Several comments on a recent answer of mine, What other useful casts can be used in C++, suggest that my understanding of C++ conversions is faulty. Just to clarify the issue, consider the following code:
#include <string>
struct A {
A( const std::string & s ) {}
};
void func( const A & a ) {
}
int main() {
func( "one" ); // error
func( A("two") ); // ok
func( std::string("three") ); // ok
}
My assertion was that the the first function call is an error, becauuse there is no conversion from a const char * to an A. There is a conversion from a string to an A, but using this would involve more than one conversion. My understanding is that this is not allowed, and this seems to be confirmed by g++ 4.4.0 & Comeau compilers. With Comeau, I get the following error:
"ComeauTest.c", line 11: error: no suitable constructor exists
to convert from "const char [4]" to "A"
func( "one" ); // error
If you can point out, where I am wrong, either here or in the original answer, preferably with reference to the C++ Standard, please do so.
And the answer from the C++ standard seems to be:
At most one user-defined conversion
(constructor or conversion function)
is implicitly applied to a single value.
Thanks to Abhay for providing the quote.
I think the answer from sharptooth is precise. The C++ Standard (SC22-N-4411.pdf) section 12.3.4 titled ‘Conversions’ makes it clear that only one implicit user-defined conversion is allowed.