Re: Working with strings in c++

"Vladimir Grigoriev" <vlad.moscow@xxxxxxx> wrote in message
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"John Carson" <jcarson_n_o_sp_am_@xxxxxxxxxxxxxxx> wrote in message
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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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