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1 change: 1 addition & 0 deletions book/en/src/SUMMARY.md
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# C++14 Core Language Features

- [Generic Lambdas](./cpp14/00-generic-lambdas.md)
- [Variable Templates](./cpp14/02-variable-templates.md)

# Additional Resources

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<div align=right>

🌎 [中文] | [English]
</div>

[中文]: ../../cpp14/02-variable-templates.html
[English]: ./02-variable-templates.html

# Variable Templates

C++14 introduces variable templates — allowing variables themselves to be parameterized by types

| Book | Video | Code | X |
| --- | --- | --- | --- |
| [cppreference-variable_template](https://en.cppreference.com/w/cpp/language/variable_template) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp14/02-variable-templates.md) | [Video Explanation]() | [Exercise Code](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp14/02-variable-templates-0.cpp) | |


**Why introduced?**

- In C++11, templates could only be applied to functions and classes
- Type traits required some_trait<T>::value everywhere
- C++14 variable templates enable pi<double> / is_same_v<T, U> syntax

## I. Basic Usage and Scenarios

### Basic Variable Template

```cpp
template <typename T>
constexpr T pi = T(3.1415926535897932385);

double d = pi<double>; // 3.1415926535897932385
float f = pi<float>; // 3.14159265f
```

### _v Type Traits

```cpp
template <typename T, typename U>
constexpr bool is_same_v = std::is_same<T, U>::value;
```

### Recursive Variable Templates

```cpp
template <int N>
constexpr int factorial_v = N * factorial_v<N - 1>;

template <>
constexpr int factorial_v<0> = 1;
```

## II. Real-World Case — Variable Templates in the STL

> The MSVC STL uses variable templates internally to replace the old `::value` trait pattern. The example below cites the vendored [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) (source: [`msvc-stl/stl/inc/xutility`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/xutility#L1458-L1462))

### _Is_iterator_v — Variable Templates Simplify Type Traits

```cpp
// MSVC STL · msvc-stl/stl/inc/xutility (abridged)
template <class _Ty, class = void>
constexpr bool _Is_iterator_v = false;

template <class _Ty>
constexpr bool _Is_iterator_v<_Ty, void_t<_Iter_cat_t<_Ty>>> = true;

// External code uses _Is_iterator_v<_Ty> directly, no ::value needed
template <class _Ty>
struct _Is_iterator : bool_constant<_Is_iterator_v<_Ty>> {};
```

`_Is_iterator_v` is a variable template — defaulting to false, with a specialization to true when `_Ty` has an `_Iter_cat_t`, replacing the traditional `_Is_iterator<T>::value` pattern. The variable template language mechanism introduced in C++14 enabled the STL to replace `::value` trait classes with `_v`-suffixed variables, paving the way for C++17 standardized `_v` traits like `is_same_v`

## III. Notes

Variable templates support both full and partial specialization (unlike function templates, which only support full specialization). Don't overuse.

## IV. Exercise Code

### Exercise Topics

- 0 - [Basic Variable Templates — pi constant and compile-time config](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp14/02-variable-templates-0.cpp)
- 1 - [_v Type Traits — implementing is_void_v and is_same_v](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp14/02-variable-templates-1.cpp)

### Auto-Checker Command

```
d2x checker variable-templates
```

## V. Other

- [Discussion Forum](https://forum.d2learn.org/category/20)
- [d2mcpp Tutorial Repository](https://github.com/mcpp-community/d2mcpp)
- [Tutorial Video List](https://space.bilibili.com/65858958/lists/5208246)
- [Tutorial Support Tool - xlings](https://github.com/openxlings/xlings)
1 change: 1 addition & 0 deletions book/src/SUMMARY.md
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# C++14核心语言特性

- [泛型 lambda - generic lambdas](./cpp14/00-generic-lambdas.md)
- [变量模板 - variable templates](./cpp14/02-variable-templates.md)

# 其他

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<div align=right>

🌎 [中文] | [English]
</div>

[中文]: ./02-variable-templates.html
[English]: ../en/cpp14/02-variable-templates.html

# 变量模板 - variable templates

变量模板是 C++14 引入的语法扩展, 允许变量本身成为模板, 在特化时根据类型参数产生不同的值或对象

| Book | Video | Code | X |
| --- | --- | --- | --- |
| [cppreference-variable_template](https://en.cppreference.com/w/cpp/language/variable_template) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/02-variable-templates.md) | [视频解读]() | [练习代码](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp14/02-variable-templates-0.cpp) | |


