Can Multiple Implicit User-Defined Conversions Be Applied in C ?

C Implicit Conversions: Clarification

In a recent discussion, the validity of implicit conversions in C has been questioned. Specifically, the issue revolves around whether multiple implicit user-defined conversions are allowed. To shed light on this matter, let's examine the following code:

<code class="cpp">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
}</code>

As stated in the original question, the first function call results in an error because there is no conversion from a const char * to an A. While a conversion from a string to an A exists, applying this involves multiple conversions, which is not permitted. This is corroborated by the C Standard:

<code class="cpp">4 At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.</code>

In this case, the presence of two implicit conversions (string to const char and const char to A) violates this rule, leading to a compiler error. Therefore, the assertion that multiple implicit user-defined conversions are prohibited in C is accurate.

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