C/C++ Question about New syntax of C++

[yungman]

I think the reason why it's not an error to omit the default constructors, is that in this program, the compiler never actually looks for a default constructor. The member declarations A1 a1; and B1 b1; by themselves do not invoke any constructors. That happens only when T1's constructor is invoked; that constructor explicitly invokes the non-default constructors A1 (int k) and B1 (int k), so the compiler never needs to look for a default constructor A1 () or B1 ().

Try removing either b1( tk ) or a1( tk ) from the initializer list for T1( int k ) and see what happens. This will force the compiler to look for a default constructor.

Here's a simpler program that illustrates the behavior that you're seeing:

#include <iostream>
using namespace std;

struct A {
    int ka;
    A( int k ) { ka = k; cout << "A: " << ka << endl; }
};

int main() {
    cout << "I'm not constructing any objects of type A." << endl;
    return 0;
}

This compiles successfully for me. Now, in main(), first add A a; which looks for a default constructor (and edit the output to cout if you like ). Then change it to A a(3); .

Thanks for the reply.

But it sure gave me a compiler error if I write it in my old way. I have to delete it to make it work.

#include <iostream>
using namespace std;
struct A1
{
    int ka;
    A1( int k)
    { ka = k; cout << "A1: " << ka << endl; }
};
struct B1
{
   int kb;
    B1( int k )
    { kb = k; cout << "B1: " << kb << endl; }
};
struct T1
{
    int tk;
//I cannot put A1 a1; B1 b1; here unless I write a default constructor.
    T1(int k)
    {
        tk = k;
        B1 b1(tk);
        A1 a1(tk);
    }
};
int main()
{
    T1 t1( 3 );
}

Please read line 18. This is a working program, but if I put A1 a1; B1 b1; in line 18, compiler will give me error unless I actually write in the default constructor.

If you look at the new syntax, compiler does not flag an error. That program does not have a default constructor, It is inconsistent. It's the new syntax that is different.

Thanks

 

[Jarvis323]

Thanks for the reply.

But it sure gave me a compiler error if I write it in my old way. I have to delete it to make it work.

#include <iostream>
using namespace std;
struct A1
{
    int ka;
    A1( int k)
    { ka = k; cout << "A1: " << ka << endl; }
};
struct B1
{
   int kb;
    B1( int k )
    { kb = k; cout << "B1: " << kb << endl; }
};
struct T1
{
    int tk;
//I cannot put A1 a1; B1 b1; here unless I write a default constructor.
    T1(int k)
    {
        tk = k;
        B1 b1(tk);
        A1 a1(tk);
    }
};
int main()
{
    T1 t1( 3 );
}

Please read line 18. This is a working program, but if I put A1 a1; B1 b1; in line 18, compiler will give me error unless I actually write in the default constructor.

If you look at the new syntax, compiler does not flag an error. That program does not have a default constructor, It is inconsistent. It's the new syntax that is different.

Thanks

In this version, a1 and b1 aren't member variables. That's not a working version. a1, b1 aren't part of T anymore. They're local variables inside T's constructor and are gone once T's constructor finishes.