How Can I Reliably Detect the Existence of Specific Member Variables in a C Class?

Detecting the Existence of Specific Member Variables in a Class

In code, checking for the presence of particular member variables in a class can be challenging. This can be crucial when implementing generic algorithms or working with classes with varying member names.

Problem Statement:

The provided code aims to address this issue by checking for the existence of "x" (or "X") and "y" (or "Y") in a class template argument. This is particularly useful in classes like MFC's CPoint or GDI 's PointF, where member variable names differ.

The original solution used different templated functions for "x" and "X." However, this resulted in compilation issues in Visual Studio.

Universal Solution:

To achieve a universal solution, we can utilize SFINAE (Substitution Failure is Not an Error). The modified code below demonstrates this approach:

template <typename T>
struct HasX : std::false_type { };

template <typename T>
struct HasX<T, decltype((void)T::x, 0)> : std::true_type { };

This code exploits SFINAE by attempting to substitute "T::x" in the "decltype(...)." If "T::x" is valid, the specialization for HasX<T, decltype((void) T::x, 0)> will be used, resulting in HasX<T>::value being true; otherwise, it will be false.

This solution is not restricted to C 11 and provides a generic method for detecting member variables in any class.

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