JeroenDM / cpp-questions

cpp-questions

for each + remove if

You have a vector with timestamped data. You want to execute a task for all data that has a timestamp in the past compared to the current time. In addition processed elements should be removed from the vector. Is there a better way to do this than the following loop, that modifies the loop iterator?

    int current_time = 4;

    auto it = work.begin();
    while (it != work.end()) {
        if (it->timestamp > current_time) {
            do_work(it->id, it->parameters);
            it = work.erase(it);
        }
        else {
            it++;
        }
    }

Minimal working example (compiler explorer):

#include <iostream>
#include <vector>
#include <string>

struct Data {
    int id;
    int timestamp;
    std::string parameters;
};

void do_work(int id, const std::string& params) {
    std::cout << "Working on: " << id << ": " << params << "\n";
}

int main()
{
    std::vector<Data> work = {
        {1, 4, "one"},
        {2, 5, "two"},
        {3, 2, "three"},
        {4, 9, "four"},
    };

    int current_time = 4;

    auto it = work.begin();
    while (it != work.end()) {
        if (it->timestamp > current_time) {
            do_work(it->id, it->parameters);
            it = work.erase(it);
        }
        else {
            it++;
        }
    }

    std::cout << "\nAfter loop:\n";

    for (auto x : work) {
        std::cout << x.id << ": " << x.parameters << "\n";
    }
}

answer The C++ Primer book [1] has some advice on this on page 354, section "Writing loops that change a container". It points out that the operations insert and erase return an iterator that should be used to refresh the loop iterator accordingly. In addition the next section reminds us to update the end() iterator. The code above follows all this advice.

Real-time programming and threads

This talk mentions that of all the related primitives, only std::atomic is real-time safe. Why is this and how to handle this?