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When I compile the given code it doesn't produce any error or warnings. My question here is shouldn't the compiler produce error when compiling the following line *err = "Error message"; because we are dereferencing a pointer to pointer to constant char and assigning a string to it.

Is it allowable to assign anything inside a pointer anything other than address and exactly what is happening in this given scenario?

#include <stdio.h>

void set_error(const char**);

int main(int argc, const char* argv[])
{
    const char* err;
    set_error(&err);
    printf("%s",err);
    return 0;
}


void set_error(const char** err1) 
{
     *err1 = "Error message";
}
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4
  • It's a const char**, you can modify the individual pointers (strings in the string array, conceptually), just not the characters of individual strings. You are thinking of char * const * (or something). Commented Dec 5, 2013 at 14:34
  • err is a non-const pointer to a non-const pointer to a const char. So yes, you can modify it. Commented Dec 5, 2013 at 14:34
  • Frist question I see with complaining that there is no warning nor errors and everything works fine ;) (+1) Commented Dec 5, 2013 at 14:34
  • C and C++ are different programming languages. Pick one. Don't attempt to program in two different languages at the same time. A C program compiled on a C++ compiler will be crap, and a C++ program that only uses C programming practices will also be crap (from a C++ programmer's point of view). Commented Dec 5, 2013 at 15:02

3 Answers 3

Reset to default
9 
const char** err1

That's a pointer to a non-constant pointer to a constant object. Dereferencing it gives a non-constant pointer (to a constant object), which can be assigned to.

To prevent assigning to the const char*, that would also have to be const:

const char * const * err1
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1 Comment

Wow really subtle! I will always prefer STL containers any day.
4

"Error message" is not a std::string. It's a const char[]. All string literals in C++ are const char[]. In C, they're char[].

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2 Comments

But in C, you should still treat them as if they have const char[] type. Everything else is poor programming practice.
@Lundin I would say that in C you must still treat them as if they have const char[] type, because the implementation is allowed to put string literals into read-only memory.
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Is it allowable to assign anything inside a pointer anything other than address and exactly what is happening in this given scenario?

You can assign pointer to a pointer. You think about pointer as an address, that's fine to understand concept, but do not mix it with data type. Data type is a pointer, not address. For example to assign address in memory to a pointer you need to cast it to a pointer:

char *pointer = reinterpret_cast<char *>( 0xA000000 );

You may ask how this would compile?

int array[10];
int *ptr = array;

That comes from C - array can be implicitly converted to a pointer to the first element. So it is pointer to pointer assignment again. Now about string literal with double quotes. It is an array as well:

const char str[] = "str";
const char str[] = { 's', 't', 'r', '\0' };

These 2 statements are pretty much the same. And as array can be implicitly converted to pointer to the first element it is fine to assign it to const char *

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