Multiple variable inialization

[Swartz]

Hi there.

I was wondering why the following two constructions dont work as expected:

1.) bool a, b, c, d = false;

2.) bool a, b, c, d;
a, b, c, d = false;

But the following works.
bool a, b, c, d;
a = b = c = d = false;

TIA

 

[Gianni Mariani]

Hi there.

I was wondering why the following two constructions dont work as expected:

1.) bool a, b, c, d = false;

This means this :

bool a;
bool b;
bool c;
bool d = false;

2.) bool a, b, c, d;
a, b, c, d = false;

This is using the "comma" operator.

It may be re-written as;

a; b; c; d=false; // well alomost

But the following works.
bool a, b, c, d;
a = b = c = d = false;

This means :

(a=(b=(c=(d=false))));


You can do somthing like:

template < typename w_type, typename w_init_type, w_init_type w_val >
class AT_IType
{
public:

w_type m_value;

inline AT_IType()
: m_value( w_val )
{
}

inline operator w_type & ()
{
return m_value;
}

inline operator const w_type & () const
{
return m_value;
}

inline w_type & operator = ( const w_type & i_value )
{
m_value = i_value;

return m_value;
}

};

AT_IType <bool,bool,false> a, b, c, d;

 

[Rolf Magnus]

Hi there.

I was wondering why the following two constructions dont work as
expected:

That depends on what you expect. They work as expected here.

 

[tom_usenet]

Hi there.

I was wondering why the following two constructions dont work as expected:

1.) bool a, b, c, d = false;

bool a = false, b = false, c = false, d = false;

You have to initialize each one separately. Often the advice in C++ is
to have only 1 variable per declaration. e.g.

bool a = false;
bool b = false;
bool c = false;
bool d = false;

is clearer.

2.) bool a, b, c, d;
a, b, c, d = false;

The comma operator is used to separate expressions and return the
right hand expression. So that is exquivalent to:
evaluate a, discard the value
evaluate b, discard the value
evaluate c, discard the value
evaluate "d = false", discard the value.
So only d is modified.

Using the comma operator in an unusual way:
a = false, b = false, c = false, d = false;
or in conventional syntax:
a = false; b = false; c = false; d = false;

But the following works.
bool a, b, c, d;
a = b = c = d = false;

It should be obvious why that works, but for the others you just have
to understand declaration syntax and the comma operator.

Tom

C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html