C2672 'std::construct_at': no matching overloaded function found main

September 12, 2024

Build error on very simple code using C++ 20 Visual Studio 2022 64bit version 17.11.3

Seems to be getting error in the compiler code base. As far as I can tell the code is fine. I know about not using the using this is just learning code at this time.

#include <cstdio>
#include <iostream>
#include <vector>
#include <fstream>
#include <string>
#include <sstream>

using std::vector;


vector<int> read_input() {
    std::string line;
    std::ifstream fp;
    vector<int> data;
    int word;

    fp.open("input.txt");
    if (fp) {
        while (getline(fp, line, '\n')) {
            vector<int> tempdata;
            std::istringstream ss(line);
            while (ss >> word) {
                tempdata.push_back(word);
            }
            data.emplace_back(tempdata);
        }
        
        fp.close();
    }
    else
        perror("File opening failed");
    
    return data;
}

Data in input.txt
43 32 21 23 43 98 90 65 53 42 69 79 82 7 1 63 64 90 87 96 4 55 3 7 2 1 67 89 543
567 985 34 975 890 4567 23 33 9 56 345 9865

>Solution :

You are trying to insert a vector<int> into a vector<int>. That does not work. A vector<int> is not convertible to int.

You can use vector::insert with iterators from tempdata instead. This tells it to insert it at the end of data:

data.insert(data.end(), tempdata.begin(), tempdata.end());
//          ^^^^^^^^^^
//       insert before this

Since C++23, you can also use vector::insert_range:

data.insert_range(data.end(), tempdata);

If you actually want one vector<int> per line in the file, you need to make the vector that you insert into a vector<vector<int>> (and modify the return value of read_input accordingly). Here’s a full example of that:

#include <cstdio>
#include <fstream>
#include <iostream>
#include <ranges>
#include <sstream>
#include <string>
#include <vector>

std::vector<std::vector<int>> read_input() {
    std::vector<std::vector<int>> data;   

    if (std::ifstream fp("input.txt"); fp) {
        std::string line;

        while (std::getline(fp, line, '\n')) {
            std::istringstream ss(line);
            // default construct a vector<int> in data
            // and get a reference to it:
            auto& current_vec_int = data.emplace_back();
            int word;
            while (ss >> word) {
                current_vec_int.emplace_back(word);
            }
        }

        // fp.close(); // no need, it closes automatically
    } else {
        std::perror("File opening failed");
    }

    return data;
}

int main() {
    auto input = read_input();
    for (auto& vec_int : input) {
        for (int val : vec_int) {
            std::cout << val << '\n';
        }
        std::cout << '\n';
    }
}