error: cannot convert 'std::optional<int>' to 'const int' in initialization

Question

#include <vector>
#include <ranges>
#include <algorithm>
#include <functional>
using namespace std;

int main()
{
    const vector<int> v = {1, 2, 3};
    const int n = ranges::fold_left_first(v | views::transform([](int i){return i*i;}), plus<int>())
    return 0;
}

Compiled using g++14.

prog.cc: In function 'int main()':
prog.cc:10:42: error: cannot convert 'std::optional<int>' to 'const int' in initialization
   10 |     const int n = ranges::fold_left_first(v | views::transform([](int i){return i*i;}), plus<int>())
      |                   ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                          |
      |                                          std::optional<int>

You can access the optional either with it's member functions has_value() / value() or using pointer syntax: if(v) { n = *v;} — nick
– nick, Commented Mar 12, 2024 at 11:35
You can use auto result. Then you can print the type of result using typeid(result).name(), based on that, you can handle it (call has_value()). — kiner_shah
– kiner_shah, Commented Mar 12, 2024 at 11:49

Kamil Pajak · Accepted Answer · 2024-03-12 11:43:05Z

2

Function ranges::fold_left_first returns std::optional<int> rather then int:

https://en.cppreference.com/w/cpp/algorithm/ranges/fold_left_first

Here's the corrected code:

#include <iostream>
#include <vector>
#include <ranges>
#include <algorithm>
#include <optional>
using namespace std;

int main() {
    const vector<int> v = {1, 2, 3};
    std::optional<int> result = ranges::fold_left_first(v | views::transform([](int i) { return i * i; }), plus<>());

    // Check if the result has a value and use it
    if (result.has_value()) {
        const int n = result.value();
        cout << "The sum of the squares is: " << n << endl;
    } else {
        cout << "The vector is empty." << endl;
    }

    return 0;
}

edited Mar 12, 2024 at 11:43

answered Mar 12, 2024 at 11:36

Kamil Pajak

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