error when Custom Sorting a vector of tuples

Question

I try to sort a vector of tuple but I have a strange error:

typedef std::tuple<const std::string, const std::string, const unsigned int> nodesfound;

std::vector<nodesfound> nf;

fil_vector(nf);

std::sort(nf.begin(), nf.end(), [](nodesfound const &n1, nodesfound const &n2) {
        return std::get<2>(n1) < std::get<2>(n2);
    });

the error is:

/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/tuple:299:17: error: no viable overloaded '='_M_head(*this) = std::forward<_Head>(_M_head(__in));

if I remove the sort line my program is perfectly fine

ildjarn · Accepted Answer · 2016-07-16 10:59:26Z

1

For sort to work elements must necessarily be assignable; however, your tuple elements are all const, so obviously not assignable. Drop the consts:

using nodesfound = std::tuple<std::string, std::string, unsigned>;

std::vector<nodesfound> nf;
fill_vector(nf);
std::sort(
    nf.begin(), nf.end(),
    [](nodesfound const& n1, nodesfound const& n2) {
        return std::get<2>(n1) < std::get<2>(n2);
    }
);

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edited Jul 16, 2016 at 10:59

answered Jul 16, 2016 at 10:50

ildjarn

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1 Comment

Fractale Over a year ago

std::sort work with tuple, why the inside of the tuple shoud be const? sort can just swap tuple in the vector? thanck's a lot

rahnema1 · Accepted Answer · 2016-07-16 11:12:53Z

0

other way is to get index of sorted elements:

typedef std::tuple< const std::string,  const std::string,  const unsigned int> nodesfound;
std::vector<nodesfound> nf;
fil_vector(nf);
std::vector<int> index(nf.size());
std::iota( index.begin(), index.end(), 0 );
std::sort(index.begin(), index.end(), [&](int n1, int n2) {
    return std::get<2>(nf[n1]) < std::get<2>(nf[n2]);
});

answered Jul 16, 2016 at 11:12

rahnema1

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