How to leave a declared function undefined in C++ like with `undefined` in Haskell?

In Haskell there is a constant called undefined that you can use to declare a function without defining it (i.e. a function prototype with an empty body) as with squarein

square :: Int -> Int   -- declaration
square = undefined     -- empty definition

main = putStrLn.show.square $ 3

This is tremendously useful to defer the work on square and focus on getting the main function right first, because the Haskell compiler makes sure that the whole file compiles as if square was defined.

The C++ equivalent is

#import <iostream>
int square(int x){
  //TODO incomplete
  return 0;
}

int main() {
  std::cout << square(3);
}

My intention is to invoke a compiler like clang++ as a typechecker for main alone and work on square later. Imagine that square really is one of many not-yet-defined complex functions that return a complex data structure with a non-trivial constructor. I would have to write a lot of code to create returnable objects just to get the functions to compile.

Is there something quick-and-dirty in C++ similar to undefined?

Thank you @molbdnilo. Using throw is concise and works perfectly:

int square(int x) {
  throw("undefined");
}