ceph/src/boost/libs/type_index/examples/constexpr14_namespace_check.cpp

   1 // Copyright 2013-2016 Antony Polukhin
   2
   3 // Distributed under the Boost Software License, Version 1.0.
   4 // (See the accompanying file LICENSE_1_0.txt
   5 // or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)
   6
   7 #include <boost/config.hpp>
   8
   9 #if !defined(BOOST_NO_CXX14_CONSTEXPR) && !defined(BOOST_NO_CXX11_CONSTEXPR)
  10 // Implementation of this function is not essential for the example
  11 template <std::size_t N>
  12 constexpr bool starts_with(const char* name, const char (&ns)[N]) noexcept {
  13     for (std::size_t i = 0; i < N - 1; ++i)
  14         if (name[i] != ns[i])
  15             return false;
  16
  17     return true;
  18 }
  19
  20 //[type_index_constexpr14_namespace_example
  21 /*`
  22     The following example shows that `boost::typeindex::ctti_type_index` is usable at compile time on
  23     a C++14 compatible compilers.
  24
  25     In this example we'll create and use a constexpr function that checks namespace of the provided type.
  26 */
  27
  28 #include <boost/type_index/ctti_type_index.hpp>
  29
  30 // Helper function that returns true if `name` starts with `substr`
  31 template <std::size_t N>
  32 constexpr bool starts_with(const char* name, const char (&substr)[N]) noexcept;
  33
  34
  35 // Function that returns true if `T` declared in namespace `ns`
  36 template <class T, std::size_t N>
  37 constexpr bool in_namespace(const char (&ns)[N]) noexcept {
  38     const char* name = boost::typeindex::ctti_type_index::type_id<T>().raw_name();
  39
  40     // Some compilers add `class ` or `struct ` before the namespace, so we need to skip those words first
  41     if (starts_with(name, "class ")) {
  42         name += sizeof("class ") - 1;
  43     } else if (starts_with(name, "struct ")) {
  44         name += sizeof("struct ") - 1;
  45     }
  46
  47     return starts_with(name, ns) && starts_with(name + N - 1, "::");
  48 }
  49
  50 /*`
  51     Now when we have that wonderfull function, we can do static assertions and other compile-time validations:
  52 */
  53
  54 namespace my_project {
  55     struct serializer {
  56         template <class T>
  57         void serialize(const T& value) {
  58             static_assert(
  59                 in_namespace<T>("my_project::types") || in_namespace<T>("my_project::types_ext"),
  60                 "Only types from namespaces `my_project::types` and `my_project::types_ext` are allowed to be serialized using `my_project::serializer`"
  61             );
  62
  63             // Actual implementation of the serialization goes below
  64             // ...
  65         }
  66     };
  67
  68     namespace types {
  69         struct foo{};
  70         struct bar{};
  71     }
  72 } // namespace my_project
  73
  74 int main() {
  75     my_project::serializer s;
  76     my_project::types::foo f;
  77     my_project::types::bar b;
  78
  79     s.serialize(f);
  80     s.serialize(b);
  81
  82     // short sh = 0;
  83     // s.serialize(sh); // Fails the static_assert!
  84 }
  85 //] [/type_index_constexpr14_namespace_example]
  86
  87 #else // #if !defined(BOOST_NO_CXX14_CONSTEXPR) && !defined(BOOST_NO_CXX11_CONSTEXPR)
  88
  89 int main() {}
  90
  91 #endif
  92