How can class fields be initialized?

Question

A bit of a basic question, but I'm having difficulty tracking down a definitive answer.

Are initializer lists the only way to initialize class fields in C++, apart from assignment in methods?

In case I'm using the wrong terminology, here's what I mean:

class Test
{
public:
    Test(): MyField(47) { }  // acceptable
    int MyField;
};

class Test
{
public:
    int MyField = 47; // invalid: only static const integral data members allowed
};

EDIT: in particular, is there a nice way to initialize a struct field with a struct initializer? For example:

struct MyStruct { int Number, const char* Text };

MyStruct struct1 = {};  // acceptable: zeroed
MyStruct struct2 = { 47, "Blah" } // acceptable

class MyClass
{
    MyStruct struct3 = ???  // not acceptable
};

In C++ the struct instance with empty braces will normally not be zeroed, but will contain garbage values. Exceptions may be in a debug environment or similarly controlled situation. — Staffan E, Jul 16 '10 at 12:11
@SEinarsson - oh... do you have a reference for that? The empty braces are recommended all over the web for initializing structs in C++. — Roman Starkov, Jul 16 '10 at 12:58
Wait, now I'm not so certain. I've found people on the web saying I'm wrong but no formal definition for the {} pattern. The following question implies that structs-classes in C++ have zeroing default constructors, when used in a class initializer (see second answer) http://stackoverflow.com/questions/112085/is-this-c-structure-initialization-trick-safe Just disregard my above comment until I can clarify this. — Staffan E, Jul 19 '10 at 07:41

score 6 · Accepted Answer · answered Jul 16 '10 at 12:05

6

In C++x0 the second way should work also.

Are initializer lists the only way to initialize class fields in C++?

In your case with your compiler: Yes.

answered Jul 16 '10 at 12:05

BeachBlocker

246
1
3

sje397 · Answer 2 · 2010-07-16T12:10:51.130

4

Static members can be initialised differently:

class Test {
    ....
    static int x;
};

int Test::x = 5;

I don't know if you call this 'nice', but you can initialise struct members fairly cleanly like so:

struct stype {
const char *str;
int val;
};

stype initialSVal = {
"hi",
7
};

class Test {
public:
    Test(): s(initialSVal) {}
    stype s;
};

edited Jul 16 '10 at 12:10

answered Jul 16 '10 at 12:05

sje397

41,293
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score 1 · Answer 3 · answered Jul 16 '10 at 12:17

Just to mention that in some cases, you have no choice but to use initializer lists to set a member's value on construction:

class A
{
 private:

  int b;
  const int c;

 public:

 A() :
  b(1),
  c(1)
 {
  // Here you could also do:
  b = 1; // This would be a reassignation, not an initialization.
        // But not:
  c = 1; // You can't : c is a const member.
 }
};

score 0 · Answer 4 · edited May 23 '17 at 11:53

0

The recommended and preferred way is to initialize all fields in the constructor, exactly like in your first example. This is valid also for structs. See here: Initializing static struct tm in a class

edited May 23 '17 at 11:53

Community

1
1

answered Jul 16 '10 at 12:04

m_pGladiator

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How can class fields be initialized?

4 Answers4