Any possible issues if const char * is converted to char * in a function call

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In fun1() I am getting a string and I need to pass this string data to a function fun2() which takes argument char *

function2 prototype is as below

void fun2(char *p);
I am calling fun2 from fun1 as below

void fun1()
{
fun2((char *)str.c_str());
}
Is there any chance from the conversion from const char * to char * does make any potential issues

Posted 7-Mar-15 4:09am

krish0969

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nv3 7-Mar-15 12:15pm

Well, what did you think the "const" attribute is good for? The const says: "Don't touch this!" and the compiler gives you a warning if you try nonetheless.

If the author of fun2 just forgot to specify the argument as "const", your attempt might go through without consequences. But if fun2 really needs a (modifiable) "char*", you should take utmost care. Solution1 tells you what to do in this case.

CPallini 7-Mar-15 15:11pm

(virtual) 5.

nv3 8-Mar-15 4:59am

Than you, Carlo.

2 solutions

Solution 2

You can get a modifiable C string from a C++ string by simply taking the address of the first character. Use

C#

void fun1()
{
    fun2(&str[0]);
}

(the only catch is if fun2 happens to modify the terminating null character of its C string argument, the null appended by the C++ string in this situation is not modifiable)

Posted 7-Mar-15 8:45am

CubbiMew

Updated 7-Mar-15 8:49am

v3

Comments

Andreas Gieriet 7-Mar-15 14:57pm

This does not answer the question.
1) it is about the consequences of casting away constness
2) if str is const, each element of the str is const again.
Regards
Andi

CubbiMew 7-Mar-15 14:59pm

I am not answering the question title, I am answering the question content.

Andreas Gieriet 7-Mar-15 15:06pm

The question is: "[...] Is there any chance from the conversion from const char * to char * does make any potential issues [...]".
Cheers
Andi

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OriginalGriff · Accepted Answer · 2015-03-07T04:30:00

Solution 1

Yes.
A const char* can't be converted to a char* because it would open up the possiblity of changing the value of constants: in this case potentially changing (say) a user prompt from "hello user!" to "HeLlO UsEr!" and making the fixed prompt change half way through the program.
If you need to pass a const char* to a function taking a char*, you need to allocate some char memory, and copy the constant string into it - then you can pass the non-const copy to your function.

Posted 7-Mar-15 4:30am

OriginalGriff

Comments

Andreas Gieriet 7-Mar-15 14:58pm

My 5!
Cheers
Andi

CPallini 7-Mar-15 15:11pm

5.

krish0969 8-Mar-15 7:46am

Thanks for the response.For me if the value is changed in the fun2 its not really impact my functionality.I just want to know is there any impact like memory corruption

OriginalGriff 8-Mar-15 8:03am

No, I think you misunderstand.
When I said "can't be converted" it's because the system won't let you - as soon as you try to change anything your application will crash because you are trying to write to a read only memory segment!
Think of it like this:
If you have a function:

void myFunc(char * str)
{
*str = '\0';
}

what happens when you call it:

myFunc("hello");

It's like trying to do this:

void Add(int* pi)
{
*pi +=10;
}

And calling it like this:

Add(&42);

Constants are called that for a reason! :laugh:
So the system is set up to not allow you to remove the "const" attribute.