#include <iostream>

// Why does this source code not compile?
//
// First, examine the code and try to
// identify the problem without compiling it.
//
// If you cannot find the problem, compile
// the code and use the compiler's error
// message as a hint.
//
// If you still cannot find the problem,
// scroll down.

// Function declaration
int main();

// Function definition
int main() {
    std::cout << Hello World! << std::endl;

    return 0;
}

// The sequence of characters
//
//     Hello World!
//
// is intended to be a string. In C++, a string
// literal must be enclosed in double quotes:
//
//     "Hello World!"
//
// Without the double quotes, the compiler does
// not interpret 'Hello World!' as text to be
// printed. Instead, it tries to interpret the
// characters as C++ source code. In particular,
// 'Hello' and 'World' are treated as identifiers,
// while '!' is the logical NOT operator.
//
// Note that '!' is not a factorial operator.
// In fact, C++ has no factorial operator.
//
// The correct statement is therefore
//
//     std::cout << "Hello World!" << std::endl;
//
// Exercise: What happens if you include only the
// opening double quote but omit the closing one?
// What happens if you include only the closing
// double quote but omit the opening one?
//
// Compare the compiler's error messages. Can you
// explain why they are different?