如何在C ++中的函数中将静态数组初始化为特定值? [英] How to initialize a static array to certain value in a function in c++?

问题描述

我正在尝试在函数中初始化静态数组.

I am trying to init a static array in a function.

int query(int x, int y) {
    static int res[100][100]; // need to be initialized to -1
    if (res[x][y] == -1) {
        res[x][y] = time_consuming_work(x, y);
    }
    return res[x][y];
}

我该如何实现?

推荐答案

首先，我强烈建议从C数组移到std::array.如果执行此操作，则可以有一个函数来执行初始化(否则就不能，因为函数不能返回C数组):

First of all, I strongly recommend moving from C arrays to std::array. If you do this you can have a function to perform the initialization (otherwise you can't, as a function cannot return C arrays):

constexpr std::array<std::array<int, 100>, 100> init_query_array()
{
    std::array<std::array<int, 100>, 100> r{};
    for (auto& line : r)
        for (auto& e : line)
            e = -1;
    return r;
}

int query(int x, int y) {
    static std::array<std::array<int, 100>, 100> res = init_query_array();

    if (res[x][y] == -1) {
        res[x][y] = time_consuming_work(x, y);
    }
    return res[x][y];
}

我实际上更喜欢的另一个选项是在lambda中执行初始化:

Another option, that I actually like more is to perform the init in a lambda:

int query(int x, int y) {
    static auto res = [] {
        std::array<std::array<int, 100>, 100> r;
        for (auto& line : r)
            for (auto& e : line)
                e = -1;
        return r;
    }();

    if (res[x][y] == -1) {
        res[x][y] = time_consuming_work(x, y);
    }
    return res[x][y];
}

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