<div dir="ltr"><p style="margin:0px 0px 1em;padding:0px;border:0px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-weight:400;font-stretch:inherit;line-height:inherit;font-family:Arial,"Helvetica Neue",Helvetica,sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(36,39,41);letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">I was looking through clang's c++ standard library, and found this class in the shared_ptr class.</p><pre style="margin:0px 0px 1em;padding:5px;border:0px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-weight:400;font-stretch:inherit;line-height:inherit;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;font-size:13px;vertical-align:baseline;box-sizing:inherit;width:auto;max-height:600px;overflow:auto;background-color:rgb(239,240,241);word-wrap:normal;color:rgb(36,39,41);letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><code style="margin:0px;padding:0px;border:0px;font-style:inherit;font-variant:inherit;font-weight:inherit;font-stretch:inherit;line-height:inherit;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;font-size:13px;vertical-align:baseline;box-sizing:inherit;background-color:rgb(239,240,241);white-space:inherit">class shared_ptr
...
private:
        struct __nat {int __for_bool_;};
...
};
</code></pre><p style="margin:0px 0px 1em;padding:0px;border:0px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-weight:400;font-stretch:inherit;line-height:inherit;font-family:Arial,"Helvetica Neue",Helvetica,sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(36,39,41);letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">and I understand that this class is used to detect whether type conversion is possible at compile time, but its member __for_bool_ is never used anywhere in the class or the weak_ptr counterpart. So my question is, what is the point of __for_bool_, why not simply use an empty class for the same purpose?</p><p style="margin:0px 0px 1em;padding:0px;border:0px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-weight:400;font-stretch:inherit;line-height:inherit;font-family:Arial,"Helvetica Neue",Helvetica,sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(36,39,41);letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">I'm sure the standard library authors definitely knows better than me. Please help.</p><p style="margin:0px 0px 1em;padding:0px;border:0px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-weight:400;font-stretch:inherit;line-height:inherit;font-family:Arial,"Helvetica Neue",Helvetica,sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(36,39,41);letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">Thanks,<br>Zeyang</p></div>