Check if constant is defined at runtime in Obj-C

To, for example, access variables in a NSDictionary Cocoa frameworks often define keys, such as UIKeyboardBoundsUserInfoKey. How can I check if a key is defined at 开发者_运维问答runtime? I found examples on how to check for classes and functions, but not for constants.

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Jasarien's answer is roughly correct, but is prone to issues under LLVM 1.5 where the compiler will optimise the if-statement away.

You should also be comparing the address of the constant to NULL, rather than nil (nil has different semantics).

A more accurate solution is this:

BOOL isKeyboardBoundsKeyAvailable = (&UIKeyboardBoundsUserInfoKey != NULL);
if (isKeyboardBoundsKeyAvailable) {
  // UIKeyboardBoundsUserInfoKey defined
}

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Check it's pointer against nil, like this

if (&UIKeyboardBoundsUserInfoKey != nil)
{
    //Key exists
}