Why is emplace_back failing even though the direct constructor works? [duplicate]

Question

Why is it that I can call a constructor directly, but when I use exactly the same arguments in emplace_back, I get an error "No matching member function for call to 'emplace_back'"? To get the error, I need to be passing a list to the constructor. There must be something very simple going on here, but I can't find it in a search.

Here is a minimal example.

class A {
public:
    A(std::list<double> c){
    }
};

void test(){
    A an_A = A({0.0});               //No error
    std::list<A> list_of_As;
    list_of_As.emplace_back({0.0});  //Error
}

Answer 1

The problem with {} is that it has several types it can describe. Now, when you don't specify the type, the compiler need to deduce which input type you ment to send, and it's unable to do that. For this reason you have to specify the type of the {}.

void test(){
    A an_A = A({0.0});
    std::list<A> list_of_As;
    list_of_As.emplace_back(list<double>{0.0});
}

Another options:

for the next class:

class A {
public:
    A(list<double> c, int a){
    }
};

You can use:

list_of_As.emplace_back(list<double>{0.0}, 1);
//list_of_As.emplace_back(A({0.0}, 1)); <-- Avoid using this. Explanation is right after this section.
list_of_As.emplace_back(initializer_list<double>{0.0}, 1);

Emplace Vs Push

is there any good reason not to solve the problem with A({0.0}) rather than list{0.0} ?

Yes. emplace_back used for creating the objects of the list in place instead of copy/move them later to the list. There is no reason for using emplace_back like it was push_back. The right way to use emplace_back is to send it the arguments of the object's constructor, and not the object itself.

push_back will move your object into the list/vector if only it moved by reference (rvalue) and if the element type has a move assignment operator.

For more reading about move/copy and push_back/emplace_back