Question

I am reviewing for my final, and I cannot figure out why this question is what it is.

Assume the following class declaration:

class Testing {
 public:
       Testing(int n);
       void Show(const Testing& w, int a = 10);
       int value;
 private:
       int DoThis();
 };

Assume the following lines of code are attempting in a main() program, and that x is of type Testing and has been propertly created.

x.Show(18); Legal or Illegal

The answer is legal, I understand that the 2nd parameter is not needed because of the = 10,but since 18 is not of type Testing isn’t that an invalid parameter?

Answer 1

Testing has a non-explicit constructor that takes an int. Therefore, an int can be implicitely converted to a Testing by constructing a temporary object.

Since Show takes a const Testing & (and not just a Testing &), you can pass a temporary to it. Finally, the second parameter is optional, so you don’t have to specify a value for that.

This whole mechanism is what allows you do this, by the way:

void f(const std::string &str);
// ...
f("Hello");

Here, "Hello" is of type const char (&)[6], which decays to a const char *, but you can construct a std::string from a const char *, thus permitting the use of a const char * where a std::string parameter is needed.

Keep in mind that this constructs a temporary, and therefore is only valid for parameters that are passed by value or by const reference (it will fail for references). Also, the constructor must not be marked as explicit.