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Q&A What does a variable followed by parentheses ("ptr()") mean?

void (*ptr)() defines a function pointer. It says that ptr is a pointer to a function. But that function must have a void return type, and take an arbitrary number of parameters (that's what the em...

posted 3y ago by hkotsubo‭  ·  edited 2y ago by Martin Bonner‭

Answer
#4: Post edited by user avatar Martin Bonner‭ · 2022-03-31T11:50:30Z (over 2 years ago)
This is C not C++, empty parens imply an arbitrary number of arguments, not "no arguments".
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (and it works because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`). It has the same effect as calling `PrintHello()`, and the parentheses are needed because it's a function call. But the function takes no parameters, thus the empty parentheses.
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take an arbitrary number of parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (and it works because `PrintHello` matches the signature: it has a `void` return type). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`). It has the same effect as calling `PrintHello()`, and the parentheses are needed because it's a function call. But the function takes no parameters, thus the empty parentheses.
#3: Post edited by user avatar hkotsubo‭ · 2021-12-05T15:02:48Z (almost 3 years ago)
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`). It has the same effect as calling `PrintHello()`, and the parentheses are needed because it's a function call. But the function takes no parameters, thus the empty parentheses.
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (and it works because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`). It has the same effect as calling `PrintHello()`, and the parentheses are needed because it's a function call. But the function takes no parameters, thus the empty parentheses.
#2: Post edited by user avatar hkotsubo‭ · 2021-12-05T15:00:59Z (almost 3 years ago)
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`).
  • `void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).
  • Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.
  • Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`). It has the same effect as calling `PrintHello()`, and the parentheses are needed because it's a function call. But the function takes no parameters, thus the empty parentheses.
#1: Initial revision by user avatar hkotsubo‭ · 2021-12-05T14:59:05Z (almost 3 years ago)
`void (*ptr)()` defines a function pointer. It says that `ptr` is a pointer to a function. But that function must have a `void` return type, and take no parameters (that's what the empty parentheses defines).

Then, `ptr = PrintHello` assigns the `PrintHello` function to the `ptr` pointer (because `PrintHello` matches the signature: it has a `void` return type and takes no parameters). So, now `ptr` is pointing to `PrintHello`.

Finally, `ptr()` is calling the function that `ptr` points to (in this case, `PrintHello`).