Why Does Overload Resolution Fail When Streaming an Object via Implicit Conversion to String?

Overload Resolution Failure When Streaming Object via Implicit Conversion to String

The code snippet provided defines a NameType struct with an operator std::string() overload and a Person struct containing a NameType instance named name. When trying to stream the Person object using the std::cout << p.name statement, the compiler fails to resolve the overload due to an error in finding a matching operator<< for the Person::name expression.

The issue arises because of a C 98 rule that prohibits implicit conversions when one of the function arguments contains template parameters involved in template argument deduction. In this case, the desired overload is a template instantiation with template parameters charT, traits, and Allocator. Since all arguments contain template parameters involved in argument deduction, implicit conversion is not permitted, resulting in the overload resolution failure.

To resolve this issue, an explicit overload declaration for operator<< that handles the Person struct should be provided. This can be achieved by defining a custom overload function with the following signature:

<code class="cpp">std::ostream& operator<<(std::ostream& os, const Person& p);</code>

By providing an explicit overload, the compiler can resolve the overload and successfully stream the Person object.

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