Assignment using std::make_unique<>

Question

Can someone explain this C++ wizardy:

#include <memory>

class Base
{
    public:
    Base(){}
};
class SubClassA : public Base
{
    public:
    SubClassA(){}
};
class SubClassB : public Base
{
    public:
    SubClassB(){}
};
int main()
{
    std::unique_ptr<Base> base;
    bool condition = true;

    if(condition) {
        base = std::make_unique<SubClassA>();  //This works
    }
    else {
        base = std::make_unique<SubClassB>();  //This works
    }
    
    base = (condition) ? 
        std::make_unique<SubClassA>(): //Does not work
        std::make_unique<SubClassB>();
    
    return 0;
}

This code produces the error:

main.cpp: In function ‘int main()’:
main.cpp:41:24: error: operands to ?: have different types ‘std::_MakeUniq::__single_object {aka std::unique_ptr >}’ and ‘std::_MakeUniq::__single_object {aka std::unique_ptr >}’
     base = (condition) ?
            ~~~~~~~~~~~~^
         std::make_unique<SubClassA>():
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         std::make_unique<SubClassB>();
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

How can I do the assignment with the ternary operator ?:

Aplet123 · Accepted Answer · 2020-12-24 02:33:59Z

2

C++ cannot infer that you want the expression to be a std::unique_ptr<Base>, so you can cast one of them and C++ can then infer the cast for the other:

base = condition ?
    (std::unique_ptr<Base>) std::make_unique<SubClassA>():
    std::make_unique<SubClassB>();

answered Dec 24, 2020 at 2:33

Aplet123

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