C++ how to use emplace_back for user defined structure

Question

I am trying to make use of emplace_back for my user defined structure:

#include <cstdint>
#include <vector>
#include <string>

struct IDNumber
{
    IDNumber(std::vector<int> d) : id(d){}
    std::vector<int> id;
};
struct Def
{
    Def(std::initializer_list<int> id) : mid(id){}
    IDNumber mid;
};

struct Student
{
    std::vector<Def> ent;
};

int main()

{
 Student a;
 a.ent.emplace_back({ {2000} });
}

I get compilation issues:

error: no matching function for call to 'std::vector<EntryDef>::emplace_back'

Answer 1

The comment by @PiotrSkotnicki:

emplace_back is function template which tries to deduce the types of arguments. an initializer list does not have a type, so deduction fails.

Clarifies the problem.

An alternative way to "fix" this issue is to pass an rvalue of the needed type as argument of the constructor, instead of the initializer list:

EntryDef(ID &&id, FType ft, … ) : mid(std::forward<ID>(id)), ftype(ft), … {}

Called as:

Def a;
a.ent.emplace_back(ID{ 2, 1 }, FType::FD_NONE, …);

Live example HERE.