char pointer to array conversions to char pointer to pointer

Question

I have a function:

void add(char const**);

And I invoke it as follow:

template<typename T>
void anotherAdd(T const& t) {
  add(&t);
}
...
anotherAdd("some string");

As a result I get the error:

 no known conversion for argument 1 from 'const char (*)[10]' to 'const char**'

Why the conversion can't be made?

Because I think the following is true:

"some string" <=> char const* =>
&"some string" <=> char const**

Why have you made a template function that relies on a function that only works on a char const **? I you want this to work, you will have to cast T const& t to a char const **. — antonijn
– antonijn, Commented Mar 18, 2013 at 14:26
You will need to explain what you are actually trying to achieve, since what you are doing in the code doesn't really make sense. — interjay
– interjay, Commented Mar 18, 2013 at 14:31
Are you asking how to get your code to work, or are you asking why that conversion can't be made? — Drew Dormann
– Drew Dormann, Commented Mar 18, 2013 at 14:32
I removed my downvote, since you kind of salvaged your question with the last edit. — antonijn
– antonijn, Commented Mar 18, 2013 at 14:37

Drew Dormann · Accepted Answer · 2013-03-18 17:31:00Z

1

Arrays are not pointers.

This code expects a pointer to a pointer

void add(char const**);

The compiler is telling you that it can't produce a pointer to a pointer because your code has no pointer to point to. You're effectively trying to evaluate &"some string", which has no valid meaning.

This code will work, because it creates the missing char const* that you're trying to take the address of.

template<typename T>
void anotherAdd(T const& t) {
  char const *pt = &t; // Now there's a pointer that you can take the address of.
  add(&pt);
}

edited Mar 18, 2013 at 17:31

answered Mar 18, 2013 at 14:38

Drew Dormann

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3 Comments

user14416 Over a year ago

And what is &"some string"? is not it char const**? Because const char[] is const char *.

Drew Dormann Over a year ago

@user14416 "const char[] is const char *" No, it isn't. Arrays are not pointers. You're getting an error because you're assuming that if you have an array, you have a pointer variable somewhere.

user14416 Over a year ago

Ok, the follwoing answer (cplusplus.com/forum/articles/10) says that it is true.

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