C Programming – char* vs const char* as Function Parameters

The latter prevents you from modifying the_string inside print_string. It would actually be appropriate here, but perhaps the verbosity put off the developer.

char* the_string : I can change which char the_string points to, and I can modify the char to which it points.

const char* the_string : I can change which char the_string points to, but I cannot modify the char to which it points.

char* const the_string : I cannot change which char the_string points to, but I can modify the char to which it points.

const char* const the_string : I cannot change which char the_string points to, nor can I modify the char to which it points.

The email you linked to about "String" is wrong (and confusing).

Basically:

char* is a pointer to an unbounded array of characters. Traditionally we consider such an array to be a C-string if it contains a set of valid characters followed by a \0. There's no limit to the size of the array.

const char* is a pointer to an unbounded array of immutable characters.

string* is a pointer to a std::string object, and is entirely different. This is a smart object that encapsulates a string. Using std::string instead of C-strings can make your life loads easier, even though they've got some rough edges and a lot of nasty gotchas; they're well worth looking into, but they're not relevant to the question.

"String" is a special expression that returns a const char* pointing at the specific C-string (note: this is not actually true, but it's a simplification that lets me answer the question concisely).

A char* can be automatically cast to a const char*, but not vice versa. Note that old C++ compilers had a special exception to the type rules to let you do this:

char* s = "String";

...without producing a type error; this was for C compatibility. Modern C++ compilers won't let you do it (such as recent gccs). They require this:

const char* s = "String";

So. The problem here is that you've got:

throw "All parameters must be greater than 0.";

...but then you're trying to catch it with:

catch(char* e)

This doesn't work, because the throw is throwing a const char*, which can't be cast to the type specified in the catch, so it isn't getting caught.

This is why changing the catch to:

catch (const char* e)

...makes it work.