C and C++ : Difference between Casting and Conversion

Question

Is there any difference between line 2 and line 3 in the following code? What does compiler do in each case?

char ch = 'A';     //line 1
int  i = ch;       //line 2
int  j = (int) ch; //iine 3

In general, what is the difference between Casting and Conversion (in C and C++)?

Answer 1

There's no difference in the final effect.

A cast is to use explicit, general, built-in cast notation for conversion.

Although in some cases we say "up-cast" when we mean an implicit conversion from Derived* to Base* (or from Derived& to Base&).

And in some cases one defines new cast notation.

The above definition of the terminology is just an operational definition, that is, it's not a definition where you can reason out that something is a cast. Casts are just those that are defined as casts. :-) For example, bool(x) is a cast, while !!x, which does the same and also is explicit notation, is not a cast.

In C++ you can and preferably should use the named casts static_cast, const_cast, dynamic_cast and reinterpret_cast, with possible exception for explicit casting of arithmetic built-in types. One reason is that a C style cast (Other*)p, or in C++-specific notation OtherPtr( p ), can do different things depending on context, and in particular, when the code is slightly changed the meaning of a C style cast can change. Another reason is that it's difficult to search for C style casts.

That said, the best is to avoid casts to the degree possible.

Cheers & hth.,

Answer 2

Both of them are conversions/casts, in line 2 it's just implicit, while on line 3 it's explicit, there's no functional difference.

Answer 3

End result is the same (that is both your int are valued at 65).

now, line 3 allows the reader (or whomever might have to maintain the code) - to spot the C cast; which is in my humble opinion a plus.

if this code is part of a C++ app, it would be even better to use static_cast for at least 2 reasons:

1. it is much easier to find static_cast in your application that a C style one; in addition to be clearer to understand your intention to someone else reading the code
2. the C++ cast syntax is lengthy, which helps limiting casting some times (casting is still sometimes needed of course :). If you expand from character to things, to do conversion between string and numbers you will have to use something like streams per example anyway

   hope it helps