我该如何解决错误：从&#39;void&#39;转换为非标量类型&#39;std :: vector <int> 要求

Question

I get a error like this :

conversion from ‘void’ to non-scalar type ‘std::vector<int>’ requested

and this a function I get this error from:

   vector<int> concat(vector<int> vector1, vector<int> vector2)
{
     return vector1.insert(vector1.end(), vector2.begin(), vector2.end());
}

I used the insert() function from algorithm library.

Answer 1

The method insert returns iterator.

It has the following declaration

template <class InputIterator>
iterator insert(const_iterator position, 
                InputIterator first, InputIterator last);

The function interface is not logically consistent.

Either declare the function the following way

std::vector<int> & concat( std::vector<int> &vector1, const std::vector<int> &vector2 )
{
    vector1.insert(vector1.end(), vector2.begin(), vector2.end());
    return vector1;
}

or the following way

std::vector<int> concat( const std::vector<int> &vector1, const std::vector<int> &vector2 )
{
    std::vector<int> result( vector1 );
    result.insert(result.end(), vector2.begin(), vector2.end());
    return result;
}