What is the difference between the following two methods of transmitting parameters?

template & lt; TypeName ... Args & gt; SMTH (Args & amp; & amp; ... args): Base (Args ...) {}

template & lt; TypeName ... Args & gt; SMTH (Args & amp; & amp ... args): base (std :: forward & lt; args & gt; (args) ...) {}

Answer 1, Authority 100%

Suppose that the base constructor accepts a MOVE-only parameter value:

struct base {
 Base (STD :: UNIQUE_PTR & LT; Int & GT; Value);
};

In this case, the first option is not compiled:

template & lt; TypeName ... Args & gt; SMTH (Args & amp; & amp; ... args): Base (Args ...) {}
                 Attempt to make a copy ^^^^

Here you can write STD :: Move and it will work:

template & lt; TypeName ... Args & gt; SMTH (Args & amp; & amp; ... args): Base (std :: Move (Args) ...) {}

However, if Base two constructors, one of which moves the value, and the other takes the link:

struct base {
 Base (STD :: UNIQUE_PTR & LT; int & gt; & amp; & amp; value);
 BASE (STD :: UNIQUE_PTR & LT; Int & gt; & amp; value);
};

And if we want smth to base it is those types of arguments with which it was called, then std :: Move we don’t Suitable – it will always turn the argument in the R-VALUE link.

For this, there is std :: forward & lt; t & gt; , which saves the original type of argument:

struct smth: base {
 TEMPLATE & LT; TypeName ... Args & GT; SMTH (Args & amp; & amp ... args): base (std :: forward & lt; args & gt; (args) ...) {}
};
STD :: UNIQUE_PTR & LT; INT & GT; p;
SMTH S1 (P); // OK, will be called Base (STD :: UNIQUE_PTR & LT; INT & GT; & amp;);
SMTH S2 (ST :: MOVE (P)); // OK, will be called Base (STD :: UNIQUE_PTR & LT; INT & GT; & amp; & amp;);

Note: STD :: FORWARD & LT; T & GT; (T) only works with “universal references”, i.e. where T – this parameter is a function template (designer), and t – the argument of this function with the type T & amp; & amp; (a special exception to the language such type T & amp; & amp; is not an r-value reference, and makes T repeat type that passed into the function)

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