Why does `Output::Print(\"Hello World\")` print \"True\" instead of \"Hello World\"?

String Literal Overload Conflict: Bool vs. std::string

In C , class methods can be overloaded to accept different parameter types. However, sometimes a string literal may unexpectedly matched the boolean type overload instead of the expected std::string overload.

Problem description:

Suppose we have an Output class defined as follows:

<code class="cpp">class Output
{
public:
    static void Print(bool value)
    {
        std::cout << (value ? "True" : "False");
    }

    static void Print(std::string value)
    {
        std::cout << value;
    }
};</code>

When calling Output::Print("Hello World "), the output result is "True" instead of the expected "Hello World".

Problem analysis:

Although we defined the std::string overload, "Hello World" is actually a character array constant that can be implicitly converted to bool. The compiler prefers this standard conversion over the user-defined std::string conversion constructor.

According to the C standard (§13.3.3.2/2), standard conversion orders prevail over user-defined conversion orders. Therefore, the compiler chooses the bool overload because it has better conversion order.

Workaround:

In order to explicitly call the std::string overload, we need to explicitly pass "Hello World" as std::string:

<code class="cpp">Output::Print(std::string("Hello World"));</code>

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