Option+shift+dash, on a default English Mac keyboard. Easy to remember—"modify [option] the dash [dash] to make the biggest common form of it [shift]", or else you can think of it as modifying the underscore (shift + dash) to sit higher on the line (with option).

That works well until the following doesn't compile

int x = 1 — 2;

  #define — -

fixes that issue for C.