Lambda in constructor

Question

I have a class which uses lambda in one of its constructor, and I am having trouble understanding how it gets executed

using pred = std::function<bool(int)>;
using pred_list = std::vector<pred>;

class check
{
    private:
        std::string const _description;
    public:
        check(std::string, msec duration = msec{0});
        check(std::string, pred_list, msec duration = msec{0});
        check(std::string, pred, msec duration = msec{0});
};

The constructors

check::check(std::string d, pred_list p, msec dur)
: _description{d}, _duration{dur}, _predicates{p}, _pred_pass{false}
, _deadline{msec::max()}
{};

check::check(std::string d, pred p, msec dur)
: _description{d}, _duration{dur}, _predicates{1,p}, _pred_pass(false)
{};

check::check(std::string s, msec dur)
: check(s, [](int i) { return i > 0; }, dur) 
{};

Constructor in question

check::check(std::string s, msec dur)
: check(s, [](int i) { return i > 0; }, dur) 
{};

If I create an object of type check using the following

check db_intl{"Test", db_dur};

The following constructor gets called

check::check(std::string s, msec dur)
: check(s, [](int i) { return i > 0; }, dur) 
{};

How is this constructor able to call the other constructor with the lambda if that lambda is not being used?

1201ProgramAlarm · Accepted Answer · 2019-06-14 19:06:46Z

1

Since pred is typedefed to std::function<bool(int)>, the lambda will be converted to a pred, and construction is then delegated to the 3rd constructor (check(std::string, pred, msec duration = msec{0}). The lambda will then be stored in the _predicates container and (presumably) called later when the predicates are invoked.

answered Jun 14, 2019 at 19:06

1201ProgramAlarm

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2 Comments

zer0c00l Over a year ago

_predicates is defined as pred_list const _predicates; If i create an object of type check like this check db_intl{"Test", db_dur}; It is only passing 2 parameters, and in the third constructor there is no local variable which can be used for the lambda

1201ProgramAlarm Over a year ago

@zer0c00l The two parameter constructor will delegate to the three parameter one; see the "Delegating constructor" section on cppreference. The lambda is not using any local variables.

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