C++ How to Initialize member object?

Question

Is there some way to initialize(?) member variable c of type C in the first part of the below example? Or must I use the new() method shown in the second part of the example?

Class B takes class A as an injected dependency. So does class C. Class B is additionally composed of class C.

How do I get the injected A to B's member C?

Part 1

class A { // ...; };

class C {
public:
    C(A &a) : a(a) {}   // constructor
};


// Does not work as is.  Want to make compiler manage C lifetime.
class B {
public:
    B(A &a);    // constructor

    C c(a);     // member variable
};

// constructor
B::B(A &a) : a(a) {
}

Part 2

// Works, but requires programmer to manage C's lifetime.
class B {
public:
    B(A &a);    // constructor

    C *c;       // member variable
};

// constructor
B::B(A &a) : a(a) {
    c = new C(a);
}

Several good answers below! My apologies for the confusing example. I have up-voted all of the good answers and questions. Unfortunately I can only mark one answer as the accepted answer, so I am choosing the first one which gave me the "ah-ha" moment in which I saw the solution to my real problem, which was more complex than my lame example here.

Usually you don't need `new`. And you never use `new/free` in conjunction. And yes, C++ provides means to initialize objects. — juanchopanza, Feb 10 '15 at 16:35
[Don't use `free` with `new`](http://stackoverflow.com/questions/4061514/c-object-created-with-new-destroyed-with-free-how-bad-is-this). — Emil Laine, Feb 10 '15 at 16:36
Sorry, bad example. I don't want to use `new` at all. Can it be done? How? (will edit question) — CXJ, Feb 10 '15 at 16:38
Also, your class `C` doesn't have any data members called `a`. — juanchopanza, Feb 10 '15 at 16:42

score 3 · Accepted Answer · answered Feb 10 '15 at 16:38

3

Member variables are initialized in constructor's initialization list (before body) so you need to do that:

B::B(A &a)
    : c(a) // Calls constructor C(a) on member c
{}

answered Feb 10 '15 at 16:38

StenSoft

9,369
25
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I see my confusion now. Too many trees in the forest for me to see the forest, as the saying goes. What I really needed was: `public: B(A &a) : a(a), c(a)`. – CXJ Feb 10 '15 at 16:42

score 3 · Answer 2 · answered Feb 10 '15 at 16:42

You almost have it:

class B {
public:
    B(A &a);

    C c(a); //see note 1
};

B::B(A &a) : a(a) { //see note 2
}

Note 1:

There are two problems with C c(a); here:

a is not in scope. a only exists within the scope of the constructor, so c needs to be initialized from there.
Until C++11, non-static data member initializers (NSDMIs) were prohibited. Even in C++11, though, you must use an equals sign (C c = value;) or braces (C c{value};) when initializing an NSDMI.

Note 2:

You've almost got this right:

B::B(A &a) : a(a)

You're trying to initialize a data member called a with the argument given to the constructor. You actually want to initialize c like this, not a non-existent a:

B::B(A &a) : c(a)

The lifetime of c will be that of the instance of the B class. Using dynamic memory management is certainly not necessary.

score 1 · Answer 3 · answered Feb 10 '15 at 16:39

1

"How do I get the injected A to B's member C?"

You can do so using B's constructor member initializer list

class B {
public:
    B(A &a) : c(a) {
           // ^^^^
    }

    C c; // <<< It's not possible to initialize members in their
         //     declaration.
};

answered Feb 10 '15 at 16:39

πάντα ῥεῖ

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I see my confusion now. Too many trees in the forest for me to see the forest, as the saying goes. What I really needed was: `public: B(A &a) : a(a), c(a)`. Or actually, in my real code `public: B(A &a) : a(a), c(a), d(a, c)`. – CXJ Feb 10 '15 at 16:42

score 1 · Answer 4 · answered Feb 10 '15 at 16:44

1

The following:

C(A &a) : a(a) {}

will not compile since a (the first a in the initialization list) is not a member variable of C.

Same applies to the constructor of B.

answered Feb 10 '15 at 16:44

Emil Laine

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My apologies for the incomplete example. In my real problem, a is also a member. – CXJ Feb 10 '15 at 16:49

C++ How to Initialize member object?

4 Answers4

Note 1:

Note 2:

Linked