C++ STL map, whose key is shared_ptr<struct tm>

Question

For one of my projects, I need to use shared_ptr to struct tm as the key to a STL map. Below is my testing code. In the for loop, there are two ways to create a shared_ptr: 1) TmSPtr tm_ptr = std::make_shared(* tminfo); 2) TmSPtr tm_ptr(tminfo). Both can compile; however during run-time, the 2nd method throws an error: "* Error in `./a.out': free(): invalid pointer: 0x00007f52e0222de0 * Aborted (core dumped)", indicating that it tries to free memory that does not exist. I am still quite new to smart pointers, so hopefully can get some insight from the forum.

Apologies that I may have included more headers than needed

#include <iostream>
#include <map>
#include <algorithm>
#include <iterator>
#include <memory>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

using namespace std;

typedef std::shared_ptr<struct tm> TmSPtr;

int main()
{
    cout << "\nTesting a map where key is a smart ptr to struct tm:\n";
    map<TmSPtr, int> price_series;

    for (int i = 0; i < 5; ++i) {
        time_t now;
        time(&now);
        struct tm * tminfo = localtime(&now);
        printf("Current local time and date: %s", asctime(tminfo));

        TmSPtr tm_ptr = std::make_shared<struct tm>(*tminfo); // TmSPtr tm_ptr(tminfo); would fail in run time

        price_series.insert(std::pair<TmSPtr, int>(tm_ptr, i));

        usleep(1000000); // 1 sec
    }
    return 0;
}

Why are you using a shared pointer as your key instead the type itself? — Qartar
I use price_series as variable name in the map. Actually I could have 1000 price series, all of which share the same time stamps. I think using struct tm is too expensive, isn't it. There is really only 1 series of time stamps. Hopefully my explanation makes sense. — tgh123
Use mktime() and store a time_t (integer) instead. Using pointers as keys is a bad idea, using shared_ptrs as keys is a terrible idea. — Qartar

Nikolai Fetissov Nikolai Fetissov · Accepted Answer · 2015-04-23T02:29:51

localtime(3) says: "The return value points to a statically allocated struct ...". That means the memory is not on the heap, so should not be de-allocated.

Your first method works because it copies the structure.

C++ STL map, whose key is shared_ptr<struct tm>

2 Answers