Question

I have an API taking some options:

void init_api(const char* options[][2]);

I am allowed to pass a NULL pointer for no options, alternatively, an options array such as this can be passed:

const char* some_options[][2] = { {"opt1", "val1"}, 
                                  {"opt2", "val2"}, 
                                  {0,0} 
                                };

This works without problems:

...
init_api(some_options);
... or ...
init_api(NULL);
...

However, this fails to compile:

const char* my_options[][2] = NULL; // error C2440: 'initializing' : cannot convert from 'int' to 'const char *[][2]'
if(...) {
  my_options = some_options; // error C2440: '=' : cannot convert from 'const char *[4][2]' to 'const char *[][2]'
}
init_api(my_options); // no error here

What is going on here? Can someone explain this?

Answer 1

To declare an empty array of array of pointers to const char, you should use:

const char* my_options[][2] = {};

You need to declare a pointer to an array of pointers to const char instead. I recommend using a typedef to simplify the syntax.

typedef const char* array_of_two_cstring[2];
array_of_two_cstring* my_options = NULL;
if (...) {
    my_options = some_options;
}
init_api(my_options);

In C++ (it is herited from C), array can be implicitly converted to pointer (only once though, that is char[] is compatible with char* but char[][] is compatible with char*[] but not `char**). However, the variable cannot be reassigned. So here you need to use a pointer instead of an array.

The init_api option accepts NULL as a parameter because for the compiler, its prototype is void init_api(char const* (*)[2]) (the first array degenerated into a pointer), and NULL is a valid pointer.