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string object initialization

 
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Christian Meier
Guest
PostPosted: Thu Jul 28, 2005 6:40 am    Post subject: Re: string object initialization Reply with quote



"Baloff" <washdc (AT) wash (DOT) edu> schrieb im Newsbeitrag
news:874qaf2zit.fsf (AT) wash (DOT) edu...
Quote:

Hello
I am not sure why my compiler will not initialize
string e1("sam");
and will initialize
string e1 = "sam";

here is my code and the error.

thanks alot

********************code********************
#include #include
using namespace std;

typedef struct
{
string firstname;
string lastname;
int age;
}
employees;

int main(){
employees e1, e2;
e1.firstname = "sam";
e1.lastname = "Jesse";
e1.age=11;

e2.firstname ("sam2");
e2.lastname("Jesse2");
e2.age(22);

These statements do not initialize, I think. As your error messages say,
operators/functions of string/int are called with these three statements.

Quote:

cout << "employee e1nt" << e1.firstname << "nt"
e1.lastname << "nt" << e1.age << endl;


cout << "employee e2nt" << e2.firstname << "nt"
e2.lastname << "nt" << e2.age << endl;
}


********************Error********************
cd /home/sam/Exercies/ThinkingInC++/Vol1/C03/15/
make -k
g++ -g -c -o main.o main.cpp
main.cpp: In function `int main()':
main.cpp:20: error: no match for call to `(std::string) (const char[5])'
main.cpp:21: error: no match for call to `(std::string) (const char[7])'
main.cpp:22: error: call to non-function `employees::age'
make: *** [main.o] Error 1
make: Target `proj1' not remade because of errors.

Compilation exited abnormally with code 2 at Thu Jul 28 16:46:34



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Baloff
Guest
PostPosted: Thu Jul 28, 2005 6:52 am    Post subject: string object initialization Reply with quote



Hello
I am not sure why my compiler will not initialize
string e1("sam");
and will initialize
string e1 = "sam";

here is my code and the error.

thanks alot

********************code********************
#include <iostream>
#include <string>

using namespace std;

typedef struct
{
string firstname;
string lastname;
int age;
}
employees;

int main(){
employees e1, e2;
e1.firstname = "sam";
e1.lastname = "Jesse";
e1.age=11;

e2.firstname ("sam2");
e2.lastname("Jesse2");
e2.age(22);

cout << "employee e1nt" << e1.firstname << "nt"
<< e1.lastname << "nt" << e1.age << endl;


cout << "employee e2nt" << e2.firstname << "nt"
<< e2.lastname << "nt" << e2.age << endl;
}


********************Error********************
cd /home/sam/Exercies/ThinkingInC++/Vol1/C03/15/
make -k
g++ -g -c -o main.o main.cpp
main.cpp: In function `int main()':
main.cpp:20: error: no match for call to `(std::string) (const char[5])'
main.cpp:21: error: no match for call to `(std::string) (const char[7])'
main.cpp:22: error: call to non-function `employees::age'
make: *** [main.o] Error 1
make: Target `proj1' not remade because of errors.

Compilation exited abnormally with code 2 at Thu Jul 28 16:46:34
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upashu2
Guest
PostPosted: Thu Jul 28, 2005 7:18 am    Post subject: Re: string object initialization Reply with quote


Quote:
e2.firstname ("sam2");
e2.lastname("Jesse2");
It is not a string initialization, it is calling a function firstname()

with parameter "sam2" and no such function exists in struct employees.
In C++, You can't call consturctor in this way.
Quote:
string e1("sam");
It is perfectly fine. It is constructing the object by calling

constructor. But in object e2 , string is already constructed when u
write employees e2; with default constructor of string. and you are
thinking again calling a different constructor at same object.

In this case How will compiler differentiate whether u r calling a
function or constructor?So simply, It is not allowed in C++.

to call constructor of memeber objects, use initialization list of
your container. For example:
struct employees
{
string firstname;
string lastname;
int age;
employess() : firstname("Upashu2"),lastname("Balooff"),age(22) {
//////
} ////defining default constructor of
employees which call constructors
///////////////////////////////////////of member objects to initialize
them.
};


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upashu2
Guest
PostPosted: Thu Jul 28, 2005 7:27 am    Post subject: Re: string object initialization Reply with quote
Also,
struct employees
{
string firstname;
string lastname;
int age;
employess() : firstname("Upashu2"),lastname("Balooff"),age(22) { }
///default construcot
employess(char* f,char* l, int a) : firstname(f),lastname(l),age(a)
{
//////
} //defining overloaded consturcotr
};

employess e2("jammes2", "jasse2",22);

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Baloff
Guest
PostPosted: Thu Jul 28, 2005 9:47 am    Post subject: Re: string object initialization Reply with quote
"upashu2" <upashu1 (AT) rediffmail (DOT) com> writes:

Quote:
Also,
struct employees
{
string firstname;
string lastname;
int age;
employess() : firstname("Upashu2"),lastname("Balooff"),age(22) { }
///default construcot
employess(char* f,char* l, int a) : firstname(f),lastname(l),age(a)
{
//////
} //defining overloaded consturcotr
};

employess e2("jammes2", "jasse2",22);

now, what is more effecent, to have a constructor
inside the struct and do e1.firstname("some-name") or use the assigning
operator (=) with no constructor in the struct?

