c++ - 模板实例化错误，MSVC 与 GCC

Question

我得到了以下模板(其中包含一个大错误):

template <class FluxVar = IloIntVar, class FluxVarArray = IloIntVarArray, 
          class DelayVar = IloIntVar, class DelayVarArray = IloIntVarArray, 
          class LoadVar = IloIntVar, class LoadVarArray = IloIntVarArray,
          class BoolVar = IloBoolVar, class BoolVarArray = IloBoolVarArray>
class MyClass {
public:
    int getAlpha () { return m_alpha ; }
private:
    int m_alpha1 ; // See the "1" here
}

在我的代码中，我做了类似的事情:

MyClass <> myClass1 ;
MyClass <IloNumVar, IloNumVarArray, IloNumVar, IloNumVarArray, IloNumVar, IloNumVarArray> myClass2 ;
MyClass <IloNumVar, IloNumVarArray, IloNumVar, IloNumVarArray, IloNumVar, IloNumVarArray, IloNumVar, IloNumVarArray> myClass3 ; 
/* Some stuff with my class BUT NEVER CALL TO myClassX.getAlpha() */

上面的代码使用 MSVC 编译但不使用 GCC，说(当然):

MyClass.h:109:39: error: m_alpha was not declared in this scope

所以我的问题是:标准对此有何规定？这是来自 MSVC 的优化吗？如果是，在这种情况下优化了什么？

我假设 MSVC 不会生成 getAlpha 的代码，因为它从未被调用过，但与 MSVC 一样，我认为这不是标准行为。

Answer 1

据我所知，MSVC 并不总是合规的，并且作为扩展，它允许对类内的成员函数进行专门化。

MSVC 在没有明确告知时不会生成代码。如果您使用该函数成员，它将触发错误，如果您显式专门化它或显式实例化它也是如此:

template <typename T>
class MyClass {
public:
  int getAlpha () { return m_alpha32 ; }
private:
  int m_alpha1 ; // See the "1" here
};

template<> class MyClass<int>{ // Explicit specialization triggers the error
public: 
  int getAlpha () { return m_alpha32 ; }
private:
  int m_alpha1 ; // See the "1" here
};

template class MyClass<int>; // Explicit instantiation, triggers the error

int main(){

  MyClass<int> obj; // doesn't trigger the error alone

  obj.getAlpha(); // trigger the error


  return 0;
}

另请看这里:http://msdn.microsoft.com/en-us/library/7y5ca42y(v=vs.110).aspx

The compiler generates code for a template class or function when the class or function is instantiated. A member function is instantiated when it is called, and a virtual member function is instantiated when its class is constructed. This can cause problems if you are building a library with templates for other users.

还有

A class template is first specialized and then instantiated by the compiler.

因此，要么不需要诊断，要么根本不生成代码。