Call constructor of member object

Question

I try to implement the amySQL, AND I want to intialize this connector intro test function.

class Foo
{
public:
    void Test();

protected:
    amy::connector mgr;

private:
    asio::io_service m;
};

void Foo::Test()
{
    mgr(m);
}

However when i want to compile i get this error:

error: no match for call to '(amy::connector {aka amy::basic_connector<amy::mysql_service>}) (asio::io_service&)'
  mgr(m);

What i do wrong here? Repository to amy sql https://github.com/liancheng/amy

You don't "initialize constructors" - constructors initialize things. — , Aug 07 '19 at 21:51
If it's `boost::asio` you are using and you have a relatively new version of boost, use `io_context` instead of `io_service`. `io_service` is just a typedef for backwards compatibility. — Ted Lyngmo, Aug 07 '19 at 22:04
Possible duplicate of [C++ Constructor](https://stackoverflow.com/questions/6724626/c-constructor) — , Aug 07 '19 at 22:44

Remy Lebeau · Accepted Answer · 2019-08-07T22:28:39.793

4

You need to initialize the mgr member inside the Foo class constructor, not in the Test() class method:

class Foo
{
public:
    Foo();
    void Test();

private:
    asio::io_service m;

protected:
    amy::connector mgr;
};

Foo::Foo() : mgr(m) // <-- initialize here!
{
}

void Foo::Test()
{
    // use mgr here as needed...
}

edited Aug 07 '19 at 22:28

answered Aug 07 '19 at 21:54

Remy Lebeau

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1

@TedLyngmo yes, it should – Remy Lebeau Aug 07 '19 at 22:28
1

Declaring `m` before `mgr` is super subtle but is crucial here, since members are initialized in order of declaration. If `m` is declared after `mgr`, then the initializer `mgr(m)` will use an unitialized object and will almost certainly cause Undefined Behavior. – alter_igel Aug 07 '19 at 22:52
@alterigel Amazing answer, you saved me. I was wondering why i'm gettin core crash, that was impossible to find out. It seems that this was the cause, order of declaration was different from order of initialization. xD – Irinel Iovan Aug 10 '19 at 03:31

score 1 · Answer 2 · edited Aug 07 '19 at 21:52

1

You could initialize the members in your class constructor's initialization list

Foo::Foo() : mgr(m) {}

edited Aug 07 '19 at 21:52

Remy Lebeau

555,201
31
458
770

answered Aug 07 '19 at 21:51

Cory Kramer

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The class name is `Foo`, not `Test`. `Test()` is a method of the `Foo` class – Remy Lebeau Aug 07 '19 at 21:52

Call constructor of member object

2 Answers2