CS253 Implicit Inclusion

Dubious Code

Is this code correct?

#include <iostream>

using namespace std;

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

hello

Bad Code

#include <iostream>

using namespace std;

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

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?
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;
}

hello

Now, we understand how this can compile.
It must be that, on this particular compiler, <iostream> includes <string>.
Can we count on that? No!
On another computer, <string> might include <iostream>.

This Could Happen

You can construct <iostream> & <string> so they don’t #include each other:

<iostream>:
    #define IOSTREAM_INCLUDED
    #ifdef STRING_INCLUDED
	#include <iostream-string-overloads>
    #endif

<string>:
    #define STRING_INCLUDED
    #ifdef IOSTREAM_INCLUDED
	#include <iostream-string-overloads>
    #endif

<iostream-string-overloads>:
    std::ostream &operator<<(std::ostream &, const std::string &);

Solution

#include <iostream>
#include <string>

using namespace std;

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

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 handwringing? No.
Compiler vendors are currently quite casual about having one header file include another.
- It is permitted by the standard.
For nearly every assignment, somebody turns in code that compiles on their computer, but fails on CSU computers.

CS253: Software Development with C++

Spring 2018

Implicit Inclusion