Can Implicit Conversions in C Involve Multiple User-Defined Conversions?

C Implicit Conversions: Clarification

Recently, comments on a previous answer regarding alternative casts in C have prompted questions about the accuracy of implicit conversions. To provide clarification, we will examine the following code segment:

#include <string>

struct A {
    A( const std::string &amp; s ) {}
};

void func( const A &amp; a ) {
}

int main() {
    func( "one" );                  // error
    func( A("two") );           // ok
    func( std::string("three") );   // ok
}

Originally, it was asserted that the first function call results in an error because no direct conversion exists from a const char * to an A. However, there is a conversion from a string to an A. Utilizing this conversion would involve more than one step, which is generally prohibited. This assertion is supported by g 4.4.0 and Comeau compilers.

Upon further investigation, the C Standard (12.3.4) sheds light on this matter, stating that at most one user-defined conversion (constructor or conversion function) can be implicitly applied to a single value. This ruling aligns with the observed behavior and clarifies the limitations of implicit conversions.

In summary, only one implicit user-defined conversion is permitted on a single value. In the provided code, the call func("one") fails because it would require multiple implicit conversions, which is not allowed.

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