Re: Working with strings in c++

From:
"John Carson" <jcarson_n_o_sp_am_@netspace.net.au>
Newsgroups:
microsoft.public.vc.language
Date:
Fri, 9 Feb 2007 02:53:11 +1100
Message-ID:
<eaTpAl5SHHA.496@TK2MSFTNGP06.phx.gbl>
"Vladimir Grigoriev" <vlad.moscow@mail.ru> wrote in message
news:%23K7IUc5SHHA.412@TK2MSFTNGP02.phx.gbl...

"John Carson" <jcarson_n_o_sp_am_@netspace.net.au> wrote in message
news:%23YjI6R4SHHA.2212@TK2MSFTNGP02.phx.gbl...

Is not the two declaration above the same?
For example what is the difference between
int i = 10;
and
int i( 10 );


No difference when you are talking about built in types. When talking
about user-defined types, however, there is a difference.

const CString myStr = "val1;val2;val3";

is supposed to involve two steps:

1. The creation of a temporary CString object initialised by
"val1;val2;val3"

2. The use of a copy constructor to initialise myStr from the temporary
CString object.

By contrast,

const CString myStr("val1;val2;val3");

involves a single step: the calling of the CString constructor to
initialise myStr.


You are wrong! In both cases the single CString constructor with
parameters is called. Neither copy constructor is called. I think you can
test this youself with a simple test program.


Try reading the answer.

"as an optimisation, the compiler is allowed (but not obliged) to
replace the two steps involved in

const CString myStr = "val1;val2;val3";

with a single call to the myStr constructor."

That is exactly what VC++ does. Nevertheless, your code should allow the
compiler to do it the two-step way, or the compiler is allowed to refuse to
compile the code --- even if it would optimise away the two steps were those
two steps possible.

To illustrate Paul's point, try compiling this:

#include <cstring>
using namespace std;

class MyString
{
    char *str;
public:
    explicit MyString(const char* arg)
    {
        str = new char[strlen(arg)+1];
        strcpy(str, arg);
    }
    MyString(const MyString& rhs)
    {
        delete[] str;
        str = new char[strlen(rhs.str)+1];
        strcpy(str, rhs.str);
    }
};

int main()
{
    MyString ms2 = "test";
    return 0;
}

To illustrate my point, try compiling this using Comeau online

http://www.comeaucomputing.com/tryitout/

#include <cstring>
using namespace std;

class MyString
{
    char *str;
public:
    MyString(const char* arg)
    {
        str = new char[strlen(arg)+1];
        strcpy(str, arg);
    }
private:
    MyString(const MyString& rhs)
    {
        delete[] str;
        str = new char[strlen(rhs.str)+1];
        strcpy(str, rhs.str);
    }
};

int main()
{
    MyString ms2 = "test";
    return 0;
}

--
John Carson

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