Question

I have a template class called Variable, with a specialized constructor for char*, which is defined as follows:

template<>
Variable<char*>::Variable(char * const &arg_value)
{
    value = new char[strlen(arg_value) + 1];
    strncpy(value, arg_value, strlen(arg_value));
    value[strlen(arg_value)] = '\0';
}

Now, I have this statement, that declares a Variable<char*>:

Variable<char*> stringVar = const_cast<char*>("Hi");

In my Variable definition, I never declared or defined a copy constructor to a const char*. However, the statement works perfectly fine. Why is this? I am quite positive that stringVar has a data-type of Variable<char*>, but this still works. Where is this assignment coming from?

Answer 1

A constructor that takes one argument allows for implicit conversions. Here’s a simpler example of your situation:

struct Foo { Foo(int, double = 0.5, char = 'a') { } };

void bar(Foo);

Foo x = 1; // OK!
bar(2);    // also OK

To inhibit this implicit conversion, say explicit:

struct Eew { explicit Eew(int) { } };

// Eew e = 3;    // error
Eew e2 = Eew(3); // OK but weird
Eew e3(3);       // correct