Understanding C++ struct with explicit and default values

Question

I have a hard time understanding the following code, can someone please elaborate.

struct WUpdate {
    explicit WUpdate(std::string id = "", std::string message = "")
        : id(std::move(id)), message(std::move(message)) {}
    explicit WUpdate(const UpgradeJob& job)
        : id(job.m.id), message(job.message) {}

    std::string id;
    std::string message;
};

I want to update the above struct to accept a new field as well (called time below) with default value -1. I have updated the code above as follows:

struct WUpdate {
    explicit WUpdate(std::string id = "", std::string message = "", int time = -1)
        : id(std::move(id)), message(std::move(message)) {}
    explicit WUpdate(const UpgradeJob& job)
        : id(job.m.id), message(job.message) {}

    std::string id;
    std::string message;
    int time{-1};
};

the code compiles but when running I can see that the value of time is always set to -1, regardless of what I try to initialise it with. I am also getting warning saying that warning: unused parameter ‘time’ pointing to the second line of updated code.

You didn't initialize it in the constructor. It has nothing to do with the explicit keyword. The warning says it. — Michael Chourdakis, May 14 '20 at 12:36
https://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor — StoryTeller - Unslander Monica, May 14 '20 at 12:38

score 2 · Accepted Answer · answered May 14 '20 at 12:59

2

the code compiles but when running I can see that the value of time is always set to -1, regardless of what I try to initialise it with.

None of the explicit keyword or that default value thing you mentioned has anything to do with time being always set to -1. You simply forgot to initialize time in the constructor as follows:

struct WUpdate {
    explicit WUpdate(std::string id = "", std::string message = "", int time = -1)
        : id(std::move(id)), message(std::move(message)), 
          time(time) /* <- Here */
    { }

    // ...
};

answered May 14 '20 at 12:59

Dean Seo

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thanks, I saw it after a while, by the way, what is that called, initialisation in parentheses? – theAlse May 14 '20 at 13:33
@theAlse It's called **member initializer lists** – Dean Seo May 14 '20 at 14:03

Understanding C++ struct with explicit and default values

1 Answers1