**为什么引入?**

- C++11 的模板只能套在函数和类上, 数值常量只能写成 static constexpr 成员或枚举
- 类型萃取大量使用 some_trait<T>::value, 每次都要写 ::value 后缀
- C++14 变量模板让 pi<double> / is_same_v<T, U> 这种写法成为可能

## 一、基础用法和场景

### 基本变量模板

```cpp
template <typename T>
constexpr T pi = T(3.1415926535897932385);

double d = pi<double>; // 3.1415926535897932385
float f = pi<float>; // 3.14159265f
int i = pi<int>; // 3
```

### 类型萃取 _v 后缀

```cpp
template <typename T, typename U>
constexpr bool is_same_v = std::is_same<T, U>::value;

static_assert(is_same_v<int, int>, "");
static_assert(!is_same_v<int, float>, "");
```

### 编译期配置值

```cpp
template <typename T>
constexpr size_t max_buffer_size = 1024;

template <>
constexpr size_t max_buffer_size<double> = 4096;
```

### 递归变量模板

```cpp
template <int N>
constexpr int factorial_v = N * factorial_v<N - 1>;

template <>
constexpr int factorial_v<0> = 1;

static_assert(factorial_v<5> == 120, "");
```

## 二、真实案例 - STL 中的变量模板

> MSVC STL 内部大量使用变量模板替代旧的 `::value` 萃取类。下面以仓库内置的 [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) 为例 (源码: [`msvc-stl/stl/inc/xutility`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/xutility#L1458-L1462))

### _Is_iterator_v — 变量模板简化类型萃取

```cpp
// MSVC STL · msvc-stl/stl/inc/xutility (有删节)
template <class _Ty, class = void>
constexpr bool _Is_iterator_v = false;

template <class _Ty>
constexpr bool _Is_iterator_v<_Ty, void_t<_Iter_cat_t<_Ty>>> = true;

// 外部通过 _Is_iterator_v<_Ty> 直接使用, 无需 ::value
template <class _Ty>
struct _Is_iterator : bool_constant<_Is_iterator_v<_Ty>> {};
```

`_Is_iterator_v` 是变量模板 — 默认 false, 当 `_Ty` 拥有 `_Iter_cat_t` 时偏特化为 true, 替代了传统的 `_Is_iterator<T>::value` 写法。C++14 引入的变量模板语言机制, 让 STL 可以用 `_v` 后缀变量替代 `::value` 萃取类, 为 C++17 标准化 `is_same_v` 等 `_v` 萃取铺平了道路

## 三、注意事项

变量模板支持全特化和偏特化(这与函数模板不同, 后者只能全特化)。不要滥用 — 有些值用 constexpr 就够了。

## 四、练习代码

### 练习代码主题

- 0 - [变量模板基础 — pi 常量和编译期配置值](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp14/02-variable-templates-0.cpp)
- 1 - [类型萃取 _v 后缀 — 实现 is_void_v 和 is_same_v](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp14/02-variable-templates-1.cpp)

### 练习代码自动检测命令

```
d2x checker variable-templates
```

## 五、其他

- [交流讨论](https://forum.d2learn.org/category/20)
- [d2mcpp教程仓库](https://github.com/mcpp-community/d2mcpp)
- [教程视频列表](https://space.bilibili.com/65858958/lists/5208246)
- [教程支持工具-xlings](https://github.com/openxlings/xlings)
46 changes: 46 additions & 0 deletions dslings/cpp14/02-variable-templates-0.cpp
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// d2mcpp: https://github.com/mcpp-community/d2mcpp
// license: Apache-2.0
// file: dslings/cpp14/02-variable-templates-0.cpp
//
// Exercise/练习: cpp14 | 02 - variable templates | 变量模板基础
//
// Tips/提示:
// - 变量模板用类型参数控制变量的值
// - template<typename T> constexpr T name = ...;
//
// Docs/文档:
// - https://en.cppreference.com/w/cpp/language/variable_template
// - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/02-variable-templates.md
//
// 练习交流讨论: http://forum.d2learn.org/category/20
//
// Auto-Checker/自动检测命令:
//
// d2x checker variable-templates
//

