How to initialize structure in C++?

Question

I am using a C-style structure which does not have any constructor, like this:

struct structName {
    int  mem1;
    int  mem2;
    char mem3;
    char mem4;
}

I am creating a variable of this structure and I want to initialize all members of the structure to zero. I found the following methods.

```
struct structName structVar = {};
```
```
struct structName structVar = {0};
```

struct structName structVar = struct structName();

For the first two methods, my compiler is giving "missing initializer for member" warning.

The third approach compiles without warnings.

Is it a valid C++ statement?
Am I missing some genuine warning/error by using method 3?
Is there any better alternative method, apart from memset()?

@101010 No, not necessarily. `struct foo { foo()=default; foo(int){} };`. — juanchopanza, Sep 01 '16 at 09:40
And what does your structure look like? Does it have any constructors? Destructors? — Some programmer dude, Sep 01 '16 at 09:40
In C++ you do not need to write `struct` there at all, so I wonder what resource you found when you "searched on internet". Prefer learning C++ from a good book... — Lightness Races in Orbit, Sep 01 '16 at 09:41
@lightness-races-in-orbit, ok, I can remove "struct". But I am more interested about the initialization. — MayurK, Sep 01 '16 at 09:43
@joachim-pileborg, my structure has no constructor/ destructor. It has some int and char members. — MayurK, Sep 01 '16 at 09:45
`= {};` is the best way. You could disable that warning. Preferable to mangling your code. — M.M, Sep 01 '16 at 09:50
[Works fine for me](http://ideone.com/G8G9G2). Please create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve) and show us. — Some programmer dude, Sep 01 '16 at 09:51
@MayurK: I understand that but it's a warning flag about what resources you're using, which is important. — Lightness Races in Orbit, Sep 01 '16 at 10:49
Is this line `struct structName structVar = struct structName();` really legal? — Carousel, Sep 01 '16 at 14:09
@Carousel, Sorry for the confusion. I have a .cpp file with C-style coding. `struct structName structVar = struct structName();` is not allowed in C but is allowed in C++. — MayurK, Sep 01 '16 at 14:42
Speaking of [learning C++ from a good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list?lq=1)... — Rob K, Oct 13 '16 at 14:44

bolov · Accepted Answer · 2016-09-01T14:16:14.770

The preferred method should be one of:

structName structVar{};
structName structVar = {};
auto structName = structVar{};

_{there are subtle differences, but not for aggregates as in your example}

This has the added advantage that it initializes structVar for any type of structName or if it cannot perform an initialization it makes the program ill-formed (the code doesn't compile) (plus it doesn't allow narrowing).

In your specific example, structName is an agregate:

C++14 draft standard:

§8.5.1 Aggregates [dcl.init.aggr]

(1) An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3)

The initialization syntax I used is called List-initialization:

§8.5.4 List-initialization [dcl.init.list]

(1) List-initialization is initialization of an object or reference from a braced-init-list. [...] An initializer list may be empty. [...]

For our aggregate this means:

§8.5.1 Aggregates [dcl.init.aggr]

(2) When an aggregate is initialized by an initializer list, as specified in 8.5.4, the elements of the initializer list are taken as initializers for the members of the aggregate, in increasing subscript or member order. Each member is copy-initialized from the corresponding initializer-clause

(7) If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from its brace-or-equal-initializer or, if there is no brace-or-equalinitializer, from an empty initializer list (8.5.4).

[ Example:
struct S { int a; const char* b; int c; int d = b[a]; };
S ss = { 1, "asdf" };
initializes ss.a with 1, ss.b with "asdf", ss.c with the value of an expression of the form int{} (that is, 0), and ss.d with the value of ss.b[ss.a] (that is, ’s’)

[...]

end example ]

So all of these are valid and do the exact same thing:

structName structVar = {};
structName structVar = {0};
structName structVar = {0, 0};

However if there is at least one initializer-clauses and less than there are members in the aggregate, gcc and clang emit a warning. It might be that you intended to initialize all members, but missed some. So the empty initializer list is the safest choice.

As a side note struct is not needed and universally not used in a declaration. So replace this:

struct structName structVar

with:

structName structVar

How to initialize structure in C++?

1 Answers1