const object and const member

[Jess]

Hello,

How can I create a constant object? I tried the following code but
failed:

#include<string>

using namespace std;

struct A{
int x;
string s;
};

int main(){
const A b;
b.x = 2;
b.s = "bs";
return 0;
}

In addition, how can I define a class/struct that has a const member
data? I tried the following but it also failed.

using namespace std;

struct A{
const int x;
string s;
};

Thanks.

[Gianni Mariani]

Jess wrote:

Hello,

How can I create a constant object? I tried the following code but
failed:

#include<string>

using namespace std;

struct A{
int x;
string s;
};

int main(){
const A b;
b.x = 2;
b.s = "bs";
return 0;
}

In addition, how can I define a class/struct that has a const member
data? I tried the following but it also failed.

using namespace std;

struct A{
const int x;
string s;
};

Thanks.

There are a number of things you can do ... I added some stuff to your
code below.

#include<string>

using namespace std;

struct A{
int x;
string s;
};

int main(){

// first way - initialize a non const object
A a;
a.x = 2;
a.s = "S";

// then initialize the class
const A c( a );

// second - use an initializer
const A b = { 2, "A" };

return 0;
}

namespace Way3
{
struct A{
const int x;
string s;

// have a constructor
A( int ia, string is ) : x( ia ), s( is ) {}
};

A a( 2, "S" );
const A b( 2, "S" );

// then initialize the class
const A c( a );

} // end namespace Way3

[Juha Nieminen]

Jess wrote:

How can I create a constant object? I tried the following code but
failed:
const A b;
b.x = 2;
b.s = "bs";

Since 'b' is const you naturally cannot modify it, as you are
trying to do there.

The only place where you can "modify" the contents of a constant
objects is in the constructor of that struct/class. So you'll have
to create a constructor for it and give those values as parameters
to the constructor.

In addition, how can I define a class/struct that has a const member
data? I tried the following but it also failed.

struct A{
const int x;
string s;
};

I see no problem with that code per se. Of course the only way
you can initialize 'x' is, once again, in the constructor of A,
so you'll have to create one. For example like this:

struct A
{
const int x;
string s;

A(int iVal = 0): x(iVal) {}
};

[Gianni Mariani]

Juha Nieminen wrote:

... the only way
you can initialize 'x' is, once again, in the constructor of A,
so you'll have to create one. For example like this:

struct A
{
const int x;
string s;

A(int iVal = 0): x(iVal) {}
};

You don't "have to" create a constructor. There are other ways to
initialize it.