能否不依赖编译器内置函数实现is_trivially_constructible库版本？

Unfortunately, the short answer is no—you can't implement std::is_trivially_constructible purely in library code without relying on compiler-specific built-ins. Here's why:

The is_trivially_constructible trait checks whether a type can be constructed with a given set of arguments using a trivial constructor. Triviality is a property defined by the C++ standard, but determining it requires intimate knowledge of how the compiler treats the type's constructor(s)—things like whether the compiler generated a default constructor automatically, if that constructor performs no operations beyond memory initialization, or if user-defined constructors meet the triviality criteria.

This kind of information isn't accessible through standard C++ library code alone. It requires compiler support to expose these internal details, which is exactly why libstdc++ uses the __is_trivially_constructible built-in for this trait.

Workarounds for GCC 4.9 #

Since you're stuck on GCC 4.9, here are some practical alternatives:

• Manual trait specialization
  Create your own custom trait and manually mark types in your codebase that are trivially constructible. For example:

  #include <type_traits>
  
  template<typename T>
  struct my_is_trivially_constructible : std::false_type {};
  
  // Specialize for primitive types
  template<> struct my_is_trivially_constructible<int> : std::true_type {};
  template<> struct my_is_trivially_constructible<float> : std::true_type {};
  template<> struct my_is_trivially_constructible<char> : std::true_type {};
  
  // Specialize for your own trivial types
  struct MyTrivialStruct { int data; };
  template<> struct my_is_trivially_constructible<MyTrivialStruct> : std::true_type {};
  

  This requires manual maintenance but works for controlled codebases.

• Use Boost.TypeTraits
  The Boost library provides boost::is_trivially_constructible, which might have better compatibility with older compilers like GCC 4.9. Note that Boost may still rely on some compiler-specific hooks under the hood, but it's a pre-built solution that avoids rolling your own.

• Upgrade your compiler (if possible)
  GCC 5 and later fully support C++14's std::is_trivially_constructible as part of the standard library. If your project constraints allow it, upgrading is the most robust long-term fix.

内容的提问来源于stack exchange，提问作者A. K.