CODEWITHSUNDEEP
c++
Joe Wilcoxson
Joe Wilcoxson

Reputation: 1500

Initializing default values in a struct

If I needed to initialize only a few select values of a C++ struct, would this be correct:

struct foo {
    foo() : a(true), b(true) {}
    bool a;
    bool b;
    bool c;
 } bar;

Am I correct to assume I would end up with one struct item called bar with elements bar.a = true, bar.b = true and an undefined bar.c?

Upvotes: 113

Views: 268695

Answers (4)

taocp
taocp

Reputation: 23664

Yes. bar.a and bar.b are set to true, but bar.c is undefined. However, certain compilers will set it to false.

See a live example here: struct demo

According to C++ standard Section 8.5.12:

if no initialization is performed, an object with automatic or dynamic storage duration has indeterminate value

For primitive built-in data types (bool, char, wchar_t, short, int, long, float, double, long double), only global variables (all static storage variables) get default value of zero if they are not explicitly initialized.

If you don't really want undefined bar.c to start with, you should also initialize it like you did for bar.a and bar.b.

Upvotes: 48

Anamnian
Anamnian

Reputation: 427

An explicit default initialization can help:

struct foo {
    bool a {};
    bool b {};
    bool c {};
 } bar;

Behavior bool a {} is same as bool b = bool(); and return false.

Upvotes: 5

Erik van Velzen
Erik van Velzen

Reputation: 7062

You don't even need to define a constructor

struct foo {
    bool a = true;
    bool b = true;
    bool c;
 } bar;

To clarify: these are called brace-or-equal-initializers (because you may also use brace initialization instead of equal sign). This is not only for aggregates: you can use this in normal class definitions. This was added in C++11.

Upvotes: 316

Tcz
Tcz

Reputation: 701

You can do it by using a constructor, like this: 

struct Date
{
int day;
int month;
int year;

Date()
{
    day=0;
    month=0;
    year=0;
}
};

or like this:

struct Date
{
int day;
int month;
int year;

Date():day(0),
       month(0),
       year(0){}
};

In your case bar.c is undefined,and its value depends on the compiler (while a and b were set to true).

Upvotes: 14

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