Why does explicit constructor of boost::shared_array cause an error?

Question

boost::shared_array<char const *> x(new char const *[n]);

In the line above (n is integer number not greater than 100) I'm creating char const**(const char**) and putting it to smart pointer x for arrays to be deleted when x is deleted. And for me it is clearly how and why this work.

boost::shared_array<char const *> x = new char const *[n];

Now lets take a look to second line. Here in my opinion we do exactly the same as in first case. Yes at first glance we may seem that here we constructing x via NULL(default value of shared_array constructors parameter) then calling operator=, but this is mistake, and as I know in this case instead of operator= will be called constructor with pointer created by new opeartor.

But in spit of this I'm getting error C2440: 'initializing' : cannot convert from 'const char **' to 'boost::shared_array<T>

The only problem I see this is the explicit constructor of boost::shared_array<T>. But I don't know what is the problem? Why does explicit constructor cause this error? And if the problem is not in explicit constructor, then where, why?

Answer 1

Your guess is correct.

What you're trying to do in the second line is implicitly calling the constructor: you want the C++ compiler to realize that there is a constructor available that accepts a T* and use it. However, since the constructor is marked as explicit, it cannot be invoked this way.

See for example the discussion at http://www.go4expert.com/forums/showthread.php?t=20756.