C++11 vector of vectors initialization - c++11

Question

In C++11, {} is preferred over () for variable initialization. However, I noticed that {} cannot correctly initialize the vector of vectors.
Given the following code, vector<vector<int>> mat2(rows, vector<int>(cols, 2)) and vector<vector<int>> mat4{rows, vector<int>(cols, 4)} work as expected, but vector<vector<int>> mat1{rows, vector<int>{cols, 1}} and vector<vector<int>> mat3(rows, vector<int>{cols, 3}) do not. Can anybody explain why?
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
string parse_matrix(const vector<vector<int>>& mat)
{
stringstream ss;
for (const auto& row : mat) {
for (const auto& num : row)
ss << std::setw(3) << num;
ss << endl;
}
return ss.str();
}
int main()
{
const int rows = 5;
const int cols = 4;
vector<vector<int>> mat1{rows, vector<int>{cols, 1}};
vector<vector<int>> mat2(rows, vector<int>(cols, 2));
vector<vector<int>> mat3(rows, vector<int>{cols, 3});
vector<vector<int>> mat4{rows, vector<int>(cols, 4)};
cout << "mat1:\n" << parse_matrix(mat1);
cout << "mat2:\n" << parse_matrix(mat2);
cout << "mat3:\n" << parse_matrix(mat3);
cout << "mat4:\n" << parse_matrix(mat4);
}
Output:
$ g++ -Wall -std=c++14 -o vector_test2 vector_test2.cc
$ ./vector_test2
mat1:
4 1
4 1
4 1
4 1
4 1
mat2:
2 2 2 2
2 2 2 2
2 2 2 2
2 2 2 2
2 2 2 2
mat3:
4 3
4 3
4 3
4 3
4 3
mat4:
4 4 4 4
4 4 4 4
4 4 4 4
4 4 4 4
4 4 4 4

Answer 1

For a combination of reasons, uniform initialization is kind of broken for std::vector<int> (and more generally, vector of arithmetic type). Uniform initialization uses the same syntax as list-initialization (constructing from initializer_list); when the syntax is ambiguous, the constructor taking initializer_list takes precedence.
Thus, std::vector<int> v(42); means "use std::vector(size_t) constructor to create a vector of 42 zeros"; while std::vector<int> v{42}; means "use std::vector(std::initializer_list) constructor to create a vector with a single element having value 42".
Similarly, std::vector<int> v(5, 10); uses two-parameter constructor to create a vector of 5 elements, all with value 10; while std::vector<int> v{5, 10}; uses initializer_list-taking constructor and creates a vector of two elements, values 5 and 10.