使用外部函数获取和设置嵌套结构的值

Question

我面临以下问题.我有一个受保护的数据结构a map中<ID , Object>.只有一个地图数据结构和多个线程可能想要使用的地图set和get值ID.在Object如图所示的以下示例代码可被嵌套.

#include <iostream>
#include <map>
#include <algorithm>
#include <mutex>
using namespace std;
struct C {
    int r;
    char s;
};
struct B {
    int p;
    char q;
    C c;
};
struct A {
    int x;
    int y;
    B b;
    char z;
};
/*
    A
    L__ x
    |
    L__ y
    |
    L__ B
        L__p
        |
        L__q
        |
        L__C
            L__r
            |
            L__s

*/
/*  the follwing is the
    data structure to store objects of of type A 
    corresponding to an ID (int)
*/
map<int,A> Map;
mutex m;

/*  Follwing are the desired 
    api's to set and get values from the Map 
    Locks are also required to handle concurrency
*/
void getValueFromMap(int id,/* what to pass ?? */) {
    m.lock();

    /* code to get the value of the specified varible */

    m.unlock();
}
void setValueInMap(int id,/* what to pass ?? */) {
    m.lock();

    /* code to set the new value to the specified variable */

    m.unlock(); 
}
int main() {
    /* lets suppose Map has some A type objects already */
    int id = 4;
    /* assume I want to change the information (value of p) of  id = 4*/
    /* instead of doing the following */
    m.lock();
    Map[id].b.p = 12; /*update to some value */
    m.unlock();
    /* what I need the follwing */
    setValueInMap(4,/* what_to_change , what_is_the_new_value etc .. */);

    /*similarly */

    /*if I want get the value of s */
    int s;
    m.lock();
    getValueFromMap(id,/* what_to_get,placeholder (for example &s) */);
    m.unlock();
}

我希望有一些api调用(例如setValueInMap具有固定数量的参数)set和使用函数调用的get值,map并且所有互斥体工作将api仅在这些调用中发生.虽然我只显示了一个通用的set函数来设置所有类型的成员变量struct A,但它不一定是(更多api函数调用没有问题).

如何实施？

谢谢!

Answer 1

如何使通用函数在地图上起作用:

std::map<int, A> Map;
std::mutex m;

template <typename F>
auto ActOnMap(F&& f) -> decltype(f(Map)) {
    std::lock_guard<std::mutex> locker(m);
    return f(Map);
}

(注意:你应该创建一个类来构造它并隐藏对mapand的直接访问mutex.)

然后:

/* lets suppose Map has some A type objects already */
int id = 4;
/* assume I want to change the information (value of p) of  id = 4*/
ActOnMap([id](std::map<int, A>&m){ m[id].b.p = 12;});

/*similarly */

/*if I want get the value of s */
int s = ActOnMap([id](std::map<int, A>&m){ return m[id].b.c.s;});