How to initialize a struct in accordance with C pr

Question

I want to initialize a struct element, split in declaration and initialization. This is what I have:

typedef struct MY_TYPE {
  bool flag;
  short int value;
  double stuff;
} MY_TYPE;

void function(void) {
  MY_TYPE a;
  ...
  a = { true, 15, 0.123 }
}

Is this the way to declare and initialize a local variable of MY_TYPE in accordance with C programming language standards (C89, C90, C99, C11, etc.)? Or is there anything better or at least working?

Update I ended up having a static initialization element where I set every subelement according to my needs.

Answer 1

C programming language standard ISO/IEC 9899:1999 (commonly known as C99) allows one to use a designated initializer to initialize members of a structure or union as follows:

MY_TYPE a = { .stuff = 0.456, .flag = true, .value = 123 };

It is defined in paragraph 7, section 6.7.8 Initialization of ISO/IEC 9899:1999 standard as:

If a designator has the form
. identifier
then the current object (defined below) shall have structure or union type and the identifier shall be the name of a member of that type.

Note that paragraph 9 of the same section states that:

Except where explicitly stated otherwise, for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization. Unnamed members of structure objects have indeterminate value even after initialization.

In GNU GCC implementation however omitted members are initialized as zero or zero-like type-appropriate value. As stated in section 6.27 Designated Initializers of GNU GCC documentation:

Omitted field members are implicitly initialized the same as objects that have static storage duration.

Microsoft Visual C++ compiler should support designated initializers since version 2013 according to official blog post C++ Conformance Roadmap. Paragraph Initializing unions and structs of Initializers article at MSDN Visual Studio documentation suggests that unnamed members initialized to zero-like appropriate values similarly to GNU GCC.

New ISO/IEC 9899:2011 standard (commonly known as C11) which had superseded ISO/IEC 9899:1999 retains designated initializers under section 6.7.9 Initialization. It also retains paragraph 9 unchanged.

Answer 2

I have read the Microsoft Visual Studio 2015 Documentation for Initializing Aggregate Types yet, all forms of initializing with {...} are explained there, but the initializing with dot, named ''designator'' isn't mentioned there. It does not work also.

The C99 standard chapter 6.7.8 Initialization explains the possibility of designators, but in my mind it is not really clear for complex structs. The C99 standard as pdf .

In my mind, it may be better to

1. Use the = {0};-initialization for all static data. It is less effort for the machine code.

2. Use macros for initializing, for example

   typedef MyStruct_t{ int x, int a, int b; } MyStruct; define INIT_MyStruct(A,B) { 0, A, B}

The macro can be adapted, its argument list can be independent of changed struct content. It is proper if less elements should be initialized. It is also proper for nested struct. 3. A simple form is: Initialize in a subroutine:

void init_MyStruct(MyStruct* thiz, int a, int b) {
  thiz->a = a; thiz->b = b; }

This routine looks like ObjectOriented in C. Use thiz, not this to compile it with C++ too!

MyStruct data = {0}; //all is zero!
init_MyStruct(&data, 3, 456);

Answer 3

void function(void) {
  MY_TYPE a;
  a.flag = true;
  a.value = 15;
  a.stuff = 0.123;
}

Answer 4

a = (MYTYPE){ true, 15, 0.123 };

would do fine in C99

Answer 5

You can do it with a compound literal. According to that page, it works in C99 (which also counts as ANSI C).

MY_TYPE a;

a = (MY_TYPE) { .flag = true, .value = 123, .stuff = 0.456 };
...
a = (MY_TYPE) { .value = 234, .stuff = 1.234, .flag = false };

The designations in the initializers are optional; you could also write:

a = (MY_TYPE) { true,  123, 0.456 };
...
a = (MY_TYPE) { false, 234, 1.234 };

Answer 6

Structure in C can be declared and initialized like this:

typedef struct book
{
    char title[10];
    char author[10];
    float price;
} book;

int main() {
    book b1={"DS", "Ajay", 250.0};

    printf("%s \t %s \t %f", b1.title, b1.author, b1.price);

    return 0;
}