Create an object within a class

Projects Programming
1

I already found many posts talking about this very problem but could not understand any of them. I wanted a simple explanation why this does not work:

class foo {
  public:
    foo(uint8_t x) {
      Serial.begin(115200);
      Serial.println(x);
    }
};

class bar {
  private:
    foo foo_1(23);  // << I get an error here error: expected ',' or '...' before numeric constant
};


void setup() {
  bar bar_1;
}
void loop() {

}

Am I missing something here? I feel like I knew the answer before but every other post regarding problems related to this I can't understand. I wanted the bare minimum example of how to get it to work with "foo" and "bar"

Another thing is, how does this work and the first does not??:

class foo {
  public:
    foo() {
      Serial.begin(115200);
      Serial.println(23);
    }
};

class bar {
  private:
    foo foo_1;
};


void setup() {
  bar bar_1;
}
void loop() {

}

Any help will be greatly appreciated!

2

Use curly braces, not parentheses:

foo foo_1 {23};

Or in this case, since you have just a single argument, you could also just use foo foo_1 = 23.

Note that you're specifying a default value for a member variable.

3

ohhh, now I feel dumb. Thank you very much.
Although, what is the difference between foo foo_1 {23}; and foo foo_1 = foo(23);?
it seems to also work

4

The former directly initializes the member variable with 23, the latter first creates a temporary foo object, and then copies that to the member variable. Although that copy probably gets elided anyway.
I prefer the first one, because I do not like to repeat the type name foo.

For future reference, initialization in C++ is a complex topic:

https://en.cppreference.com/w/cpp/language/initialization

5

I was expecting an initializer list to be required:

class foo {
  public:
    foo(uint8_t x) {
      Serial.begin(115200);
      Serial.println(x);
    }
};

class bar {
  public:
    bar() : foo_1(23) {}
  private:
    foo foo_1;
};


void setup() {
  bar bar_1;
}

void loop() {
}
6

Default member initialization is just syntactic sugar that automatically adds the initializer to the initializer lists of all constructors that don't explicitly initialize the member.