#include <d2x/cpp/common.hpp>

template <typename T>
constexpr D2X_YOUR_ANSWER pi = T(3.1415926535897932385);

template <typename T>
constexpr size_t max_buffer = D2X_YOUR_ANSWER;

template <>
constexpr size_t max_buffer<D2X_YOUR_ANSWER> = 4096;

int main() {

d2x_assert_eq(pi<double>, 3.1415926535897932385);
d2x_assert(pi<float> > 3.14f);
d2x_assert(pi<float> < 3.142f);

d2x_assert_eq(max_buffer<int>, 1024);
d2x_assert_eq(max_buffer<char>, 1024);
d2x_assert_eq(max_buffer<double>, D2X_YOUR_ANSWER);

D2X_WAIT

return 0;
}
56 changes: 56 additions & 0 deletions dslings/cpp14/02-variable-templates-1.cpp
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// d2mcpp: https://github.com/mcpp-community/d2mcpp
// license: Apache-2.0
// file: dslings/cpp14/02-variable-templates-1.cpp
//
// Exercise/练习: cpp14 | 02 - variable templates | _v 类型萃取
//
// Tips/提示:
// - _v 后缀的类型萃取本质就是变量模板, 省去了 ::value
// - template<typename T> constexpr bool xxx_v = xxx<T>::value;
//
// Docs/文档:
// - https://en.cppreference.com/w/cpp/language/variable_template
// - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/02-variable-templates.md
//
// 练习交流讨论: http://forum.d2learn.org/category/20
//
// Auto-Checker/自动检测命令:
//
// d2x checker variable-templates
//

#include <d2x/cpp/common.hpp>
#include <type_traits>

template <typename T>
constexpr bool is_void_v = D2X_YOUR_ANSWER;

template <typename T, typename U>
constexpr D2X_YOUR_ANSWER is_same_v = std::is_same<T, U>::value;

template <int N>
constexpr int factorial_v = D2X_YOUR_ANSWER;

template <>
constexpr int factorial_v<0> = D2X_YOUR_ANSWER;

int main() {

static_assert(is_void_v<void>, "");
d2x_assert(is_void_v<void>);
d2x_assert(!is_void_v<int>);

static_assert(is_same_v<int, int>, "");
d2x_assert((is_same_v<int, int>));
d2x_assert((!is_same_v<int, float>));
d2x_assert((is_same_v<D2X_YOUR_ANSWER, char>));

static_assert(factorial_v<5> == 120, "");
d2x_assert_eq(factorial_v<5>, 120);
d2x_assert_eq(factorial_v<0>, 1);
d2x_assert_eq(factorial_v<3>, 6);

D2X_WAIT

return 0;
}
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Expand Up @@ -9,3 +9,13 @@ target("cpp14-00-generic-lambdas-0")
target("cpp14-00-generic-lambdas-1")
set_kind("binary")
add_files("00-generic-lambdas-1.cpp")

-- target: cpp14-02-variable-templates

target("cpp14-02-variable-templates-0")
set_kind("binary")
add_files("02-variable-templates-0.cpp")

target("cpp14-02-variable-templates-1")
set_kind("binary")
add_files("02-variable-templates-1.cpp")
46 changes: 46 additions & 0 deletions dslings/en/cpp14/02-variable-templates-0.cpp
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// d2mcpp: https://github.com/mcpp-community/d2mcpp
// license: Apache-2.0
// file: dslings/en/cpp14/02-variable-templates-0.cpp
//
// Exercise: cpp14 | 02 - variable templates | basic variable templates
//
// Tips:
// - Variable templates use type parameters to control values
// - template<typename T> constexpr T name = ...;
//
// Docs:
// - https://en.cppreference.com/w/cpp/language/variable_template
// - https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp14/02-variable-templates.md
//
// Discussion Forum: http://forum.d2learn.org/category/20
//
// Auto-Checker:
//
// d2x checker variable-templates
//

#include <d2x/cpp/common.hpp>

template <typename T>
constexpr D2X_YOUR_ANSWER pi = T(3.1415926535897932385);

template <typename T>
constexpr size_t max_buffer = D2X_YOUR_ANSWER;

template <>
constexpr size_t max_buffer<D2X_YOUR_ANSWER> = 4096;

int main() {

d2x_assert_eq(pi<double>, 3.1415926535897932385);
d2x_assert(pi<float> > 3.14f);
d2x_assert(pi<float> < 3.142f);

d2x_assert_eq(max_buffer<int>, 1024);
d2x_assert_eq(max_buffer<char>, 1024);
d2x_assert_eq(max_buffer<double>, D2X_YOUR_ANSWER);

D2X_WAIT

return 0;
}
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