Fun C++ Quiz!

Originally published on Cohost.

Why doesn't the following code run? (comments represent different files):

// foo.hpp
template <typename T>
struct Foo {
  static void foo(void);
}
// foo.cpp
#include <iostream>
#include "foo.hpp"
template <>
struct Foo<int> {
  static void foo() { std::cout << "foo\n"; }
}
// main.cpp
#include "foo.hpp"
int main(void) {
  Foo<int>::foo();
}

Answer

From this SO post. Basically, foo.cpp needs to be

#include <iostream>
#include "foo.hpp"
template struct Foo<int>;
void Foo<int>::foo() { std::cout << "foo\n"; }

because otherwise, member function definitions are secretly inline, and since there is no usage site in the same compilation unit they won't get exported as symbols. Declaring them like this lets them be non-inline, and the symbol can be used in the other compilation unit.

hell language btw.