[feat] hw03 done #5
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archibate
approved these changes
Jan 3, 2022
| // 请实现自动匹配容器中具体类型的加法！10 分 | ||
| return std::visit([&](auto const&x){ | ||
| return x + b; | ||
| }, a); | ||
| } | ||
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| template <class T1, class T2> | ||
| std::ostream &operator<<(std::ostream &os, std::variant<T1, T2> const &a) { | ||
| // 请实现自动匹配容器中具体类型的打印！10 分 |
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| template <class T1, class T2, class T3> | ||
| decltype(auto) operator+(std::variant<T1, T2> const &a, std::vector<T3> const &b) { | ||
| // 请实现自动匹配容器中具体类型的加法！10 分 |
main.cpp
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| } | ||
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| template <class T1, class T2, class T3> | ||
| decltype(auto) operator+(std::variant<T1, T2> const &a, std::vector<T3> const &b) { |
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如果 variant 里的 T1, T2 不是 vector 呢？
main.cpp
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| } | ||
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| template <class T1, class T2> | ||
| std::variant<T1, T2> operator+(std::variant<T1, T2> const &a, std::variant<T1, T2> const &b) { | ||
| decltype(auto) operator+(std::variant<T1, T2> const &a, std::variant<T1, T2> const &b) { |
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Suggested change
| decltype(auto) operator+(std::variant<T1, T2> const &a, std::variant<T1, T2> const &b) { | |
| auto operator+(std::variant<T1, T2> const &a, std::variant<T1, T2> const &b) { |
decltype(auto) 是用来自动匹配引用类型的，建议用 auto，因为这里只需要匹配值类型。
main.cpp
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| // 请实现列表的逐元素加法！10 分 | ||
| // 例如 {1, 2} + {3, 4} = {4, 6} | ||
| using T0 = decltype(T1{} + T2{}); | ||
| std::vector<T0> res; | ||
| for(size_t i=0; i<a.size()&&i<b.size(); i++){ |
There was a problem hiding this comment.
Suggested change
| for(size_t i=0; i<a.size()&&i<b.size(); i++){ | |
| for(size_t i=0; i<std::min(a.size(),b.size()); i++){ |
方便编译器识别出不变量从而优化。
| @@ -16,19 +20,48 @@ std::ostream &operator<<(std::ostream &os, std::vector<T> const &a) { | |||
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| // 请修复这个函数的定义：10 分 | |||
| template <class T1, class T2> | |||
| std::vector<T0> operator+(std::vector<T1> const &a, std::vector<T2> const &b) { | |||
| decltype(auto) operator+(std::vector<T1> const &a, std::vector<T2> const &b) { | |||
| // 请实现列表的逐元素加法！10 分 | |||
| #include <iostream> | ||
| #include <vector> | ||
| #include <variant> | ||
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| #include "cpp_type_name.h" | ||
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| // 请修复这个函数的定义：10 分 |
| @@ -16,19 +20,48 @@ std::ostream &operator<<(std::ostream &os, std::vector<T> const &a) { | |||
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| // 请修复这个函数的定义：10 分 | |||
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d = d + c + e时，decltype(auto) operator+(std::variant<T1, T2> const &a, std::variant<T1, T2> const &b)没办法对std::vector<double>+std::variant<int, double>进行处理，因为前者已经被特化为两个variant相加。所以对std::vector<double>和std::variant<int, double>顺序进行排列组合，增加了两个重载函数。decltype(auto)，但是这里还是有疑问，为了防止滥用自动推导，应该什么时候用decltype(auto)，什么时候不用呢？std::visit配合lambda非常好用！