Parse this Expression !

Can anyone parse this following expression f开发者_StackOverflow中文版or me

#define Mask(x) (*((int *)&(x)))

I applied the popular right-left rule to solve but cant.. :(

Thanks a bunch ahead :)

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This just reads out the first sizeof (int) bytes at the address of the argument, and returns them as an integer.

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This defines a macro Mask that interprets its argument as an int.

&(x) - address of x...

(int *)&(x) - ...interpreted as a pointer to int

*((int *)&(x)) - the value at that pointer

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You need to think inside out:

• Find the address of x.
• cast this to an integer pointer
• dereference this pointer to return the value.

As such, int y = Mask(something); returns an integer interpretation of something.

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To keep it simple:

#define Mask(x) reinterpret_cast<int&>(x)

I am assuming that there is no const_cast, that is, that the argument x is of non-const type, else there would be an extra const_cast in there.

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This kind of macro can cast a float value to a DWORD in it's binary form and vice versa. This can be used with libraries that have functions using DWORD as generic input types.

An example would be SetRenderState() in DirectX :

HRESULT SetRenderState(
  D3DRENDERSTATETYPE state,
  DWORD value
);

In this particular case, some state require you to give a float value. Now, trying to pass 6.78f directly to that function would truncate 6.78f to an integer which would be 6. What we want is the binary form 0x40D8F5C3 so that the library will be able to cast it back to 6.78f.

That's what we call a reinterpret cast. It's platform dependent and potentialy dangerous unless you know what you are doing.