Re: Does C++ allow you to define explicit casts?

Ben Wing <ben(a)666.com&gt; writes: Yes you can.. C++ has an "explicit" keyword for this purpose. The below example does it by defining an explicit copy constructor and letting the compiler generate the conversion operators from that. [Note that if you allow the uncommented conversions to int below, you not strictly need the explicit copy constructors as] struct charcount; struct bytecount; struct charcount { int count; charcount() { } charcount(int i) : count(i) { } // Allow type conversions explicit charcount(bytecount bc); int getcount() { return count;} // operator int() { return getcount(); } }; struct bytecount { int count; bytecount() { } bytecount(int i) : count(i) { } // Allow explicit type conversions through copy explicit bytecount(charcount cc) : count(cc.getcount()) { } int getcount() { return count ;} // operator int() { return getcount(); } }; inline charcount::charcount(bytecount bc) : count(bc.getcount()) { } void foo(bytecount b); int main() { bytecount b1 = 50; bytecount b2; charcount c1 = 100; // Explit conversions (C style) c1 = (charcount)b1; b2 = (bytecount)c1; foo((bytecount)c1); // Explict conversion (C++ style) c1 = charcount(b1); b2 = bytecount(c1); // Conversion to int works if the convertor operators are defined above // int i1 = c1; // int i2 = b1; // These fail to compile because // 1. We made the copy contructor explicit // 2. Convertsion via int requires two conversions, if defined c1 = b1; b2 = c1; foo(c1); } cd /tmp/ g++-3.0 bytecount.cpp -o bytecount bytecount.cpp: In function `int main()': bytecount.cpp:49: no match for `charcount& = bytecount&' operator bytecount.cpp:5: candidates are: charcount& charcount::operator=(const charcount&) bytecount.cpp:50: no match for `bytecount& = charcount&' operator bytecount.cpp:16: candidates are: bytecount& bytecount::operator=(const bytecount&) bytecount.cpp:51: conversion from `charcount' to non-scalar type `bytecount' requested Compilation exited abnormally with code 1 at Sun Apr 14 16:28:06