C++ C++；11变量包前的默认模板参数

为什么模板类参数后面的参数有默认值，前面的变量也必须有默认值？

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这与可变模板无关：

template<class First, class Second> struct X {};

X<int, double> // instantiates X with First == int, Second == double

如果现在第三个参数没有默认参数，则无法使用默认参数：

template<class First, class Second = double> struct X {};

X<int> // instantiates X with First == int, Second == double

template<class First, class Second = double, class Third> struct X {};

X<int, char> // tries to instantiate X with First == int, Second == char,
             // Third == ??

template<class First, class Second = double, class... Pack> struct X {};

X<int> // instantiates X with First == int, Second == double, Pack == (empty)
X<int, char> // First == int, Second == char, Pack == (empty)
X<int, char, short, bool> // First == int, Second == char, Pack == {short, bool}

模板结构X{}；
X//尝试用First==int，Second==char实例化X，
//第三==？？

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模板参数包可以为空，因此它可以跟随带有默认参数的参数：

template<class First, class Second = double> struct X {};

X<int> // instantiates X with First == int, Second == double

template<class First, class Second = double, class Third> struct X {};

X<int, char> // tries to instantiate X with First == int, Second == char,
             // Third == ??

template<class First, class Second = double, class... Pack> struct X {};

X<int> // instantiates X with First == int, Second == double, Pack == (empty)
X<int, char> // First == int, Second == char, Pack == (empty)
X<int, char, short, bool> // First == int, Second == char, Pack == {short, bool}

模板结构X{}；
X//实例化X，第一个==int，第二个==double，Pack==（空）
X//First==int，Second==char，Pack==（空）
X//First==int，Second==char，Pack=={short，bool}

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在OP的示例中：

template<class A = int, class B, class ...Args> struct Dat {};
         ^~~~~~~        ^~~~~~~  ^~~~~~~~~~~~~
         |              |        |no argument required
         |              |an argument IS required
         |no argument required

template struct Dat{}；
^~~~~~~        ^~~~~~~  ^~~~~~~~~~~~~
|| |无需任何参数
||需要一个参数
|不需要任何参数

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因此，您必须始终为第二个参数提供参数，因此，您也需要为第一个参数提供参数。无法使用第一个参数的默认参数。

这与可变模板无关：

template<class First, class Second> struct X {};

X<int, double> // instantiates X with First == int, Second == double

如果现在第三个参数没有默认参数，则无法使用默认参数：

template<class First, class Second = double> struct X {};

X<int> // instantiates X with First == int, Second == double

template<class First, class Second = double, class Third> struct X {};

X<int, char> // tries to instantiate X with First == int, Second == char,
             // Third == ??

template<class First, class Second = double, class... Pack> struct X {};

X<int> // instantiates X with First == int, Second == double, Pack == (empty)
X<int, char> // First == int, Second == char, Pack == (empty)
X<int, char, short, bool> // First == int, Second == char, Pack == {short, bool}

模板结构X{}；
X//尝试用First==int，Second==char实例化X，
//第三==？？

---

模板参数包可以为空，因此它可以跟随带有默认参数的参数：

template<class First, class Second = double> struct X {};

X<int> // instantiates X with First == int, Second == double

template<class First, class Second = double, class Third> struct X {};

X<int, char> // tries to instantiate X with First == int, Second == char,
             // Third == ??

template<class First, class Second = double, class... Pack> struct X {};

X<int> // instantiates X with First == int, Second == double, Pack == (empty)
X<int, char> // First == int, Second == char, Pack == (empty)
X<int, char, short, bool> // First == int, Second == char, Pack == {short, bool}

模板结构X{}；
X//实例化X，第一个==int，第二个==double，Pack==（空）
X//First==int，Second==char，Pack==（空）
X//First==int，Second==char，Pack=={short，bool}

---

在OP的示例中：

template<class A = int, class B, class ...Args> struct Dat {};
         ^~~~~~~        ^~~~~~~  ^~~~~~~~~~~~~
         |              |        |no argument required
         |              |an argument IS required
         |no argument required

template struct Dat{}；
^~~~~~~        ^~~~~~~  ^~~~~~~~~~~~~
|| |无需任何参数
||需要一个参数
|不需要任何参数

---

因此，您必须始终为第二个参数提供参数，因此，您也需要为第一个参数提供参数。无法使用第一个参数的默认参数。

在默认参数之后不能有非默认参数。这是一种语言，默认参数对于不显式传递一个参数非常有用。例如

模板结构X{}；X/*==X*/

。参数用于始终以相同的顺序填充参数，因此，如果需要为下一个参数传递参数，则无法使用默认参数——您需要为两个参数传递参数。可变模板参数与此无关（）。在默认参数之后不能有非默认参数。这是一种语言，默认参数对于不显式传递一个参数非常有用。例如

模板结构X{}；X/*==X*/

。参数用于始终以相同的顺序填充参数，因此如果需要为下一个参数传递参数，则无法使用默认参数——您需要为两个参数传递参数。可变模板参数与此无关（）。