callable_traits - C++ Type Traits for All Callable Types

This header-only library provides type traits to inspect any callable type, like functions, member-functions, functors, and lambdas.

• Features
• Supported Traits
• Usage

Features

• Header-only
• Works for any and all callable types
• Full support for all detectable qualifiers a callable can have
• C++17

Supported Traits

Type	Name	Description
bool	is_member_function	True when callable is a member function, unless it is the call operator (operator()), false otherwise.
bool	is_functor	True when callable is the call operator (operator()), or a lambda, false otherwise.
bool	is_const	True when callable has const specifier. Not applicable for regular functions.
bool	is_volatile	True when callable has volatile specifier. Not applicable for regular functions.
bool	is_lvalue_reference	True when callable has & specifier. Not applicable for regular functions.
bool	is_rvalue_reference	True when callable has && specifier. Not applicable for regular functions.
bool	is_noexcept	True when callable has noexcept specifier.
bool	is_variadic	True when callable is a variadic function.
std::size_t	arity	The number of arguments the callable takes. Does not take variadicness into account.
R	result_type	The type that the callable returns.
A0	arg0_type	The type of the first argument of the callable. Only exists if arity > 0.
A1	arg1_type	The type of the second argument of the callable. Only exists if arity > 1.
A2	arg2_type	The type of the third argument of the callable. Only exists if arity > 2.
A	arg_type<idx>	The type of the idxth argument of the callable. Only exists if arity > 0 and idx < arity.

Usage

Callable traits can be used by passing the type of some callable as template argument to callable_traits like so:

#include <callable_traits.hpp>

using traits = callable_traits<SomeCallableType>;

Then the applicable traits listed in Supported Traits can be queried on traits like:

static_assert(traits::arity == 1);

For a more complete example see examples/callable_traits_example.cpp.