How to initialize a struct to 0 in C++

Question

Here is a related C answer that doesn't work (as a zero initializer for a struct) in C++: Initializing a struct to 0. One of the solutions presented is this:

myStruct _m1 = {0}; 

This works fine in C, but it doesn't work in C++. :( :

error: cannot initialize a member subobject of type 'myScope::MyStruct' with an rvalue of type 'int'`.

How do you zero-initialize a struct in C++?

Related:

1. Initializing a struct to 0 in C: Initializing a struct to 0
2. Update: (an adjacent, but NOT duplicate question which also turns out to be very useful) Initialization with empty curly braces

See also:

1. [adjacently-related, but is for C-style arrays, not structs] How to initialize all members of an array to the same value?

To the people voting to close this question:

My question is not a duplicate of this other question (Initialization with empty curly braces), as this other question isn't asking about the various ways to initialize a struct in C++ and why the C way doesn't work, rather, they are asking why does the C++ keyword explicit break one of their initialization techniques. Two distinct questions. Not duplicate.

Follow-up question I asked:

1. Why doesn't initializing a C++ struct to = {0} set all of its members to 0?

Answer 1

Before we start:

1. Let me point out that a lot of the confusion around this syntax comes because in C and C++ you can use the = {0} syntax to initialize all members of a C-style array to zero! See here: https://en.cppreference.com/w/c/language/array_initialization. This works:

   int z[3] = {0}; // z has type int[3] and holds all zeroes
   

   But, that syntax does not work the same for structs, which are entirely different animals than C-style arrays.

2. See also my follow-up question I asked after writing this answer below: Why doesn't initializing a C++ struct to = {0} set all of its members to 0?

---

Back to the answer:

I figured it out: to get it to compile, just delete the zero:

# does NOT work
myStruct _m1 = {0}; 

# works!
myStruct _m1 = {};

It now compiles. However, I ran a bunch of tests to check some things in my struct_initialization.cpp file in my eRCaGuy_hello_world repo, and that does NOT initialize all elements of the struct to zero! Rather, it initializes the struct to its default values. To run my tests and see for yourself, clone my repo above and run eRCaGuy_hello_world/cpp/run_struct_initialization.sh.

Assuming you have this struct:

typedef struct
{
    int num1 = 100;
    int num2 = -100;
    int num3;
    int num4 = 150;
} data_t;

Note: the typedef above is a carry-over from when I was testing this stuff in C instead of C++ (although the default struct values are not allowed in C, of course). For C++, this is preferred instead:

struct data_t
{
    int num1 = 100;
    int num2 = -100;
    int num3;
    int num4 = 150;
};

So please ignore it wherever I unnecessarily use typedef to define the structs below.

Anyway, if I declare one of the above data_t structs, and then do this:

data_t d2 = {};
printf("d2.num1 = %i\nd2.num2 = %i\nd2.num3 = %i\nd2.num4 = %i\n\n",
       d2.num1, d2.num2, d2.num3, d2.num4);

...the output will be:

d2.num1 = 100
d2.num2 = -100
d2.num3 = 0
d2.num4 = 150

And I'm not even sure if d2.num3 is zero because it was initialized to zero or because it was left uninitialized, and that memory location happened to contain zero.

As explained here: https://en.cppreference.com/w/cpp/language/zero_initialization, you can also do this:

myStruct _m1{};

In the example above, this code:

data_t d2{};
printf("d2.num1 = %i\nd2.num2 = %i\nd2.num3 = %i\nd2.num4 = %i\n\n",
       d2.num1, d2.num2, d2.num3, d2.num4);

...would produce output identical to what I showed above.

Even in cases where setting the struct to = {0} DOES work, such as this:

// Does NOT do what I expected! Only sets the FIRST value in the struct to zero! 
// The rest seem to use default values.
data_t d3 = {0};
printf("d3.num1 = %i\nd3.num2 = %i\nd3.num3 = %i\nd3.num4 = %i\n\n",
       d3.num1, d3.num2, d3.num3, d3.num4);

...the output is still not what I expected, as it only sets the first value to zero! (I don't understand why):

d3.num1 = 0
d3.num2 = -100
d3.num3 = 0
d3.num4 = 150

On C-style arrays, however (NOT structs), these semantics work fine. Refer to this answer here (How to initialize all members of an array to the same value?). The following lines, therefore, both set all elements of the C-style array to zero when using C++:

uint8_t buffer[100] = {0}; // sets all elements to 0 in C OR C++
uint8_t buffer[100] = {};  // sets all elements to 0 in C++ only (won't compile in C)

So, after much experimentation, it looks like the following several ways are the ONLY ways to zero-initialize a struct, PERIOD. If you know differently, please comment and/or leave your own answer here.

