const char* vs char*

Question

Here is what my book says:

char *p="Hello"; //pointer is variable, string is constant *p='M'; // error p="bye"; // works

Well, in C my second line does not give me any error, Neither in C++.

I am using Turbo C++ on windows 7. Now is the above thing true if i try in gcc or something else.

Also on the similar lines if the above code is correctly interpreted by the book,

#include<iostream.h> void display(char*); void display(const char*); void main() { char* ch1="hello"; const char *ch2="bye"; display(ch1); display(ch2); } void display(char* p) { cout << p << endl; } void display(const char* p) { cout << p << endl; }

Now is my book considering char* and const char* the same because if it is then the above code shall not work, since arguments will be the same?

(Though i get the output Hello bye on turbo +windows.)

Which one is correct?

String literals are stored in immutable memory, and C++ strongly encourages using only const char * to refer to them. — Kerrek SB
– Kerrek SB, Commented Nov 12, 2011 at 3:44
@KerrekSB What if i dont use const char* ? Does it automatically typecast it or remains as such? — Kraken
– Kraken, Commented Nov 12, 2011 at 3:48
GCC tells you that there's a "deprecated conversion from const char * to char *". No matter what you have, writing to the data is always undefined behaviour. — Kerrek SB
– Kerrek SB, Commented Nov 12, 2011 at 3:53

ruakh · Accepted Answer · 2011-11-12 03:58:49Z

The language specification is not a promise that compilers make to you, but a mutual contract that you and compilers both sign onto. (Metaphorically speaking, of course. Obviously it's not a literally legally binding contract.) If you violate the contract by writing *p='M';, then you've triggered "undefined behavior", and you can't expect any specific behavior from the compiler: maybe it will be strict about it and give you a compile error, maybe it will just go wonky at run-time . . . you didn't hold up your end of the bargain, and it's allowed to do literally whatever it wants now. See also: http://catb.org/jargon/html/N/nasal-demons.html.

halfer · Accepted Answer · 2020-08-16 11:16:24Z

Question:

“Now is my book considering char* and const char* the same because if it is then the above code shall not work, since arguments will be the same?”

Well you’re probably misrepresenting your book.

It probably does not consider those types to be the same, because they are not the same.

Now to your code:

#include<iostream.h>

[iostream.h] is not part of standard C++. C++ was standardized in 1998, and that standardization dispensed with the ARM era's [iostream.h]. So a modern compiler will likely choke on that.

As a workaround, when you get yourself a less antique compiler, you might do …

 #include <iostream> using namespace std;

Next,

void display(char*); void display(const char*);

Declaring functions at the top of the file generally just yields extra work. It often means maintaining two declarations of the function. When you could have dealt with just one declaration.

void main()

In standard C and standard C++ main is not allowed to have any other result type than int.

Visual C++ is one compiler that, as a language extension, permits void.

However, it’s quite silly to use that possibility since it is more to write and just makes the code non-standard and likely to not compile with other compilers.

{ char* ch1="hello";

By C++11 rules the above will not compile. It was deprecated in C++98 and removed in C++11. However, AFAIK current compilers still allow it, but some have a warning that can be turned on.

const char *ch2="bye"; display(ch1); display(ch2); }

The above is OK, although it would not hurt to add an extra const, like

 const char* const ch2="bye";

iammilind · Accepted Answer · 2011-11-12 04:23:51Z

char* ch1="hello";

is deprecated in C++. The correct way is const char*. Because "hello" is stored in a read only region. Also, int main() or int main(int, char**) is the correct way of defining main().

Don't use Turbo C, it's an outdated compiler. Go for gcc or MSVC. Also, don't use books which provides such incorrect information.

Bo Persson · Accepted Answer · 2011-11-12 09:38:29Z

char *p="Hello"; //pointer is variable, string is constant

This code is allowed, because it has been valid since long before the C language got a const keyword.

In order not to break old code, the C and C++ standards just documented that this should work like before.

However, if you write new code you should use the proper form const char*.

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