Why does `func(\"one\")` cause an error in C implicit conversions?

C Implicit Conversions

In the context of C , the concept of implicit conversions has been a subject of discussion. A recent answer on "What other useful casts can be used in C ?" raised questions about the correct understanding of conversions in C .

Consider the following code snippet:

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

In this snippet, the first function call, func("one"), results in an error. This is because there is no direct conversion from a const char * to an A. While there is a conversion from a string to an A, using it would involve multiple implicit conversions, which is not permitted according to C standards.

The C Standard (SC22-N-4411.pdf) in section 12.3.4 "Conversions" states:

4 At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.

This means that only one implicit user-defined conversion can be applied when performing a conversion. In the first function call, both the conversion from const char * to string and the conversion from string to A are user-defined conversions. Since more than one conversion is required, the compiler raises an error.

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