thanks

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upashu2
Guest
PostPosted: Thu Jul 28, 2005 10:25 am    Post subject: Re: string object initialization Reply with quote
Quote:
to have a constructor
inside the struct and do e1.firstname("some-name")
You cann't do e1.firstname("......"); read my prevoius post.post #3

What u can do is employees e1("......"); see my post #4.
Quote:
employess(char* f,char* l, int a) : firstname(f),lastname(l),age(a) {}
Using iniatialization list in constructor call the copy constructor of

object (here for strings) .
Quote:
employess(char* f,char* l, int a) { firstname = f; lastname =l; age =a;}
OR
employees e1; e1.firstname = "....."
This results first constructing the string object (firstname, lastname)

with their default constructor. after that when u assign the value
using "=" , it calls overladed assignment operator to copy the value
from rightside object to leftside object. it will take Two steps to
initialize the object.

In your case , if u don't want to define the constructor, simply use
employees e1; e1.firstname ="...."; Don't think to write
e1.firstname(....) even by mistake , It is syntax of function call,
not initalization, you will never found such statements in C++ anywhere.


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Jakob Bieling
Guest
PostPosted: Thu Jul 28, 2005 10:32 am    Post subject: Re: string object initialization Reply with quote
"Baloff" <washdc (AT) wash (DOT) edu> wrote


Quote:
int main(){
employees e1, e2;

At this point, all members of e1 and e2 have been initialized.

Quote:
e1.firstname = "sam";
e1.lastname = "Jesse";
e1.age=11;

All of the above are assignments.

Quote:
e2.firstname ("sam2");
e2.lastname("Jesse2");
e2.age(22);

Tho you think you are initializing, the objects are already
initialized above. You cannot initialize twice. That is why this syntax
is interpreted as a function call.

To answer your other question in a follow-up post: It is better to
have a constructor inside the struct, which initializes the strings
using the initializer list. The compiler might optimize your code above,
so it is just as fast, so the constructor with initializer-list might
not be faster with all compilers, but it will always be at least as fast
as the method above.

hth
--
jb

(reply address in rot13, unscramble first)



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Karl Heinz Buchegger
Guest
PostPosted: Thu Jul 28, 2005 11:56 am    Post subject: Re: string object initialization Reply with quote
Baloff wrote:
Quote:

"upashu2" <upashu1 (AT) rediffmail (DOT) com> writes:

Also,
struct employees
{
string firstname;
string lastname;
int age;
employess() : firstname("Upashu2"),lastname("Balooff"),age(22) { }
///default construcot
employess(char* f,char* l, int a) : firstname(f),lastname(l),age(a)
{
//////
} //defining overloaded consturcotr
};

employess e2("jammes2", "jasse2",22);

now, what is more effecent, to have a constructor
inside the struct and do e1.firstname("some-name") or use the assigning
operator (=) with no constructor in the struct?


In general questions about efficiency can aonly be answered with:
you have to try it on your specific platform.

But in this case, think about the following.

If you do (in a version with no constructor in the struct)

employess e1;
e1.firstname = "jammes2";
e1.lastname = "jasse2";
e1.age = 22;

What is going on?
Well. First of all the e1 object comes into existence. For this the members
get initialized. In this specific case this means:
* firstname initializes to an empty string
* lastname initializes to an empty string
* age does nothing, since it is a builtin type
it contains a random value

Then the assignments are performed, which do
* change firstname from an empty string to "jammes2"
* change lastname from an empty string to "jasse2"
* age changes from some unspeficied random value to 22

Now contrast this with the version using a constructor in the struct.
When you do:
empoyess e2( "jammes2", "jasse2",22);
what is going on.

Well. First of all the e2 object comes into existence. For this the members
get intialized. In this specfic case this means:
* firstname instializes to "jammes2"
* lastname initializes to "jasse2"
* age initializes to 22

and that's it. Same end result.

Now compare both variants and answer the question:
Which of those 2 variants is likely to be more efficient
then the other?

--
Karl Heinz Buchegger
[email]kbuchegg (AT) gascad (DOT) at[/email]

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Peter Julian
Guest
PostPosted: Thu Jul 28, 2005 2:50 pm    Post subject: Re: string object initialization Reply with quote

"Baloff" <washdc (AT) wash (DOT) edu> wrote

Quote:

Hello
I am not sure why my compiler will not initialize
string e1("sam");
and will initialize
string e1 = "sam";

You are not requesting an initialization with e1.firstname("sam2") above.
That is, until you provide the ctor that will carry out such initialization.
Since you haven't provided a ctor with parameters, the compiler is looking
for a function that matches that signature.

However, you can create a class that initializes its members through
parameters in the ctor. Note the initialization lists.

<snip>

#include <string>

class Employees
{
std::string firstname;
std::string lastname;
int age;
public:
Employees()
: firstname("unknown"), lastname("unknown"), age(0) { }
Employees(std::string fn, std::string ln, int n)
: firstname(fn), lastname(ln), age(n) { }
~Employees() { }

};

int main()
{
Employees e1("George", "Smith", 30);
}



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