Is there a convenience constructor in C++?

Question

Is it possible for an overloaded constructor to somehow call another constructor within the class, similar to the code below?

class A {
public:
    A(std::string str) : m_str(str) {}
    A(int i) { *this = std::move(A(std::to_string(i))); }

    std::string m_str;
};

The code above works, yet I am afraid that calling this in the constructor might lead to undefined behavior.

If it does could you please explain why and also suggest a better alternative?

This is a minimal example. I want to overload a constructor that accepts 7 parameters, 4 of which have default values and I want to avoid explicitly setting the defaults in more than one place. — Makaronodentro, Feb 08 '17 at 13:56
@Makaronodentro: But you've still got to give the defaults twice, no? — Lightness Races in Orbit, Feb 08 '17 at 15:50
@LightnessRacesinOrbit no, with defaults: A(std::string str1, str::string str2="abc") : m_str1(str1), m_str2(str2) {} delegating: A(int i) : A(std::move(std::to_string(i))) {} — Makaronodentro, Feb 08 '17 at 16:57
`std::move(std::to_string(i))` That "move" is redundant. `std::to_string(i)` is already an rvalue. Recall that `std::move` doesn't actually perform a move — Lightness Races in Orbit, Feb 08 '17 at 17:20

acraig5075 · Accepted Answer · 2017-02-08T14:10:06.553

29

C++11 introduced delegating constructors:

class A
    {
    public:
    std::string m_str;
    A(std::string str) : m_str(str) {} // target constructor
    A(int i) : A(std::to_string(i)) {} // delegating constructor
    };

edited Feb 08 '17 at 14:10

answered Feb 08 '17 at 13:44

acraig5075

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Might be worth it to `m_str(std::move(str))`. – juanchopanza Feb 08 '17 at 13:53
It might be worth noting that the target constructor is called before the delegating constructor, so in the body of the latter, you can assume the former was called. – MartinVeronneau Feb 08 '17 at 14:59

Is there a convenience constructor in C++?

1 Answers1