How do I use std::vector::emplace_back with vector<vector<int>>?
c++, c++11, emplace, stdvector, vector
Solution
The problem is that function template arguments doesn't deduce `std::initializer_list` from a braced-init-list (like `{ 1, 2 }`).
Example:
#include <initializer_list>
#include <type_traits>
template<typename T>
void func(T arg) {
}
int main() {
auto init_list = {1, 2}; // This works because of a special rule
static_assert(std::is_same<decltype(init_list), std::initializer_list<int>>::value, "not same");
func(std::initializer_list<int>{1, 2}); // Ok. Has explicit type.
func({1, 2}); // This doesn't because there's no rule for function
// template argument to deduce std::initializer_list
// in this form.
}
Live example
`std::vector::emplace_back()` is a function template with its arguments being deduced. So passing it `{1, 2}` will not work because it couldn't deduce it. Putting an explicit type to it
res.emplace_back(std::initializer_list<int>{1,2});
would make it work.
Live example
Problem
``` vector<vector<int>> res; res.emplace_back({1,2}); // change to res.push_back({1,2}); would work ``` This gives me the error: ``` main.cpp:61:25: error: no matching function for call to ‘std::vector<std::vector<int> >::emplace_back(<brace-enclosed initializer list>)’ main.cpp:61:25: note: candidate is: In file included from /usr/include/c++/4.7/vector:70:0, from /usr/include/c++/4.7/bits/random.h:34, from /usr/include/c++/4.7/random:50, from /usr/include/c++/4.7/bits/stl_algo.h:67, from /usr/include/c++/4.7/algorithm:63, from miscalgoc.hpp:1, from main.cpp:1: /usr/include/c++/4.7/bits/vector.tcc:92:7: note: void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {}; _Tp = std::vector<int>; _Alloc = std::allocator<std::vector<int> >] ``` How do I make this work? Also, why is an allocator needed here?