Re: Class Destroys itself straight away!

From:

David Wilkinson <no-reply@effisols.com>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 26 Jan 2008 12:40:03 -0500

Message-ID:

<#KyfCKEYIHA.3400@TK2MSFTNGP03.phx.gbl>

Gerry Hickman wrote:

Hi,

I have a Class who's constructor accepts an STL wstring, but it seems to
kill itself before it can be used. If I initialize with int, it works as
expected. Here's the sample code and the output. Note the Destructor is
firing twice.

#include "stdafx.h"
using namespace std;

class MyClass
{
public:
    MyClass(wstring);
    ~MyClass(void);
    void SayHello(void);
};

int _tmain(int argc, _TCHAR* argv[])
{
    wstring wsInput = _T("Passed in String");
    MyClass mc = MyClass(wsInput);
    // Class kills itself here!
    wcout << "About to use Class" << endl;
    mc.SayHello();
    return 0;
}

MyClass::MyClass(wstring wsNewString)
{
    wcout << "Constructing Class" << endl;
}

MyClass::~MyClass(void)
{
    wcout << "Destructing Class" << endl;
}

void MyClass::SayHello()
{
    wcout << "Say Hello" << endl;
}

// Output From Program

Constructing Class
Destructing Class <-- WHY??
About to use Class
Say Hello
Destructing Class

Gerry:

This is happening because you are making unnecessary copies. Try like this:

class MyClass
{
public:
     MyClass(const wstring&);
     ~MyClass(void);
     void SayHello(void);
};

int _tmain(int argc, _TCHAR* argv[])
{
     wstring wsInput = L"Passed in String";
     MyClass mc(wsInput);
     wcout << L"About to use Class" << endl;
     mc.SayHello();
     return 0;
}

Always pass input parameters by const reference if you can.

You can also do just

MyClass mc(L"Passed in String");

BTW, you should not use _T("") with wstring; use L"". If you want
"build-agnostic" code you can do things like

typdef std::basic_string<TCHAR> tstring;

--
David Wilkinson
Visual C++ MVP