C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  String literals in C++          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Neelesh Guest Posted: Thu Oct 20, 2005 2:04 pm    Post subject: String literals in C++ Hi, I have a confusion regarding what is the exact datatype of a "string literal" in C++. The following code makes the point clear - #include <string> using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? ii) When I just say "hello" as a literal does it represent any particular type? Thanks, ~Neelesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Thu Oct 20, 2005 5:24 pm    Post subject: Re: String literals in C++ Neelesh wrote: Quote: I have a confusion regarding what is the exact datatype of a "string literal" in C++. It's an array of const char. The size is equal to the number of characters plus one for the terminating null char plus at least one for multibyte encoding of each of universal characters. It has static storage duration. Quote: The following code makes the point clear - #include using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. It is not "incorrect". It is "deprecated". Should be supported by all compilers at this time, though. Quote: } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? First and fourth, array to pointer. Third, array to pointer and user- defined (string has a c-tor from const char*). Quote: ii) When I just say "hello" as a literal does it represent any particular type? Yes, It's "const char[6]". V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Thu Oct 20, 2005 5:27 pm    Post subject: Re: String literals in C++ * Neelesh: Quote: I have a confusion regarding what is the exact datatype of a "string literal" in C++. array of constant char, or in C++'ish, 'char const [N]'. Quote: The following code makes the point clear - #include using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. Actually the last _is_ formally correct, but only due to a C compatibility feature; it's not something one should do except for interfacing to legacy C code. Quote: } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? All of them. Unless the second one doesn't count as a "conversion". There are so many special meanings of words, and somebody else can look that up! Quote: ii) When I just say "hello" as a literal does it represent any particular type? 'char const [6]', with 6 instead of 5 because of the terminating zero. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Thu Oct 20, 2005 7:33 pm    Post subject: Re: String literals in C++ Neelesh wrote: Quote: Hi, I have a confusion regarding what is the exact datatype of a "string literal" in C++. The following code makes the point clear - #include using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? ii) When I just say "hello" as a literal does it represent any particular type? IIRC, the standard requires that a string literal is a const array of char. However, some older (pre-standard) versions of C did not support the const keyword, so the type of a string literal is simple array of char. I'm not sure offhand if draft versions of the C++ standard (before it was ratified) ever required that a string literal be a const array, but it is probable some older compilers treated a string literal as a non-const array of char for reasons of backward compatibility to C. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Fri Oct 21, 2005 8:25 am    Post subject: Re: String literals in C++ Neelesh wrote: Quote: Hi, I have a confusion regarding what is the exact datatype of a "string literal" in C++. The following code makes the point clear - #include using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? ii) When I just say "hello" as a literal does it represent any particular type? IIRC, the standard requires that a string literal is a const array of char. However, some older (pre-standard) versions of C did not support the const keyword, so the type of a string literal is simple array of char. I'm not sure offhand if draft versions of the C++ standard (before it was ratified) ever required that a string literal be a const array, but it is probable some older compilers treated a string literal as a non-const array of char for reasons of backward compatibility to C. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tony Delroy Guest Posted: Fri Oct 21, 2005 4:28 pm    Post subject: Re: String literals in C++ Quote: const char *x = "hello"; results in memory allocation for both a six-character string literal, and a pointer "x" that you're unlikely to need (it could be moved along the string, but unless you plan to do so exactly once, or there are quirks to your string layout that you plan to exploit, you'll need a second variable to assist with moving towards the start of the string, so you might as well declare a named string literal and a pointer to it, rather than two pointers and an unnamed string literal). i.e. const char y[] = "hello"; const char* p = y; Quote: char y[] = "hello"; better to say const char where possible and sufficient, or you may end up with run-time copying from read-only memory to read-write memory. Quote: string z = "hello"; Assuming std::string, this is equivalent to string("hello"), and quite different from the character array approaches in that at run-time it will dynamically allocate memory from the heap and then copy the characters from the string literal. Quote: char *w = "hello" ; As mentioned by others, this is allowed for backwards compatibility with C. Consider all the C headers declaring things like: void fn(char* p_string); It would be very painful to have to type: fn(const_cast<const char*>("hello")); in order to call them. Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dietmar Kuehl Guest Posted: Fri Oct 21, 2005 7:34 pm    Post subject: Re: String literals in C++ Neelesh wrote: Quote: I have a confusion regarding what is the exact datatype of a "string literal" in C++. This is something which is not really often explained in C++ text books. However, it is rather simple: the type is 'char const[n]' where 'n' is the number of characters in the string literal, including the terminating zero character. That is, "foo" has the type 'char const[4]'. Quote: const char *x = "hello"; // allowed This takes advantage of the normal decaying of array types to pointer types. Quote: char y[] = "hello"; // allowed This is entirely unrelated to the type of the string literal: the above line involves to character array, namely the string literal "hello" and the character array 'y' whose size is determined from the string literal and which is initialized by copying the literal's elements. Quote: string z = "hello"; // allowed This is similar to the previous example in that it is entirely unrelated to the type of the string literal: a variable is initialized using a conversion operator taking a pointer to the string literal which is obtained as the result from decaying the array into an array. Quote: char *w = "hello" ; // Incorrect, but still g++ allows this. All conforming compilers shall allow this although it breaks const correctness. This is a special exception applying to string literals which was added to avoid invalidating legacy code: it was considered too common that string literals are assigned to 'char*' variables or passed to functions taking this type as argument. New code should never do anything like this. Quote: i) In which of these cases a type conversion is involved during the initialization? In all of these cases. In fact, you cannot initialize a variable with a string literal without a conversion! The closes you can get is initializing a reference with string literal but even this has a different type than the string literal itself: it is a reference to string literal rather than the string literal itself: char const (&ref)[6] = "hello"; Quote: ii) When I just say "hello" as a literal does it represent any particular type? It has a particular type: 'char const[6]'. -- <mailto:dietmar_kuehl (AT) yahoo (DOT) com> <http://www.dietmar-kuehl.de/> <http://www.eai-systems.com> - Efficient Artificial Intelligence [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Sat Oct 22, 2005 1:49 pm    Post subject: Re: String literals in C++ Alf P. Steinbach wrote: Quote: * Neelesh: I have a confusion regarding what is the exact datatype of a "string literal" in C++. array of constant char, or in C++'ish, 'char const [N]'. The following code makes the point clear - #include using namespace std; int main() { const char *x = "hello"; // allowed char y[] = "hello"; // allowed string z = "hello"; // allowed char *w = "hello" ; // Incorrect, but still g++ allows this. Actually the last _is_ formally correct, but only due to a C compatibility feature; it's not something one should do except for interfacing to legacy C code. } The RHS of all these expressions is same. My questions are i) In which of these cases a type conversion is involved during the initialization? All of them. Unless the second one doesn't count as a "conversion". There are so many special meanings of words, and somebody else can look that up! The second doesn't count as a conversion, because conversions occur in expressions, and there is no expression:-). Seriously, from a standards point of view, the type of the token may be string literal, but the string doesn't work like a string literal otherwise. In particular, it *doesn't* have a type char const[]; in fact, it doesn't have a type at all. It's just a special way of writing: { 'h', 'e', 'l', 'l', 'o', '' }. (Sort of, anyway. The important point is that there isn't a char const[] which you can access. Typically, at least when initializing static char[], there isn't a char const[] at all.) Quote: ii) When I just say "hello" as a literal does it represent any particular type? 'char const [6]', with 6 instead of 5 because of the terminating zero. Except when it is used to initialize a character array. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Sat Oct 22, 2005 6:09 pm    Post subject: Re: String literals in C++ * kanze: Quote: * Alf P. Steinbach: * Neelesh: i) In which of these cases a type conversion is involved during the initialization? All of them. Unless the second one doesn't count as a "conversion". There are so many special meanings of words, and somebody else can look that up! The second doesn't count as a conversion, because conversions occur in expressions, and there is no expression:-) Thanks, but now I note that Alf has said he's somewhat agnostic on the issue, baddin ingles languge as he is, Victor has kept silent but with an implied no, James has said clearly no, and Dietmar has said clearly yes. This can get interesting. ;-) Watch the next episode of ... C O N V E R S I O N! -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Mon Oct 24, 2005 12:14 am    Post subject: Re: String literals in C++ Alf P. Steinbach wrote: Quote: * kanze: * Alf P. Steinbach: * Neelesh: i) In which of these cases a type conversion is involved during the initialization? All of them. Unless the second one doesn't count as a "conversion". There are so many special meanings of words, and somebody else can look that up! The second doesn't count as a conversion, because conversions occur in expressions, and there is no expression:-) Thanks, but now I note that Alf has said he's somewhat agnostic on the issue, I noted that too. I just like arguing about nits:-). In the end, of course, what counts is what the thing does, not what you call it, and I think we all agree as to what it does. Quote: baddin ingles languge as he is, Victor has kept silent but with an implied no, James has said clearly no, and Dietmar has said clearly yes. I thought that Dietmar also said no. He described what happens here without using the word "conversion". Quote: This can get interesting. ;-) Watch the next episode of ... C O N V E R S I O N! Glad to see that your sense of humor is keeping things in perspective. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Mon Oct 24, 2005 9:00 am    Post subject: Re: String literals in C++ * James Kanze: Quote: I thought that Dietmar also said no. He described what happens here without using the word "conversion". No, he wrote "In all of these cases.". Quote: This can get interesting. ;-) Watch the next episode of ... C O N V E R S I O N! And here we go... ;-) -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? 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