constructor question

[technologist]

Hi,

I provide a constructor or not and the program executes the same. What is the purpose of initializing var_1 & var_2 through a constructor?

#include<iostream>
//this code will handle request from main to access private values
//can work without the constructor

class Pass_it
{
private:
	int var_1 = 120;
	int var_2 = 98;
public:
	Pass_it()
{
		var_1, var_2;
}
	void print_it()
	{
		std::cout<<var_1<<"  "<<var_2;
	}

};


int main()
{
Pass_it pass_it;
pass_it.print_it();

	return 0;
}

120  98

[jlb]

I provide a constructor or not and the program executes the same

Yes you provided a constructor, however that constructor isn't actually doing anything and is therefore unnecessary. You used the assignment to initialize the variables on lines 8 and 9, not the constructor.

[technologist]

ok, how would I then use a public method to retrieve private values? That's were I am stuck.

[technologist]

I'll answer my own question there (below - public method retrieving private values). I think I'm trying to force the constructor issue because I am studying them. I'll have to wait for the need to develop would be a better strategy.

#include<iostream>

class Pass_it
{
private:
	int var_1 = 6;
	int var_2 = 7;
public:

	void print_it()
	{
		std::cout<<var_1<<"  "<<var_2;
	}

};


int main()
{
Pass_it pass_it;
pass_it.print_it();
	return 0;
}

Generically what circumstances would I be most prone to use constructors.

Thx

[TheIdeasMan]

ok, how would I then use a public method to retrieve private values? That's were I am stuck.

You are doing that just fine (printing them). But you don't have a ctr to set any member variables, that is the purpose of them.

public:
	Pass_it(const int var1Arg, const int var2Arg)
         :  //colon introduces initializer list
          var_1 (var1Arg),  // make sure to set all members variables here
          var_2 (var2Arg)   // in the same order as listed in the private section
          {
           //do validation here
           }

If you mean retrieving the variables as in returning them, just use a get function, or a function with parameters as references, or return a std::tuple.

[TheIdeasMan]

Note that :

private:
	int var_1 = 6;
	int var_2 = 7;

In C++11 is the same as using an initializer list with those values. But the ctr I had shown is better, because one can set any values via the arguments.

Generically what circumstances would I be most prone to use constructors.

For two reasons:

1 to initialize all the member variables
2 to validate the member variables

The second reason leads to exceptions, but not everyone is keen on them:
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines.html#Re-invariant

CppcoreGuidlines wrote:

Note

But what do we do if we are writing a program where exceptions cannot be used? First challenge that assumption; there are many anti-exceptions myths around. We know of only a few good reasons:

We are on a system so small that the exception support would eat up most of our 2K or memory.
We are in a hard-real-time system and we don’t have tools that guarantee us that an exception is handled within the required time.
We are in a system with tons of legacy code using lots of pointers in difficult-to-understand ways (in particular without a recognizable ownership strategy) so that exceptions could cause leaks.
We get fired if we challenge our manager’s ancient wisdom.

The last 2 reasons may be the most common :+)

[TheIdeasMan]

There are a bunch of conceptual ideas that go with ctr's, Google these:

Invariant
Special member functions. Implicit, defaulted, deleted.
Rule of 0, 3, 5

It's late at my end, catch you later ZZZZzzzzzzzzzz..........

[technologist]

Thx ID-will catch up on the suggestions and reading.

Later : )