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CS253 Symbol Ambiguity

The __cplusplus symbol

% cp ~/Example/version.cc .
% cat version.cc
#include <iostream>
int main() {
    std::cout << __cplusplus << '\n';
}

% g++ -std=c++98 -Wall version.cc && ./a.out
199711

% g++ -std=c++11 -Wall version.cc && ./a.out
201103

• __cplusplus, an int, is the YYYYMM of the C++ standard in use.
• Released in 1997, ratified in 1998?

using namespace, C++ 98

#include <iostream>
#include <string>
#include <utility>

using namespace std;

void move(const string &s) {
    cout << "moving " << s << '\n';
}

int main() {
    cout << __cplusplus << '\n';
    move("umbrella");
}

199711
moving umbrella

• move wants std::string; we gave it const char *.
• A ctor conversion exists between the two, so it works.

using namespace, C++ 11

#include <iostream>
#include <string>
#include <utility>

using namespace std;

void move(const string &s) {
    cout << "moving " << s << '\n';
}

int main() {
    cout << __cplusplus << '\n';
    move("umbrella");
}

201103

• Only changed -std=c++98 to -std=c++11.

using namespace, C++ 11

#include <iostream>
#include <string>
#include <utility>

using namespace std;

void move(const string &s) {
    cout << "moving " << s << '\n';
}

int main() {
    cout << __cplusplus << '\n';
    move("umbrella");
}

201103

• C++ 11’s std::move is a better match for "umbrella".
• New compiler, different behavior. 😱

Explicit std::, C++ 98

#include <iostream>
#include <string>
#include <utility>

void move(const std::string &s) {
    std::cout << "moving " << s << '\n';
}

int main() {
    std::cout << __cplusplus << '\n';
    move("umbrella");
}

199711
moving umbrella

• Some avoid using namespace, and sprinkle their code with std::.
• It's not pretty«

Explicit std::, C++ 11

#include <iostream>
#include <string>
#include <utility>

void move(const std::string &s) {
    std::cout << "moving " << s << '\n';
}

int main() {
    std::cout << __cplusplus << '\n';
    move("umbrella");
}

201103
moving umbrella

• However, it works.

Selective using, C++ 98

#include <iostream>
#include <string>
#include <utility>

using std::string;
using std::cout;

void move(const string &s) {
    cout << "moving " << s << '\n';
}

int main() {
    cout << __cplusplus << '\n';
    move("umbrella");
}

199711
moving umbrella

• Some people apply selective using declarations.
• It's not pretty, either, but at least the tedious part is all together.

Selective using, C++ 11

#include <iostream>
#include <string>
#include <utility>

using std::string;
using std::cout;

void move(const string &s) {
    cout << "moving " << s << '\n';
}

int main() {
    cout << __cplusplus << '\n';
    move("umbrella");
}

201103
moving umbrella

• And it works.

☼☂⛅☁☔

Do you carry an umbrella every day, or only when the weather report predicts rain?

If you carry an umbrella every day, then you’re always dry, but you have the bother of carrying an umbrella all the time.

If you only carry it when rain is predicted, then you’ll get wet once in a while. However, you don’t have to lug around an umbrella when you don’t need it.

It’s a trade-off. Which price do you want to pay? Constant carrying, or occasional moisture?

Trade-offs

Similarly …

• Without any using at all, you pay the price of your code being littered with std:: prefixes.

    
cout << setw(5) << setfill('*') << hex << 42 << endl;
std::cout << std::setw(5) << std::setfill('*') << std::hex << 42 << std::endl;
***2a
***2a

• With selective using declarations, you have to keep adding them as you change your code. They’re all at the top of the file, so they don’t get in the way of reading the real code.

    #include <iostream>
    using std::cin;
    using std::cout;
    using std::getline;

• With using namespace std, you’ll have a rare nasty problem.

    using namespace std;

Trade-offs

Your choice! My opinions:

• std::cout << x << std::endl makes the code difficult to read. It’s hard to get past the litter of std:: to see the real code. Perhaps you get used to it.

• Prof. Draper favors using std::cin, and he’s one of the two finest C++ programmers in the CS Department, so there’s a lot to be said for that.
• I throw caution to the winds with using namespace std.
  • I haven’t had a problem … yet.