C++ vector initialization by using istream_iterator

Question

my test.cpp:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

using namespace std;

int main()
{
    vector<int> vec(istream_iterator<int>(cin), istream_iterator<int>());
    // no error using std::cin
    // vector<int> vec(istream_iterator<int>(std::cin), istream_iterator<int>());
    sort(vec.begin(), vec.end());
    return 0;
}

complile with g++ and here is what I get:

test.cpp:12:11: error: request for member ‘begin’ in ‘vec’, which is of non-class type ‘std::vector<int>(std::istream_iterator<int>, std::istream_iterator<int> (*)())’
   12 |  sort(vec.begin(), vec.end());

[Most vexing parse](https://en.wikipedia.org/wiki/Most_vexing_parse), `vector vec{istream_iterator(cin), istream_iterator()};` should work. — rafix07, Feb 10 '22 at 05:57
If using an older compiler version prior to C++11 support, you can use the more outdated approach to resolving ambiguity by enclosing the first parameter in parentheses: `vector vec((istream_iterator(cin)), istream_iterator());` — paddy, Feb 10 '22 at 06:04

score 1 · Answer 1 · edited Feb 10 '22 at 06:11

When I compiled this code with the options -Wall -Wextra -Werror, I got the following error messages:

iter-vec.cpp: In function ‘int main()’:
iter-vec.cpp:10:20: error: parentheses were disambiguated as a function declaration [-Werror=vexing-parse]
   10 |     vector<int> vec(istream_iterator<int>(cin), istream_iterator<int>());
      |                    
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
iter-vec.cpp:10:20: note: replace parentheses with braces to declare a variable
   10 |     vector<int> vec(istream_iterator<int>(cin), istream_iterator<int>());
      |                    
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                    -
      |                    {                                                   
-
      |                                                                        
}
iter-vec.cpp:13:14: error: request for member ‘begin’ in ‘vec’, which is of non-class type ‘std::vector<int>(std::istream_iterator<int>, std::istream_iterator<int> (*)())’
   13 |     sort(vec.begin(), vec.end());
      |              ^~~~~
iter-vec.cpp:13:27: error: request for member ‘end’ in ‘vec’, which is of non-class type ‘std::vector<int>(std::istream_iterator<int>, std::istream_iterator<int> (*)())’
   13 |     sort(vec.begin(), vec.end());
      |                           ^~~
cc1plus: all warnings being treated as errors

From this I concluded that it was a problem of the initialization of vec being ambiguous. When I removed the using namespace std; clause and added the appropriate std:: markers, I got the following to compile cleanly:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

int main()
{
    std::vector<int> vec(std::istream_iterator<int>(std::cin), 
                         std::istream_iterator<int>());
    std::sort(vec.begin(), vec.end());
    return 0;
}

I may not have gotten all the details correct, but I am fairly certain that this was the cause of the problem.

Welcome to the *most-vexing-parse*, which does provide a bit of head-scratching if you don't have it fresh in your mind. — David C. Rankin, Feb 10 '22 at 07:05

C++ vector initialization by using istream_iterator

1 Answers1