type_name

C++ type_name utilities for pretty-printing type names.

Originally inspired by Howard Hinnant's type_name code, posted in reponse to:

"How can I see the type deduced for a template type parameter?"
StackOverflow, August 2013.

The implementation is based on C++ typeinfo (RTTI, to become constexpr in C++20).

  typeid(T).name()

On GCC, Clang and compilers using the Itanium ABI the result is a mangled name.
On these platforms the name is demangled using the abi::__cxa_demangle() function.

C++17 <type_name_rt>

1. type_name_str<T>()
   Returns a std::string copy of the demangled typeid name.
   On each call it does all the work, and cleans it all up
   (i.e. it frees any demangle allocation once copied from).

2. type_name_rt<T>
   A std::string_view global constant (a view into the
   demangle buffer, on CXXABI, which is not ever free'd).
   All work is done in static initialization, before main()

• Failure is signaled by an empty return value; ""
  (indicates a demangle failure as typeid is assumed failsafe).

Requirements

C++17 for string_view, constexpr-if and __has_include
RTTI, the compiler's runtime type information, must be enabled.

Dependencies

From std:

<cstring> for std::strlen
<string>,<string_view> as the return values.
<type_traits> for std::conditional.
<typeinfo> (RTTI) for typeid(T).name(), an implementation-defined name.
<cstdlib> for std::free.
<memory> for std::unique_ptr

Platform dependency:

<cxxabi.h> for demangling (on CXXABI platforms only - GCC, Clang, etc.)

Usage example

Because the type is supplied as a template parameter, decltype(expr) is required
to query the type of an expression:

    const volatile char abc[1][2][3]{};
    std::cout << type_name_rt<decltype(abc)>();

...produces output (different format is possible on different platforms):

  char const volatile [1][2][3]

Design notes

Firstly, note that the type argument has to be provided as a template parameter; type deduction from a regular function argument is not sufficient because deduction cannot distinguish all possible passed types. In other words, it is not possible to wrap decltype() or implement it otherwise.

In principal, with the type known at compile time, a fully constexpr implementation of type_name<T> should be possible, for example as a constexpr variable template whose value is some compile-time string type containing the human-readable type name.

In practice, when based on the C++ typeid() operator, a fully constexpr implementation is not generally possible.

Howard Hinnant's C++11 type_name code

template <typename T>
std::string
type_name()
{
    using TR = typename std::remove_reference<T>::type;
    std::unique_ptr<char, void(*)(void*)> own (
    #ifndef _MSC_VER
        abi::__cxa_demangle (typeid(TR).name(), nullptr,
            nullptr, nullptr),
    #else
            nullptr,
    #endif
            std::free
    );
    std::string r = own != nullptr ? own.get() : typeid(TR).name();
    if (std::is_const<TR>::value)
        r += " const";
    if (std::is_volatile<TR>::value)
        r += " volatile";
    if (std::is_lvalue_reference<T>::value)
        r += "&";
    else if (std::is_rvalue_reference<T>::value)
        r += "&&";
    return r;
}