Why new visual studio take string literals as pointer to constant?

Question

I have this constructor for my class student.

student::student( char* rollno ,   char* name , date dob) :
    rollno(rollno), name(name),dob(dob)
{
}

When I write student s1( "l1f18bscs0322" , "usama" , { 13,7,1998 }); in main, it accepts it in my university I think because its using Visual studio 2013, but It gives error in home as I am using visual studio 2019. It says no instance of constructor matches the argument list it takes value in double quotes as pointer to constant. What will be the alternate to pass values to this constructor? Because even If I change my constructor protype to constant to pointer so it will give new error that I cannot initialize my non constant members with constant.

Literal string have *always* been constant arrays of characters in C++. That's in the C++ standard itself. Older versions of the MSVC++ compiler might have non-portable extensions to the language that allowed it. — Some programmer dude, Nov 30 '20 at 15:43
You can't convert string literals (like "usama") to `char*`. You need a `const char*`. — François Andrieux, Nov 30 '20 at 15:43
Because it always was one, now MSVC is following the current c++ standard. — πάντα ῥεῖ, Nov 30 '20 at 15:44
Relevant: [What is the type of string literals in C and C++?](https://stackoverflow.com/q/2245664/580083) — Daniel Langr, Nov 30 '20 at 15:44
@Someprogrammerdude Until C++11 the conversion was only deprecated. The standard had an exception allowing this conversion, I guess for compatibility with C. — François Andrieux, Nov 30 '20 at 15:45
[VS 2017 Doesn't Implicitly Convert const char* to char*](https://stackoverflow.com/questions/48554625/vs-2017-doesnt-implicitly-convert-const-char-to-char) — user7860670, Nov 30 '20 at 15:46
Visual Studio 2013 did not fully support the c++11 standard. — drescherjm, Nov 30 '20 at 15:47
Does this answer your question? [What is the type of string literals in C and C++?](https://stackoverflow.com/questions/2245664/what-is-the-type-of-string-literals-in-c-and-c) — rsjaffe, Nov 30 '20 at 16:20

score 7 · Accepted Answer · answered Nov 30 '20 at 15:46

7

Because it is!

The type of a string literal, in C++, is const char[N] (where N is the length of the string including its null terminator). That decays to const char*, not char*.

In earlier versions of C++ (pre-2011), you were permitted to "forget" the const for compatibility with C, though doing so was deprecated (and attempting to "modify" the literal had undefined behaviour). Since then you must write it.

You've upgraded to a version of Visual Studio that enforces this rule, possibly only because its default C++ standard is C++11 or later.

It's a good thing; add the const.

If your course material uses char* here, it is out-of-date, and was always "wrong" in a sense.

even If I change my constructor protype to constant to pointer so it will give new error that I cannot initialize my non constant members with constant

You need to do the same thing to your member variables.

answered Nov 30 '20 at 15:46

Asteroids With Wings

17,071
2
21
35

But what if I don't want my members to be a constant and I want their value can be changed later? – Usama Javed Nov 30 '20 at 15:53
1

@UsamaJaved Then you'll need them to be something other than pointers to string literals. A `std::string` would be ideal, if your course permits it. Failing that, there's old-fashioned `strdup` and `free` (but yuck!). That being said, it seems unlikely that you really want to modify individual characters of these strings, and can probably just assign _different_ string literals to the pointers later? We don't know enough about your goal and requirements to make further concrete suggestions at the moment. – Asteroids With Wings Nov 30 '20 at 15:55
@UsamaJaved, then you make a copy of the input and store the copy. – R Sahu Nov 30 '20 at 15:55
2

not the pointers are constant just the "point-to-data". You are not allowed to change "usama" (it is stored in const storage) – Bernd Nov 30 '20 at 15:55
Got it, Thank you everyone. – Usama Javed Nov 30 '20 at 15:57
@UsamaJaved You bet. Have fun! – Asteroids With Wings Nov 30 '20 at 16:01

Why new visual studio take string literals as pointer to constant?

1 Answers1