error with vector, unique_ptr, and push_back

Question

I am learning smart pointers, with the following example test.cpp

#include<iostream>
#include<vector>
#include<memory>

struct abstractShape
{
    virtual void Print() const=0;
};

struct Square: public abstractShape
{
    void Print() const override{
        std::cout<<"Square\n";
    }
};

int main(){
    std::vector<std::unique_ptr<abstractShape>> shapes;
    shapes.push_back(new Square);

    return 0;
}

The above code has a compilation error "c++ -std=c++11 test.cpp":

smart_pointers_2.cpp:19:12: error: no matching member function for call to 'push_back'
    shapes.push_back(new Square);

Could someone help explain the error to me? By the way, when I change push_back to emplace_back, the compiler only gives a warning.

Answer 1

push_back expects an std::unique_ptr, when passing raw pointer like new Square, which is considered as copy-initialization, the raw pointer needs to be converted to std::unique_ptr implicitly. The implicit conversion fails because std::unique_ptr's conversion constructor from raw pointer is marked as explicit.

emplace_back works because it forwards arguments to the constructor of std::unique_ptr and construct element in direct-initialization form, which considers explicit conversion constructors.

The arguments args... are forwarded to the constructor as std::forward<Args>(args)....