Default struct initialisation in any case?

Question

I realized that in C++ you can initialize any structure with default values. I don't think that was possible under C. Is that correct?

Are there situations in which it still makes sense not to use such a default initialization? Or is it better in any case since it is safer?

I also saw that there are different ways to do this. Is One Method Better Than Another? Or is it just a matter of taste?

struct STR_Foo {
    int value{ 0 };
};

struct STR_Foo {
    int value = 0;
};

struct STR_Foo {
    int value = { 0 };
};

Title says C++, but then tags include C. Please be explicit what you need here. — ryyker
– ryyker, Commented Feb 15, 2021 at 19:47
Default initialisation is not possible in C, other than the usual "set to all 0's" initialisation for global and static variables. — user3386109
– user3386109, Commented Feb 15, 2021 at 19:47
The three different initializations could do different things depending on type. — Some programmer dude
– Some programmer dude, Commented Feb 15, 2021 at 19:49
For complex types it could be better to allocate or reserve, but not initialize, if the default value is never used. Initialization can also take cycles. — JHBonarius
– JHBonarius, Commented Feb 15, 2021 at 20:10

eerorika · Accepted Answer · 2021-02-15 20:16:50Z

I realized that in C++ you can initialize any structure with default values. I don't think that was possible under C. Is that correct?

Yes.

Are there situations in which it still makes sense not to use such a default initialization?

If you target Older standard than C++11 - or cross-compatibility with C. Default member initialisers weren't in the language before that - and aren't in C.
If you target C++11 standard and you want the class to be an aggregate. Since C++14 default member initialisers don't disqualify a class from being an aggregate.
If you need an instance of the class to be initialised later and have measured that you cannot afford the very low overhead of redundant initialisation.

I also saw that there are different ways to do this. Is One Method Better Than Another? Or is it just a matter of taste?

The choice to use = or not is stylistic when using curly brackets. Attempting to initialise without = and parentheses instead of curlies ends up being a function declaration for some cases, so you need to disambiguate by using = or curlies.

Curly brackets affect the form of initialisation in some cases. In cases where curlies and no curlies invoke the same constructor, using curlies is recommended because that syntax don't allow narrowing conversions. When curlies invoke a different constructor than no curlies, use the form that does what you need.

These apply to all initialisation; not just default members.

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