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Implicit Inclusion

CS253 Implicit Inclusion

Dubious Code

Is this code correct?

#include <iostream>

using namespace std;

int main() {
    string s = "hello\n";
    cout << s;
    return 0;
}

hello

Bad Code

#include <iostream>

using namespace std;

int main() {
    string s = "hello\n";
    cout << s;
    return 0;
}

hello

The code is not correct. A std::string was declared, but there is no #include <string>. But, still, it compiled. ☹

operator<<

• Consider the statement cout << s;. This meaning of << isn’t built into C++ like integer addition. Instead, somebody must have written a operator<< function that takes an ostream and a std::string.
• Where might the declaration of that operator<< be?
• The only sensible places are <iostream> and <string>.
• Wherever it is, it has to know about both classes: ostream and string.
• If it’s in <iostream>, then <iostream> has to #include <string>.
• If it’s in <string>, then <string> has to #include <iostream>.

Light Dawns

#include <iostream>

using namespace std;

int main() {
    string s = "hello\n";
    cout << s;
    return 0;
}

hello

• Now, we understand how this can compile.
• It must be that, on this particular compiler, <iostream> includes <string>.
• Can we count on that?

Solution

#include <iostream>
#include <string>

using namespace std;

int main() {
    string s = "hello\n";
    cout << s;
    return 0;
}

hello

• To write a portable program:
  • #include what you need.
  • Don’t count on an accident of implementation.
• It’s a shame that the compiler doesn’t tell us that we got it wrong.
• This is why computer programmers get the big bucks.

Theoretical

Is this just theoretical handwaving?

No. For nearly every assignment, somebody turns in code that compiles on their computer, but fails on CSU computers.