Empty_Template_Idiom.h

// Following class doesn't have to be templated, but we use template here to:
// 1. Make sure function's declaration and definition are in same place, so it is convenient when you update the function signature. 
// Although inline could also achieve this, but you can't prevent others from moving implementation into a cpp, while template provide stronger protect.

// 2. Code will only be instantiated if used for template functions, thus reduce the binary size
// Although modern compiler and linker could remove non-used functions, that requires additional compiler or linker time to make it happen.

// But it also have some drawbacks
// 1. Compile error message is hard to read for templates
// 2. As all code are in header file, it is more vulnerable to #defines and typedefs when included

// Header file
template <typename E = void>
class ApplicationT
{
public:
    bool Run()
    {
        //...
        return true;
    }
    // ...
};
typedef ApplicationT<> Application;

// Source file
Application app;
app.Run();