Is if ( c ) the same as if ( c == 0 ) in C++?
views:
480answers:
7
+3
A:
It's more like if ( c != 0 )
Of course, != operator can be overloaded so it's not perfectly accurate to say that those are exactly equal.
Mehrdad Afshari
2009-09-25 19:20:31
+10
A:
No, if (c) is the same as if (c != 0).
And if (!c) is the same as if (c == 0).
Jesper
2009-09-25 19:20:42
Assuming no odd overloading is taking place, which is often part of the answer in C++.
David Thornley
2009-09-25 19:38:54
Ofcourse, if the operators == or != are overloaded, anything could happen, it could even start playing the national anthem for you... ;-)
Jesper
2009-09-26 06:27:09
One of these days I'm going to make a class which overloads operators to play sound effects. It's gonna be awesome every time someone wants to sort them.
rlbond
2009-11-05 05:59:41
+9
A:
I'll break from the pack on this one... "if (c)" is closest to "if (((bool)c) == true)". For integer types, this means "if (c != 0)". As others have pointed out, overloading operator != can cause some strangeness but so can overloading "operator bool()" unless I am mistaken.
D.Shawley
2009-09-25 19:38:45
The question was originally tagged with only C, hence the above answers since there is no bool in C.
Brian R. Bondy
2009-09-25 19:44:10
Or `if(bool cond = c) ...;` to show the conversion is implicit.
Johannes Schaub - litb
2009-09-25 23:35:01
A:
This is only true for numeric values. if c is class, there must be an operator overloaded which does conversion boolean, such as in here:
#include <stdio.h>
class c_type
{
public:
operator bool()
{
return true;
}
};
int main()
{
c_type c;
if (c) printf("true");
if (!c) printf ("false");
}
galets
2009-09-25 19:47:31
+1
A:
If c is a pointer or a numeric value,
if( c )
is equivalent to
if( c != 0 )
If c is a boolean (type bool [only C++]), (edit: or a user-defined type with the overload of the operator bool())
if( c )
is equivalent to
if( c == true )
If c is nor a pointer or a numeric value neither a boolean,
if( c )
will not compile.
Patrice Bernassola
2009-09-25 20:35:30
It c is a user-defined type one can just implement `operator bool()` as D. Shawley and galets both pointed out and so it will compile just fine.
Troubadour
2009-09-25 20:39:39
A:
If c is a pointer then the test
if ( c )
is not quite the same as
if ( c != 0 )
The latter is a straightforward check of c against the 0 (null) pointer whereas the former is actually a instruction to check whether c is points to a valid object. Typically compilers produce the same code though.
Troubadour
2009-09-25 20:37:11
no, it's not... both mean absolutely the same in the case of a pointer: if( c != NULL )
Massa
2009-09-25 22:38:06
There is no way for the compile and/or the runtime to know whether an object is valid in either C or C++.You can write:" X* c = (X*)1; "Then c almost certainly points to an invalid object. And" if(c) " will be true.
Andrew Stein
2009-09-26 04:47:21
@Andrew: I doubt you can justify your statement that there is no way to check that the object is valid or not. Certainly it would require a lot of effort by the compiler to generate run-time code to check and that's why compilers don't bother and generate the same code i.e. test against the null pointer. That's why your example fails to produce a false result on (probably) all current compilers.
Troubadour
2009-09-26 08:06:23
@Massa: I'm afraid not. My reference is section 6.3.2 (Selection Statements) from "The C++ Programming Language, Special Edition" by Bjarne Stroustrup (the creator of C++). What's yours?
Troubadour
2009-09-26 08:09:50
Comparing an object pointer against a null pointer does check whether it points to a valid object, while "valid object" means "object with non-null-pointer address"
Johannes Schaub - litb
2009-09-26 11:10:48