Getting expression must have class type error as well as invalid types error. (Multithreading)

[YViera]

Hello, I'm getting an error with my program when using maps with pointers. For context, I'm trying to calculate the frequency of each character in a string that I got from the input file. Note that I am trying to implement multithreading as well. Any suggestions on fixing this error?

struct variables{

    char symbol;
    int freq; 
    string code;
    string* pointerContent;

};

void* frequency(void* inputContents){

    struct variables* contents = (struct variables*) inputContents;

    /*Step (2)*/
    std::map<char, int> priorityMap; 
    std::map<char, int>::iterator priorityItr;

    for(long j = 0; j < (contents->pointerContent->length()); j++){
        priorityMap[(contents->pointerContent).at(j)++]; //expression must have class type & invalid types ‘char[long int]’ for array subscript
    }

[dutch]

#include <iostream>
#include <string>
#include <map>
using namespace std;

struct variables
{
    char symbol;
    int freq; 
    string code;
    string* pointerContent;
};

void* frequency(void* inputContents)
{
    variables* contents = static_cast<variables*>(inputContents);
    std::map<char, int> priorityMap; 

    for (size_t j = 0; j < contents->pointerContent->size(); ++j)
        ++priorityMap[(*contents->pointerContent)[j]];
        // OR ++priorityMap[contents->pointerContent->at(j)];

    for (const auto& e: priorityMap)
        cout << e.first << ' ' << e.second << '\n';

    return nullptr;
}

int main()
{
    variables v
    {
        'a',
        42,
        "abcde",
        new string("hello")
    };
    
    frequency(static_cast<void*>(&v));

    delete v.pointerContent;
}