get and set values of a nested structure using external functions

I am facing the following problem. I have a protected data structure a map of <ID , Object>. There is only one map data structure and multiple threads might want to set and get values from the map using an ID. The Object could be nested as shown in the following sample code.

#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();
}

I want to have some api calls ( for example setValueInMap with fixed number of arguments ) to set and get values of the map using function calls and all the mutex work will happen inside those api calls only. Although I have shown only one generic set function to set all types of member variables of struct A, it need not be (more api function calls is of no problem).

How it can be implemented?

thanks!