C++ identifiers

Question

C++ Primer says:

The identifier we define in our programs may not contain 2 consecutive underscores, nor can identifier begin with an underscore followed immediately by an uppercase letter. In addition, identifiers be fined outside of a function may not begin with an underscore

All is well, but

int _c = 55;                  // outside function starts with _

int main () {

    int _A = 12;              // _ followed by uppercase letter
    cout << _A << endl;

    int __b__ =33;            // 2 consecutive __
    cout << __b__ << endl;

    cout << _c << endl;

}

Code above compiles perfectly fine on mac, g++ 4.7.1, using the following flags

g++ -pedantic -Wall -Werror -std=c++11 -O3 -funroll-loops -fprefetch-loop-arrays

What am i missing please?

When you break the "rules", it's doesn't *guarantee* that it will not work. It just may or may not. — Mysticial, Aug 18 '12 at 19:13
Related: http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier — Bart, Aug 18 '12 at 19:13
NOt everything that isn't correct must fail to compile. Much like not every sentence that you can form in the English language must automatically make sense. — Kerrek SB, Aug 18 '12 at 20:16
Also, you should avoid using identifiers that begins with an underscore and followed by *lowercase* letter (not only uppercase letters as mentioned in the book). In my compiler and OS, `_pthread_cleanup_buffer` is already used. — Adola, May 04 '22 at 13:01

score 9 · Answer 1 · edited Aug 18 '12 at 20:09

9

Crossing the street without looking out for traffic both ways doesn't guarantee that you are run over by a bus, but it is still a bad idea.

One of those days it isn't going to work...

edited Aug 18 '12 at 20:09

Bart

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answered Aug 18 '12 at 19:18

Bo Persson

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score 4 · Accepted Answer · edited Aug 18 '12 at 19:15

4

Nothing. The identifiers with a leading underscore followed by an uppercase letter and identifiers with two underscores are merely reserved.

Your compiler vendor might decide to use them at any time (for instance, it's perfectly fine for a compiler vendor to use _A as a keyword) and hence you should not use them. Often, the standard library is implemented using reserved identifiers, hence they do not result in a compile error.

edited Aug 18 '12 at 19:15

jrok

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answered Aug 18 '12 at 19:13

Anteru

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Is there a way to treat these as warnings? – James Leonard Aug 18 '12 at 19:14
Nope, that wouldn't work, as the compiler does not know what code is from the standard library and which one is yours. – Anteru Aug 18 '12 at 19:15

C++ identifiers

2 Answers2