c++ value initialization

Question

I'm reading c++ primer 4th edition by Stanley Lipmann and I'm on page 92 about value initialisation.

I just don't understand when about value initialisation. I browsed around and I know there are also default initialisation and zero-initialisation. Can anyone explain about value initialisation?

Coming to this paragraph ..

clause a)

"Some classes does not define default constructor. We cannot initialise vector of such a type by specifying only a size, we must also specify an initial value"

I do understand the above but I find that the below contradict the above sentence.

clause b)

"Element type might be of a class type that does not define any constructors. In this case, the library still creates a value-initialised object. It does so by value-initialising each member of that object"

I don't understand the clause b.

Any help is appreciated

score 8 · Answer 1 · edited May 23 '17 at 12:06

8

a) This is true, if the class defines other constructors - thereby suppressing generation of a default constructor.

struct Foo {
  Foo(int n) : mem(n) {}
  int mem;
};

This class can't be value-initialized.

b) If the class has no constructors defined, value-initialization will simply value-initialize all sub-objects (base classes and non-static members)

struct Foo {
  Foo() : mem(0) {}
  int mem;
};

struct Bar {
  Foo f;
};

Value-initialization of Bar simply means that the f member will be value-initialized.

See e.g. What do the following phrases mean in C++: zero-, default- and value-initialization?

edited May 23 '17 at 12:06

Community

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answered Apr 18 '11 at 00:07

Erik

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Can you give me some example for clause b)? – yapkm01 Apr 18 '11 at 00:10
@yapmk01: Updated answer with some samples – Erik Apr 18 '11 at 00:17
I'm new to C++. I'm from Java background. The clause b) you mentioned Foo() : mem(0) {} .. Isn't Foo() a default constructor? In Java, a default constructor is one without any parameters. Can C++ class have NO constructor at all (meaning no default constructor too)? From what i understand .. if you don't supply any constructor, the compiler will generate the default one for you. If C++ class can have NO constructor at all, how do you initialise it? – yapkm01 Apr 18 '11 at 02:42
@yapkm01: If it has *no* constructor, an implicit parameter-less constructor is generated – Erik Apr 18 '11 at 08:11

score 0 · Answer 2 · answered Apr 18 '11 at 00:32

#include <vector>
#include <string>
class fooz {
    private:
        string s;
        int n;
    public:
        fooz(string& str, int num) {
            s=str;
            n=num;
        }
        ~fooz(){}
        void gets(string& str) {str=s;}
        void getn(int& num) {num=n;}
};
vector<class fooz> vfDialpad = {
    fooz(string(""),0),
    fooz(string(""),1),
    fooz(string("abc"),2),
    fooz(string("def"),3),
    fooz(string("ghi"),4),
    fooz(string("jkl"),5),
    fooz(string("mno"),6),
    fooz(string("pqrs"),7),
    fooz(string("tuv"),8),
    fooz(string("wxyz"),9)
};

after this, both rules have been activated. this is an initializer list and it is new to C++, you may or may not find it only in newer versions of gcc. each element gets the new object initialized.

maybe I am not making the best example here, but it's a start.

c++ value initialization

2 Answers2