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[分享]《探索现代化C++》泛读笔记摘要11
2022-9-17 18:47 3373

[分享]《探索现代化C++》泛读笔记摘要11

2022-9-17 18:47
3373

《探索现代化C++》泛读笔记摘要11

Chapter 3 通用编程

Lambda

C++11 的产物

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[] (double x) {return sin(x) + cos(x);}

值捕获

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double phi = 2.5;
auto sin_phi = [phi](double x){ return sin(phi*x); };

引用捕获

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double phi=2.5,xi=0.2;
auto pxr = [&phi,&xi] (double x) {return sin(phi*x) +cos(x)*xi;};

常见捕获方式

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[=] 全部值捕获
[&] 全部引用捕获
[=,&a,&b,&c] a,b,c采用引用捕获,其他值捕获
[&,a,b,c] a,b,c采用值捕获,其他引用捕获

C++14 初始化捕获

 

能为现有变量引入新的名称以及创建新的变量。

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int x = 4;
auto l = [&r=x,i=x+1]() {r+=2; return i+1;};

变量模板

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template<size_t N>
auto derive = [] (auto f,auto x,auto h){
    auto prev = [&](auto x2) {
        return derive<N-1>(f,x2,h);
    };
 
    return (prev(x+h)-prev(x))/h;
};
 
template<>
auto derive<0>= [] (auto f,auto x,auto h){
    return f(x);
};
 
auto f = [](double x) {return 2.0*std::cos(x)+x*x;};
 
cout<<"f''(1)="<< derive<2>(f,1.0,1e-3)<<endl;
cout<<"f(5) (1)="<< derive<5>(f,1.0,1e-3)<<endl;

C++17 编译时if

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template <size_t N>
auto derive = [] (auto f, auto h) {
    if constexpr (N == 0) {
        return [f] (auto x) { return f(x); };
    } else {
        auto prev = derive<N-1>(f, h);
        return [=](auto x){ return (prev(x+h) - prev(x)) / h;};
    }
};
 
auto f = [] (double x) { return 2.0 * std::cos(x) + x*x; };
auto d2f = derive<2>(f, 1e-3);
auto d5f = derive<5>(f, 1e-3);
cout ≪ "f''(1) = " ≪ d2f(1.0) ≪ endl;
cout ≪ "f(5)(1) = " ≪ d5f(1.0) ≪ endl;

C++20 Concepts

Concepts定义一个集合,用于限定模板参数的类型。

定义一个Concepts

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template <typename Iter, typename Value>
Value accumulate(Iter first, Iter last, Value init)
requires Summable<Iter, Value>
{
    for (; first != last; ++first)
        init += *first;
    return init;
}
 
template <typename Iter, typename Value>
requires Summable<Iter, Value>
Value accumulate(Iter first, Iter last, Value init);
 
template <typename Iter, typename Value>
concept Summable =
requires (Iter it, Value init)
{
    requires std::input_iterator<Iter >;
    requires std::is_copy_constructible_v<Value>;
    Value{ init }; // Attempt a copy
    init += *it;
};

不可拷贝构造的类

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struct uncopyable
{
    uncopyable(int v) : v{v} {}
    uncopyable(const uncopyable&) = delete;
    uncopyable(uncopyable&&) = default;
    uncopyable& operator+=(const uncopyable& rhs) &
    {
        v+= rhs.v;
        return *this;
    }
    operator int() const { return v; }
    int v;
};

不可移动构造的类:

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struct unmovable
{
    unmovable(const unmovable&) = default;
    unmovable(unmovable&&) = delete;
    // ... remainder like uncopyable
};

[2023春季班]《安卓高级研修班(网课)》月薪两万班招生中~

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