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See this page as a slide show

CS253 Symbol Ambiguity

The __cplusplus symbol

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

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

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

• __cplusplus is defined to be the YYYYMM of the standard version.
• Released in 1997, ratified in 1998?

using namespace, C++98

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

using namespace std;

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

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

199711
moving umbrella

• move wants an argument of type string, but we gave it "umbrella", which is a C-style string, of type 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(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(string s) {
    cout << "moving " << s << '\n';
}

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

201103

• C++ 2011’s new std::move, a better match for the "umbrella" argument, got called!
• We changed nothing except the compiler version, and the code behavior changed. 😱

Explicit std::, c++98

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

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

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

199711
moving umbrella

• Some people avoid using namespace for this reason, and sprinkle their code with std::.
• It's not pretty.

Explicit std::, C++11

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

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

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

201103
moving umbrella

• However, it works.

Selecting using, c++98

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

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

void move(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 at the top of the program.

Selecting using, C++11

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

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

void move(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 to carry a stupid 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.

    std::cout << x << std::endl;

• With selective using declarations, you have to keep adding them as you change your code.

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

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

    using namespace std;

Trade-offs

Your choice! Personally, I find that:

• 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 and do using namespace std. I haven’t had a problem … yet.

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