The only ways possible to zero-initialize a struct in C++ are:

1. Be explicit:

    // C-style typedef'ed struct
    typedef struct
    {
        int num1 = 100;
        int num2 = -100;
        int num3;
        int num4 = 150;
    } data_t;
   
    // EXPLICITLY set every value to what you want!
    data_t d1 = {0, 0, 0, 0};
    // OR (using gcc or C++20 only)
    data_t d2 = {.num1 = 0, .num2 = 0, .num3 = 0, .num4 = 0};
   

2. Use memset() to force all bytes to zero:

    data_t d3;
    memset(&d3, 0, sizeof(d3));
   

3. Set all default values to zero in the first place:

    // C-style typedef'ed struct
    typedef struct
    {
        int num1 = 0;
        int num2 = 0;
        int num3 = 0;
        int num4 = 0;
    } data_t;
   
    // Set all values to their defaults, which are zero in
    // this case
    data_t d4 = {};
    // OR
    data_t d5{}; // same thing as above in C++
   
    // Set the FIRST value only to zero, and all the rest
    // to their defaults, which are also zero in this case
    data_t d6 = {0};
   

4. Write a constructor for the C++ struct

    // 1. Using an initializer list
    struct data
    {
        int num1;
        int num2;
        int num3;
        int num4;
   
        data() : 
            num1(0),
            num2(0),
            num3(0),
            num4(0) {}
    };
   
    data d7; // all values are zero
   
    // OR: 2. manually setting the values inside the constructor
    struct data
    {
        int num1;
        int num2;
        int num3;
        int num4;
   
        data()
        {
            num1 = 0;
            num2 = 0;
            num3 = 0;
            num4 = 0;
        }
    };
   
    data d8; // all values are zero
   

5. Use a struct with no default values, and make your object you create from it static

    typedef struct
    {
        int num1;
        int num2;
        int num3;
        int num4;
    } data_t;
   
    // `static` forces a default initialization of zero for each
    // value when no other default values are set
    static data_t d9;
   

6. So, if you have a struct with non-zero default values, and you want to zero all values, you must do it EXPLICITLY! Here are some more ways:

    // 1. Have a `constexpr` copy of the struct that you use to
    // reset other struct objects. Ex:
   
    struct data
    {
        int num1 = 1;
        int num2 = 7;
        int num3 = -10;
        int num4 = 55;
    };
   
    constexpr data DATA_ALL_ZEROS = {0, 0, 0, 0};
   
    // Now initialize d13 to all zeros using the above `constexpr` struct 
    // object
    data d13 = DATA_ALL_ZEROS; 
   
   
    // OR 2. Use a `zero()` member function to zero the values:
   
    struct data
    {
        int num1 = 1;
        int num2 = 7;
        int num3 = -10;
        int num4 = 55;
   
        zero()
        {
            num1 = 0;
            num2 = 0;
            num3 = 0;
            num4 = 0;
        }
    };
   
    data d14;
    d14.zero();
   

The big take-away here is that NONE of these: data_t d{}, data_t d = {}, and data_t d = {0}, actually set all members of a struct to zero!

1. data_t d{} sets all values to their defaults defined in the struct.
2. data_t d = {} also sets all values to their defaults.
3. And data_t d = {0} sets only the FIRST value to zero, and all other values to their defaults.

SO, BE EXPLICIT!

Note that the above key take-aways I wrote seem to contradict this documentation on cppreference.com, so it has led me to ask this follow-up question listed just below, which has proven VERY helpful to my understanding!

Going further

1. MOST USEFUL: Follow-up question of mine: Why doesn't initializing a C++ struct to = {0} set all of its members to 0?

References:

1. VERY USEFUL:
   1. https://en.cppreference.com/w/cpp/language/zero_initialization
   2. https://en.cppreference.com/w/cpp/language/aggregate_initialization
   3. https://en.cppreference.com/w/cpp/language/value_initialization
2. VERY USEFUL: Initializing all members of an array (not struct) to the same value:
   1. How to initialize all members of an array to the same value?
   2. [gcc only] How to initialize all members of an array to the same value?
3. https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/cpp/struct_initialization.cpp
   1. Clone this repo and run the code yourself with cpp/run_struct_initialization.sh

Related:

1. Initializing default values in a struct
2. *****[my own answer, which demonstrate this sort of struct modification/aggregate member reassignment within any function: leds[0] = {10, 20, 30, 40, 50};] Arduino Stack Exchange: Initializing Array of structs

Answer 2

Generically it's not possible. A class may not provide a way to access all members and in those cases the best you can do is value initialize it which will give you a default version of that class.

Answer 3

Objects in C++ are given the ability to control the values of any subobjects within them. As such, C++ has no mechanism that will force zero-initialization onto any object in general.

Objects with no user-provided constructors or default member initializers can undergo zero-initialization in two cases: if the variable is declared static, or if the object is being value-initialized. There are multiple syntaxes that will provoke value initialization of an object, including T(), T{}, and T t = {};, where applicable.

But other than that, if the creator of the type of an object does not want it to be zero-initialized, then you cannot impose that upon the object. You can ask for value-initialization or default-initialization, but whether either of these will invoke zero-initialization depends on the type.

Answer 4

In C++ for types with no constrictor (id est trivially constructible), {} always initializes all bytes to zero, identically the same as { 0 } in C.

PlainOldData pod{};

For types with a constructor, hence not trivially constructable, it makes little sense to initialize all bytes to zero as those objects are meant to control their own state. What you usually want is default initialization, which can usually be done with {}.:

Object obj{};
// or
Object obj;

You can go around this limitation and use memset to clear the memory, but be aware that it will cause a lot of problems with overwriting vtable pointers, et cetera.

memset(&obj, 0, sizeof obj); // dangerous for non-trivial objects