容器和算法的改进 --- C++20

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2024-01-24 00:01:32

容器和算法的改进 — C++20

C++ 20对容器和算法有很多的改进

std::vector 和std::string支持constexpr+ 所有容器支持consistent container erasure , contains+ 新的算法移动元素 std::shift_left+ 可以检查 std::string 的前缀和后缀

支持 `constexpr` 的容器和算法

C++ 20的std::vector和std::string支持constexpr+ 超过100多种algorithm支持constexpr

这样, 你就可以实现在编译期查找字符串子串, 在编译期对数组进行排序

#include <ranges>
#include <vector>
#include <iostream>
#include <span>
#include <format>
#include <array>
#include <algorithm>

consteval int maxEle()
{
    std::vector v{2, 4, 1, 6, 3, 8};
    std::sort(v.begin(), v.end());
    return v.back();
}

consteval bool findSub(std::string s, std::string sub)
{
    return s.find(sub, 0) != std::string::npos;
}


int main()
{
    

    constexpr int maxVal = maxEle();
    std::cout << maxVal << std::endl;

    std::cout << findSub("ianaworld", "world") << std::endl;
}

之所以能容器能在编译期能实现这些操作, 是因为使用了瞬态分配 Transient Allocation

Transient Allocation 瞬态分配

Transient Allocation: 编译期申请的内存必须在编译期就释放, 不能将编译期申请的内存在运行时使用

#include <memory>

constexpr auto correctRelease()
{
    auto* p = new int[2020];
    delete [] p;
    return 2020;
}

constexpr auto forgottenRelease()
{
    auto* p = new int[2020];
    return 2020;
}

constexpr auto falseRelease()
{
    auto* p = new int[2020];
    delete p;
    return 2020;
}

int main()
{
    constexpr int res1 = correctRelease();
    constexpr int res2 = forgottenRelease();
    constexpr int res3 = falseRelease();
}

从容其中移除元素

在C++20 前是有一些复杂的

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::cout << '\n';

    std::vector myVec{-2, 3, -5, 10, 3, 0, -5};

    for (auto ele : myVec) std::cout << ele << " ";
    std::cout << "\n\n";

    std::remove_if(myVec.begin(), myVec.end(), [](int ele) { return ele < 0; });
    for (auto ele : myVec) std::cout << ele << " ";

    std::cout << "\n\n";
}

因为没有应用新的end , 所以需要像下面这样做

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::cout << '\n';

    std::vector myVec{-2, 3, -5, 10, 3, 0, -5};

    for (auto ele : myVec) std::cout << ele << " ";
    std::cout << "\n\n";

    auto newEnd = std::remove_if(myVec.begin(), myVec.end(),
                                 [](int ele) { return ele < 0; });
    myVec.erase(newEnd, myVec.end());
    // myVec.erase(std::remove_if(myVec.begin(), myVec.end(),
    // [](int ele){ return ele < 0; }), myVec.end());
    for (auto ele : myVec) std::cout << ele << " ";

    std::cout << "\n\n";
}

C++ 20 之后

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::cout << '\n';

    std::vector myVec{-2, 3, -5, 10, 3, 0, -5};

    std::erase_if(myVec, [](int ele) { return ele >= 3; });

    for (int my_vec : myVec)
    {
        std::cout << my_vec<<"  ";
    }
}

其他容器也类似

`contains`函数

可以方便的判断容器是否包含一个元素

C++ 20 之前

#include <set>
#include <iostream>

int main()
{
    std::cout << '\n';

    std::set mySet{3, 2, 1};
    if (mySet.find(2) != mySet.end())
    {
        std::cout << "2 inside" << '\n';
    }

    std::multiset myMultiSet{3, 2, 1, 2};
    if (myMultiSet.count(2))
    {
        std::cout << "2 inside" << '\n';
    }

    std::cout << '\n';
}

比较长,并且对初学者不友好

C++ 20

#include <set>
#include <iostream>
#include <vector>

int main()
{
    std::cout << std::boolalpha;

    std::cout << '\n';

    std::set mySet{3, 2, 1};
    std::cout << mySet.contains(2) << " ";
    
}