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Compiler Warning (level 3) C4231

nonstandard extension used : 'identifier' before template explicit instantiation

When Microsoft extensions are enabled (/Ze), an extern template can be instantiated, generating this warning. Under ANSI compatibility (/Za), such instantiations cause an error.

// C4231.cpp
// compile with: /W3
template<class T, int i> class MyStack {}; // C4231
extern template MyStack< int, 4>;

int main()