );

- Returns:

    `std::bind( std::bit_not<>(), std::forward<A>(a) );`

    template<class A, class B> auto operator<<( A && a, B && b );

- Returns:

    `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`, where `fn` is a function object such that `fn(x, y)` returns `x << y`.

    template<class A, class B> auto operator>>( A && a, B && b );

- Returns:

    `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`, where `fn` is a function object such that `fn(x, y)` returns `x >> y`.

#### Additional Unary Operators

    template<class A> auto operator+( A && a );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `+x`.

    template<class A> auto operator*( A && a );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `*x`.

    template<class A> auto operator++( A && a );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `++x`.

    template<class A> auto operator--( A && a );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `--x`.

    template<class A> auto operator++( A && a, int );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `x++`.

    template<class A> auto operator--( A && a, int );

- Returns:

    `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object such that `fn(x)` returns `x--`.

#### Compound Assignment Operators

    template<class A, class B> auto operator@=( A && a, B && b );

- Returns:

    `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`, where `fn` is a function object such that `fn(x, y)` returns `x @= y`.

#### Additional Binary Operators

    template<class A, class B> auto operator->*( A && a, B && b );

- Returns:

    `std::bind( std::forward<B>(b), std::forward<A>(a) );`

  Notes:

    This operator is intended to be used with "projection" function objects such as member pointers or member functions taking zero arguments, as in `_1->*&X::m` or `_1->*&X::f`.

#### Projections

    inline constexpr /unspecified/ first{};

A function object such that `first(x)` returns `std::get<0>(x)`.

    inline constexpr /unspecified/ second{};

A function object such that `second(x)` returns `std::get<1>(x)`.

Code Example 1. Using first and second to print out a map

    void print( std::map<int, std::string> const & m )
    {
        using namespace boost::lambda2;
        std::for_each( m.begin(), m.end(), std::cout << _1->*first << ": " << _1->*second << '\n' );
    }

## Copyright and License

This documentation is copyright 2020, 2021 Peter Dimov and is distributed under the Boost Software License, Version 1.0.