constexpr for sizeof in template does not compile

问题

In Visual C++ 2012 (C++ 11) I am getting error for code

template <typename T, T... I>
struct structFoo
{
  static const T sz = sizeof...(I);        (1)
  static constexpr T size = sizeof...(I);  (2)
};

the compiler takes the line (1) but complains that the line (2) is an error: 'error C4430: missing type specifier - int assumed. Note: C++ does not support default-int.

Why?

回答1:

Microsoft did not implement support for constexpr until Visual Studio 2015.

You imply that VC++2012 is an implementation of C++11, but that's a gross oversimplification. In fact, various language and library features were added over time across multiple versions; some C++11 features, never mind C++14, are still not supported:

• https://msdn.microsoft.com/en-us/library/hh567368.aspx#corelanguagetable

来源：https://stackoverflow.com/questions/31632491/constexpr-for-sizeof-in-template-does-not-compile