Testing my knowledge: how many constructors are called

From:

AhmedB <ahmed.badran@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 2 Aug 2008 22:44:43 -0700 (PDT)

Message-ID:

<6ca64954-670f-4e7d-9804-ae2bcba88971@1g2000pre.googlegroups.com>

Hi,

I wrote this program, compiled it and ran it with the expectation that
I will get two printed lines (one from each ctor):

#include <cstdio>
class A
{
    public:
    A(int a)
    {
        fprintf(stderr, "A(int a)\n");
        i = a;
    }
    A(const A& a)
    {
        fprintf(stderr, "A(const A& a)\n");
        i = a.i;
    }
    private:
    int i;
    private:
    const A& operator= (const A&)
    {
    }
};

int main()
{
    A a = A(5);
    return 0;
}

I only got:

A(int a)

So I thought Ok, maybe the compiler is compiling the

A a = A(5); to be

A a(5) somehow,

So I went ahead and changed the code to make the copy constructor
explicit, as follows:

#include <cstdio>
class A
{
    public:
    A(int a)
    {
        fprintf(stderr, "A(int a)\n");
        i = a;
    }
    explicit A(const A& a)
    {
        fprintf(stderr, "A(const A& a)\n");
        i = a.i;
    }
    private:
    int i;
    private:
    const A& operator= (const A&)
    {
    }
};

int main()
{
    A a = A(5);
    return 0;
}

Then I tried compiling the code and got the following:

test.cc: In function =91int main()':
test.cc:42: error: no matching function for call to =91A::A(A)'
test.cc:18: note: candidates are: A::A(int)

So my question is, if my understanding is correct and this is the
sequence that is going on:
1. An A::A(int) constructor is used to convert an int to an A,
followed by
2. An A::A(const A&) constructor is used to copy the temporary A
and this is why I'm getting the compilation error when I make the
A::A(const A&) explicit, then why am I not getting two printed lines
in the first case? Is there something else going on here that I'm not
paying attention to?

This code is compiled/run on a Suse 10.3 Linux box with gcc/g++
version 4.2.1.

Regards,
Ahmed