c++c-preprocessor

Meaning of #undef in C++

I know what this means

#define M(B) (1U << ((sizeof(x) * CHAR_BIT) - B)) // CHAR_BIT=bits/byte

but I don't understand this one well:

#undef M

After this, what happens? Is M cleared, deleted, or?

Solution

  • After the #undef, it's as if the #define M... line never existed.

    int a = M(123); // error, M is undefined

#define M(B) (1U << ((sizeof(x) * CHAR_BIT) - B))

int b = M(123); // no error, M is defined

#undef M

int c = M(123); // error, M is undefined