声明二维向量大小时出错

Question

我想先声明我的二维向量，然后给它一个大小。

但为什么我会出错？

谁能解释一下？

int main() {
    vector<vector<int>> a;
    a = vector<int>(16, vector<int>(15));
    cout << a.size() << a[0].size();
}

这样做的原因是我之前不知道尺寸，但是在得到用户的输入之后，我想给它尺寸。

错误：

Char 9: error: no matching constructor for initialization of 'vector<int>'
    a = vector<int>(16, vector<int>(15));
        ^           ~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:507:7: note: candidate constructor not viable: no known conversion from 'vector<int>' to 'const std::vector<int, std::allocator<int>>::allocator_type' (aka 'const std::allocator<int>') for 2nd argument
      vector(size_type __n, const allocator_type& __a = allocator_type())
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:519:7: note: candidate constructor not viable: no known conversion from 'vector<int>' to 'const std::vector<int, std::allocator<int>>::value_type' (aka 'const int') for 2nd argument
      vector(size_type __n, const value_type& __value,
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:572:7: note: candidate constructor not viable: no known conversion from 'int' to 'const std::vector<int, std::allocator<int>>' for 1st argument
      vector(const vector& __x, const allocator_type& __a)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:604:7: note: candidate constructor not viable: no known conversion from 'int' to 'std::vector<int, std::allocator<int>>' for 1st argument
      vector(vector&& __rv, const allocator_type& __m)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:622:7: note: candidate constructor not viable: no known conversion from 'int' to 'initializer_list<std::vector<int, std::allocator<int>>::value_type>' (aka 'initializer_list<int>') for 1st argument
      vector(initializer_list<value_type> __l,
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:650:2: note: candidate template ignored: deduced conflicting types for parameter '_InputIterator' ('int' vs. 'std::vector<int, std::allocator<int>>')
        vector(_InputIterator __first, _InputIterator __last,
        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:494:7: note: candidate constructor not viable: requires single argument '__a', but 2 arguments were provided
      vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:550:7: note: candidate constructor not viable: requires single argument '__x', but 2 arguments were provided
      vector(const vector& __x)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:569:7: note: candidate constructor not viable: requires 1 argument, but 2 were provided
      vector(vector&&) noexcept = default;
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:582:7: note: candidate constructor not viable: requires 3 arguments, but 2 were provided
      vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:586:7: note: candidate constructor not viable: requires 3 arguments, but 2 were provided
      vector(vector&& __rv, const allocator_type& __m, false_type)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:484:7: note: candidate constructor not viable: requires 0 arguments, but 2 were provided
      vector() = default;
      ^
1 error generated.

Answer 1

vector<int> 不是二维向量。

而不是这个：

a = vector<int>(16, vector<int>(15));

你应该使用这个：

a = vector<vector<int>>(16, vector<int>(15));