Boost.Hana: boost::hana::is_default< Method, typename > Struct Template Reference

| Boost.Hana 1.0.0 Your standard library for metaprogramming |

boost::hana::is_default< Method, typename > Struct Template Reference
Core

## Description

### template<typename Method, typename>
struct boost::hana::is_default< Method, typename >

Returns whether a tag-dispatched method implementation is a default implementation.

Given a tag-dispatched method implementation `method_impl<T...>`, `is_default<method_impl<T...>>` returns whether `method_impl<T...>` is a default implementation. Note that if there is no default implementation for the method, then `is_default` should not be used unless a static assertion saying that "the method is not implemented" is acceptable.

## Example

// Copyright Louis Dionne 2013-2016
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/core/default.hpp>
#include <boost/hana/core/tag_of.hpp>
#include <algorithm>
#include <iterator>
#include <sstream>
#include <vector>
namespace hana = boost::hana;
// In the header defining the concept of a Printable
template <typename T>
struct print_impl : hana::default_ {
template <typename Stream, typename X>
static void apply(Stream& out, X const& x)
{ out << x; }
};
auto print = [](auto& stream, auto const& x) {
return print_impl<hana::tag_of_t<decltype(x)>>::apply(stream, x);
};
// In some other header
template <typename T>
struct print_impl<std::vector<T>> {
template <typename Stream>
static void apply(Stream& out, std::vector<T> const& xs) {
out << '[';
std::copy(begin(xs), end(xs), std::ostream_iterator<T>{out, ", "});
out << ']';
}
};
static_assert(hana::is_default<print_impl<int>>{}, "");
static_assert(!hana::is_default<print_impl<std::vector<int>>>{}, "");
int main() {
{
std::stringstream s;
print(s, std::vector<int>{1, 2, 3});
BOOST_HANA_RUNTIME_CHECK(s.str() == "[1, 2, 3, ]");
}
{
std::stringstream s;
print(s, "abcd");
BOOST_HANA_RUNTIME_CHECK(s.str() == "abcd");
}
}

Inherits false_type.