access map elemet via pointer in c-CodePudding

I switched from c to c recently and just can't figure out what I'm doing wrong here. I would like to access and set the member of a map via another function.

Her my example which you can just copy to cpp.sh or so if you like

#include <iostream>
#include <map>

using namespace std;

struct test{
    int i;
    int j;
};



void addValues(test* val){
    
    if  (val == NULL){
        val = new test();
        cout<<"new";
    }
    val->i = 10;
    val->j = 12;
}

void printVal(test* val){
    cout<<"finish " << val->i << " " << val->j;
}

int main()
{
    
    map<string, test*> bla = {{"test1",NULL}};
    
    addValues(bla.at("test1"));
    
    printVal(bla.at("test1"));

    return 0;
}

code from my project is a little bit more complex but its basically this problem. I created test in addValues() and have not deleted it. Why am I not able to print this value in printVal()? What am I missing?

Thanks in advance!

CodePudding user response：

Parameters are passed by value. Pointers are no exception to that. Your addValues modifies a local copy of the pointer when a nullptr is passed. Modifying that local copy does not affect the pointer in the map. Pass the pointer by reference:

void addValues(test*& val){
    
    if  (val == nullptr){
        val = new test();
        cout<<"new";
    }
    val->i = 10;
    val->j = 12;
}

Or better yet, do not use raw pointers in the first place. Moreover, consider to write a constructor that initializes the members of test instead of relying on the caller to initialize them.

CodePudding user response：

Example :

#include <iostream>
#include <map>

//using namespace std; NO teach yourself not to do this.

struct test 
{
    int i = 0;      // <== in c   you can initialize values of structs
    int j = 0;
};

// this instead of printVal
std::ostream& operator<<(std::ostream& os, const test& t)
{
    os << "i = " << t.i << ", j = " << t.j << "\n";
    return os;
}

int main()
{
    std::map<std::string, test> map =
    {
        {"test1",{1,1}},
        {"test2",{2,2}},
    };

    // loop over all entries in the map
    // range based for loop. 
    // each entry in the map is a key,value pair (not they key, not the value but a pair)
    // https://en.cppreference.com/w/cpp/language/range-for

    std::cout << "range based for over keyvalue pairs\n";
    for (const auto& kv : map)
    {
        // note kv.second is where we use operator<< from earlier.
        std::cout << "Key : " << kv.first << ", value : " << kv.second << "\n";
    }
    std::cout << "\n";

    // structured bindings make code more readable
    // https://en.cppreference.com/w/cpp/language/structured_binding
    std::cout << "range based for using structured bindings \n";
    for (const auto& [key, value] : map)
    {
        std::cout << "Key : " << key << ", value : " << value <<"\n";
    }
    std::cout << "\n";
   

    return 0;
}