cannot convert char** to const char**

[Brad Moore]

Hey all,

I'm getting the following compiler error from my code. I was wondering
if anyone could help me understand the concept behind it (I actually did
try and compile this degenerate example).

int foo(const char* argv[]) { return 0; }

int main(int argc, char* argv[])
{
foo(argv);
return 0;
}

Error: cannot convert parameter 1 from char** to const char**


I tried using an explicit type cast, and it worked. However, I'd like
to know the difference between the above code and the following (which
did compile):

int goo(const int x) { return 0; }
int main(int argc, char* argv[])
{
int x;
goo(x);
}

I've been reading that using const in your args list is a useful way of
noting that an argument isn't changed. If foo(const char* argv[]) is
bad form in some way, please let me know.

To reiterate, I did get the above code to work. I was just wondering
what the good programming practice is (and the concept behind it).

Thanks for your expertise and time,
-Brad

 

[Gianni Mariani]

Hey all,

I'm getting the following compiler error from my code. I was wondering
if anyone could help me understand the concept behind it (I actually did
try and compile this degenerate example).

int foo(const char* argv[]) { return 0; }

int main(int argc, char* argv[])
{
foo(argv);
return 0;
}

Error: cannot convert parameter 1 from char** to const char**

You need to consider the meaning of "**" - in this case it means a
pointer to a pointer to a [const] char. This is a double level of
indirection which allows the pointer to be modified in one place and the
object to be modified in another.

If the conversion from char** to const char** was allowed, then you
could write this code.

const char * A[2] = { "A", "B" };
char * B[2];

int main()
{
char ** b = B;
const char ** a = b; // illegal
//const char ** a = ( const char ** ) b; // BAD work-around (1)
//const char * const * a = b; // better option (2)

a[0] = A[0]; // illegal if you use (2) ok if you use (1)

b[0][0] = 'X'; // const violation - big probs if you use (1)
}

This would implicity allow const objects to be accessed in a non-const way.

I tried using an explicit type cast, and it worked. However, I'd like
to know the difference between the above code and the following (which
did compile):

int goo(const int x) { return 0; }
int main(int argc, char* argv[])
{
int x;
goo(x);
}

This is a very different issue, the earlier one had a level of
indirection, this one has none. In this case x is "copied" to a const
value parameter.

I've been reading that using const in your args list is a useful way of
noting that an argument isn't changed. If foo(const char* argv[]) is
bad form in some way, please let me know.

To reiterate, I did get the above code to work. I was just wondering
what the good programming practice is (and the concept behind it).

What you did was probably not what you wanted to do.

 

[John Harrison]

Hey all,

I'm getting the following compiler error from my code. I was wondering if
anyone could help me understand the concept behind it (I actually did try
and compile this degenerate example).

int foo(const char* argv[]) { return 0; }

int main(int argc, char* argv[])
{
foo(argv);
return 0;
}

Error: cannot convert parameter 1 from char** to const char**


I tried using an explicit type cast, and it worked. However, I'd like to
know the difference between the above code and the following (which did
compile):

int goo(const int x) { return 0; }
int main(int argc, char* argv[])
{
int x;
goo(x);
}

I've been reading that using const in your args list is a useful way of
noting that an argument isn't changed. If foo(const char* argv[]) is bad
form in some way, please let me know.

OK, first thing to note is that foo(char* argv[]) is just another way of
writing foo(char** argv). C and C++ allows you to use array notation in
function parameters but its a lie. The compiler always converts to array to
a pointer. So its better to use the pointer notation, its more truthful.

C++ says its ok to convert char* to const char* because this it harmless.
char* is a pointer to char, and const char* is a pointer to constant char
(note its the char that is constant not the pointer). Adding a const can't
do any harm. So sometimes people think that char** should be convertible to
const char** but this conversion is most definitely not harmless. Here's why

const char a = 'a'; // this is a constant it should never change
char* x;
const char** y = &x; // in reality this is illegal
*y = &a; // now x points to a
*x = 'b'; // now we have modified a

By allowing a conversion from char** to const char** we've managed to write
a sequence of statements which have modified a constant. This is obviously a
bad thing

john

 

[Brad Moore]

I want to thank everyone who responded. I understand the error in my
reasoning.

Thanks,
-Brad

 

[Default User]

John Harrison wrote:

OK, first thing to note is that foo(char* argv[]) is just another way
of writing foo(char** argv).

This is true.

C and C++ allows you to use array
notation in function parameters but its a lie. The compiler always
converts to array to a pointer. So its better to use the pointer
notation, its more truthful.

This is your personal opinion, not an objective fact. Many people
prefer the other notation, and their programs are none the less for it.



Brian Rodenborn

 

[John Harrison]

John Harrison wrote:

OK, first thing to note is that foo(char* argv[]) is just another way
of writing foo(char** argv).

This is true.

C and C++ allows you to use array
notation in function parameters but its a lie. The compiler always
converts to array to a pointer. So its better to use the pointer
notation, its more truthful.

This is your personal opinion, not an objective fact. Many people
prefer the other notation, and their programs are none the less for it.

OK, well perhaps we can at least agree that people should be aware of the
real meaning of the array notation in a function parameter. Its my
experience in this group that some newbies are not.

John