emplace_back

std::vector::emplace_back() is used to construct the new element directly at the end of the vector. This is allocated through std::allocator_traits::construct which uses parens and not braces to initialize. This means it does not do uniform or aggregate initialization.

From C++20, parentheses are allowed for aggregate initialization. So emplace_back can do aggregate initialization as well.

struct A{
    int a;
};
 
int main() {
    std::vector<A> vec;
    vec.emplace_back(1);
}

The above code doesn’t compile for C++17, but compiles for C++20.

Read More:

• C++20’s parenthesized aggregate initialization has some downsides
• Aggregate initialization - cppreference.com